■ .

ÄRID-ZONE HYDROLOGY INVESTIGATIONS WITH SOTOPE TECHNIQUES PROCEEDINGS OF AN ADVISORY GROUP MEETING, VIENNA, 6-9 NOVEMBER 1978

Ш

W) INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1980

ARID-ZONE HYDROLOGY: INVESTIGATIONS WITH ISOTOPE TECHNIQUES

STI/PUB/547

CORRIGENDUM

Paper IAEA-AG-158/17, by J.Ch. Fontes et al.

Page 237, authors’ names

F o r P. POUCHON rea d P. POUCHAN (The Contents List should be corrected accordingly)

Page 259, equation at top of page

F o r 0.3 rea d 0.13

List of Participants

Page 265, under ‘International Atomic Energy Agency (IAEA)’

Add the following name: Gonfiantini, R. Section of Isotope Hydrology, (Scientific Secretary) Division of Research and Laboratories

ARID-ZONE HYDROLOGY: INVESTIGATIONS WITH ISOTOPE TECHNIQUES The following States are Members of the International Atomic Energy Agency:

AFGHANISTAN HOLY SEE PHILIPPINES ALBANIA HUNGARY POLAND ALGERIA ICELAND PORTUGAL ARGENTINA INDIA QATAR AUSTRALIA INDONESIA ROMANIA AUSTRIA IRAN SAUDI ARABIA BANGLADESH IRAQ SENEGAL BELGIUM IRELAND SIERRA LEONE BOLIVIA ISRAEL SINGAPORE BRAZIL ITALY SOUTH AFRICA BULGARIA IVORY COAST SPAIN BURMA JAMAICA SRI LANKA BYELORUSSIAN SOVIET JAPAN SUDAN SOCIALIST REPUBLIC JORDAN SWEDEN CANADA KENYA SWITZERLAND CHILE KOREA, REPUBLIC OF SYRIAN ARAB REPUBLIC COLOMBIA KUWAIT THAILAND COSTA RICA LEBANON TUNISIA CUBA LIBERIA TURKEY CYPRUS LIBYAN ARAB JAMAHIRIYA UGANDA CZECHOSLOVAKIA LIECHTENSTEIN UKRAINIAN SOVIET SOCIALIST DEMOCRATIC KAMPUCHEA LUXEMBOURG REPUBLIC DEMOCRATIC PEOPLE’S MADAGASCAR UNION OF SOVIET SOCIALIST REPUBLIC OF KOREA MALAYSIA REPUBLICS DENMARK MALI UNITED ARAB EMIRATES DOMINICAN REPUBLIC MAURITIUS UNITED KINGDOM OF GREAT ECUADOR MEXICO BRITAIN AND NORTHERN EGYPT MONACO IRELAND EL SALVADOR MONGOLIA UNITED REPUBLIC OF ETHIOPIA MOROCCO CAMEROON FINLAND NETHERLANDS UNITED REPUBLIC OF FRANCE NEW ZEALAND TANZANIA GABON NICARAGUA . UNITED STATES OF AMERICA GERMAN DEMOCRATIC REPUBLIC NIGER URUGUAY GERMANY, FEDERAL REPUBLIC OF NIGERIA VENEZUELA GHANA NORWAY VIET NAM GREECE PAKISTAN YUGOSLAVIA GUATEMALA PANAMA ZAIRE HAITI PARAGUAY ZAMBIA PERU

The Agency’s Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters of the Agency are situated in Vienna. Its principal objective is “to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world”.

© IAEA, 1980

Permission to reproduce or translate the information contained in this publication may be obtained by writing to the International Atomic Energy Agency, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria.

Printed by the IAEA in Austria November 1980 PANEL PROCEEDINGS SERIES

ARID-ZONE HYDROLOGY:

INVESTIGATIONS

W ITH ISOTOPE TECHNIQUES

PROCEEDINGS OF AN ADVISORY GROUP MEETING ON APPLICATION OF ISOTOPE TECHIQUES IN ARID ZONES HYDROLOGY ORGANIZED BY THE INTERNATIONAL ATOMIC ENERGY AGENCY AND HELD IN VIENNA FROM 6 TO 9 NOVEMBER 1978

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1980 ARID-ZONE HYDROLOGY: INVESTIGATIONS WITH ISOTOPE TECHNIQUES IAEA, VIENNA, 1980 STI/PUB/547 ISBN 92-0-141180-4 FOREWORD

The present publication includes the papers presented at the Advisory Group Meeting on the Application of Isotope Techniques to Arid-Zone Hydrology, which was held in Vienna from 6 to 9 November 1978. Twenty-eight scientists - twenty-two invited participants and six observers — representing fourteen countries and one international organization, attended the meeting which was chaired by Professor J.Ch. Fontes of the University of Paris-Sud. It is frequently admitted that isotope hydrology has achieved the most significant results in arid zones. The most frequent applications of environmental isotope techniques are, on the basis of the papers presented at the meeting: ( 1 ) To investigate the occurrence and the mechanisms of modern recharge which, in extremely arid zones, seems to take place mainly through wadis rather than through direct infiltration of the rainfall; this latter mechanism seems to be significant only in less arid and in semi-arid areas. (2) To assess the occurrence and the characteristics of past recharge and of palaeogroundwaters, which are anon-renewable resource. (3) To provide evidence of interconnections between aquifers. All these problems are quite complex and therefore the isotopic data should be interpreted together with all available hydrological, hydrogeological and hydro­ chemical data. It should be emphasized, however, that isotopes often provide a type of information which is not possible to obtain with other techniques. For instance, tritium occurrence is a definite proof of the presence of modern water in an aquifer, and stable isotopes can be used as conservative tracers to prove or disprove interconnections between aquifers and to follow groundwater flow patterns. In conclusion, we believe that this book gives a sufficiently complete and up-to-date account of the progress made by environmental isotope techniques in arid-zone hydrology, as well as of the problems that still remain open and demand further thought and data. It is hoped therefore that hydrogeologists working in arid zones who are not familiar with the use of isotope techniques will find in this book ideas and suggestions on how to tackle some of the problems that they are facing.

CONTENTS

GROUNDWATER RECHARGE STUDIES

Precipitation, flood- and groundwaters of the Negev highlands: An isotopic study of desert hydrology (IAEA-AG-158/1 ) ...... 3 M. Levin, J.R. Gat, A. Issar Use of environmental isotopes in arid-zone hydrology (IAEA-AG-158/2)... 23 T. D in ger Interpretation of environmental isotopic groundwater data: Arid and semi-arid zones (IAEA-AG-158/3) ...... 31 M .A . G eyh A geochemical and isotopic approach to recharge evaluation in semi-arid zones: Past and present (IAEA-AG-158/4)...... 47 W.M. Edmunds, N.R.G. Walton An examination of recharge mound decay and fossil gradients in arid regional sedimentary basins (IAEA-AG-158/5)...... 69 J.W . L lo y d Environmental isotopes in North African groundwaters; and the Dahna sand-dune study, Saudi Arabia (IAEA-AG-158/6)...... 77 C. Sonntag, G. Thoma, K.O. Münnich, T. Dinçer, E. Klitzsch An injected gamma-tracer method for soil-moisture movement investigations in arid zones (IAEA-AG-158/7)...... 85 ■ A. R. Nair, S. V. Navada, S.M. Rao Aspects of the isotope hydrology of two sandstone aquifers in arid Australia (IAEA-AG-158/8) ...... 93 P.L. Airey, G.E. Calf, RE. Hartley, D. Roman Study of the leakage between two aquifers in Hermosillo, Mexico, using environmental isotopes (IAEA-AG-158/9) ...... 113 B.R. Payne, L. Quijano, L. Latorre D.

FIELD INVESTIGATIONS ON GROUNDWATER ORIGIN AND FLOW PATTERNS

Utilization of natural isotopes in the study of salination of the waters in the Pajeú River Valley, northeast Brazil (IAEA-AG-158/10)...... 133 E. Salati, E. Matsui, J.M. Leal, P. Fritz Isotope investigations as a tool for regional hydrogeological studies in the Libyan Arab Jamahiriya (IAEA-AG-158/11 ) ...... 153 D. Srdoc, Adela Sliepcevic, B. Obelic, Nada Horvatincic, H. Moser, W. Stichler Groundwater flow patterns in the western Libyan Arab Jamahiriya evaluated from isotopic data (IAEA-AG-158/12)...... 165 ‘ 0. Salem, J.H. Visser, M. Dray, R. Gonfiantini Recharge of groundwaters in arid areas: Case of the Djeffara Plain in Tripolitania, Libyan Arab Jamahiriya (IAEA-AG-158/13) ...... 181 M. Allemmoz, Ph. Olive Aspects of environmental isotope chemistry in groundwaters in Eastern Jordan (IAEA-AG-158/14) ...... 193 J. W. Lloyd A conceptual hydrochemical model for alluvial aquifers on the Saudi Arabian basement shield (IAEA-AG-158/15) ...... 205 /. W. Lloyd, P. Fritz, D. Charlesworth Isotope methods as a tool for Quaternary studies in Saudi Arabia (IAEA-AG-158/16) ...... 215 H: Hötzl, C. Job, H. Moser, W. Rauert, W. Stichler, J.G. Zötl Environmental isotope study of groundwater systems in the Republic of Djibouti (IAEA-AG-158/17) ...... 237 J.Ch. Fontes, P. Pouchon, J.F. Saliege, G.M. Zuppi

List of Participants...... 263 GROUNDWATER RECHARGE STUDIES

IAEA-AG-158/1

PRECIPITATION, FLOOD- AND GROUNDWATERS OF THE NEGEV HIGHLANDS: AN ISOTOPIC STUDY OF DESERT HYDROLOGY

M. LEVIN*, J.R. GAT**, A. ISSAR* ♦Institute for Desert Research, Ben-Gurion University of the Negev, Sdeh-Boqer

**Isotope Research Department, Weizmann Institute of Science, Rehovot, Israel

Abstract

PRECIPITATION, FLOOD- AND GROUNDWATERS OF THE NEGEV HIGHLANDS: AN ISOTOPIC STUDY OF DESERT HYDROLOGY. Precipitation in the Negev highlands was found to be surprisingly depleted of 180 and deuterium and generally characterized by “deuterium excess” values of d > 15%o. Isotopic compositions are relatively uniform over a wide area on any particular day, but differ appreciably from storm to storm. Thus, they are valuable tools for hydrographic analysis of flood-flows. Flood-flow samples, collected in Nahal-Zin and Nahal-Besor, were often even more depleted in heavy isotopes than the total rainstorm, indicating that run-off is generated selectively by high-intensity rains. The initial rush of the flood flushed away the surface salinity and saline accumulations in surface pools, but apparently does not involve sub­ surface salinity to any great extent. Recharge to groundwater appears to be accompanied by a slight evaporative enrichment of the isotopes, more so in the case of waters recharged from flood-flows. Environmental tritium can be used as an indicator of direct flood-water contributions to the aquifers.

INTRODUCTION

The Negev Desert covers about a million hectares, and forms part of the desert belt extending from the Sahara through the Sinai peninsula to the deserts of Trans-Jordan and Arabia. Northwards there is a rather rapid transition into the semi-arid areas of Israel with their more humid Mediterranean-type climate. The Negev highlands consist of a series of parallel anticlines with elevations ranging from 300 to 1000 m above sea-level. These mountains comprise mainly Senonian and Eocene chalks and Cenomanian-Turonian limestones. Older formations, such as the prominent Makhtesh Ramon, are exposed by erosion.

3 4 LEVIN et al.

The mountains are dissected by wadis, where rare floods discharge into the Mediterranean Sea or into the Arava Valley. In the highlands the rainfall varies from 50 to 150 mm annually; however, the average rainfall has little meaning because of its erratic nature and wide variations in the rain’s intensity. October to March can be considered the rainy period, which is more or less in phase with the Mediterranean climate. However, precipitation in the Negev is characterized by extreme annual variations and the influence of monsoon circulation may be a complicating feature. The Mediterranean-type regional precipitation is characterized by the “Levantine Meteoric Water” relationship of d = — 8 5180 > 15%o [ 1 ], which has been related to the air-sea exchange conditions associated with the winter cyclogenesis [2] which in turn gives rise to the winter precipitation. On the Israeli coast the mean weighted isotopic omposition (1959—1970) was about 5180 = — 4.75%o, 5d = - 22.8%c and the altitude effect has a mean value of about 1.1 X 10 ~3%o m-1 [3]. The increasing aridity in the rain-shadow valleys to the east of the mountains expresses itself in an enrichment of heavy isotopes accompanied by a decrease in the “d” parameter. This trend does not, however, continue into the desert region proper. Precipitation at Beer Sheva, situated on the border of the arid zone on the 200-mm isohyet, and also in the Sinai highlands, shows approximately the normal composition of the coastal rains, without an appreciable evaporation effect, but also with no indication of an inland or an altitude effect. This has been noted by Gat and Issar [4] and explained in general terms by the sporadic (not continuous) appearances of desert showers. The few data published on precipitation from the Negev indicate an over­ whelming predominance of heavy isotopic species [3]; in the light of our experience with rain collectors in this arid area we suspect that evaporation in the collector may have distorted these data. Some surprisingly depleted values from groundwaters in this area were tentatively related to recharge under more humid conditions, since such low 5 values in an arid zone were not then expected. The project under discussion aimed at characterizing the Negev precipitation and resultant flood-flows isotopically, and in turn relating these to the located groundwater sources, both shallow ones found in the alluvial wadi fill and larger aquifers in the chalk and limestone units. A better understanding of the hydrological regime and recharge mechanism is being sought. Collectors for rain and flood-waters which minimize the effect of evaporation were developed for this project. The study area is shown in Fig. 1.

1. INSTRUMENTATION

Standard rain recorders do not perform satisfactorily as rain collectors under arid conditions owing to losses of sample parts by evaporation. Moreover, IAEA-AG-158/1 5

FIG.l. Location map o f study area, indicating precipitation-sampling sites.

to obtain a large enough sample for environmental isotope analysis from rain amounts of only a few millimetres larger-than-standard rain collectors, in which the evaporation effects are even more severe, must be used. For this reason a special “hermetic” rain collector was developed (Fig. 2) [5], with a special provision (through a magnetic catch) for hermetic sealing of the collection flask once it has been filled by a minimum amount of 1 mm rainfall. An overflow mechanism then enables successive rain amounts to be sampled, with each additional sampler accommodating roughly 1 mm rainfall. 6 LEVIN et al.

8 0 0 rnm

FIG.2. Hermetic rain collector.

A comparison of the data for one storm, which yielded 4.3 mm rain collected by the hermetic sampler, with the concurrent collection of total rainfall by a standard rain gauge, which was emptied soon after the passage of the storm, gave the following results:

1st m m : 5 180 = - Ъ.21%о 2nd m m : 5 180 = - 4П9%о 3rd m m : 5 180 = - 5 . 5 6%o 4 th m m : s 18o = — 1.27%o A verage: 5 180 = - 5 . 2 2 %o

T he first three fractions were filled during a period of 20 minutes each. Two hours later the last sample was discharged in one minute. The total daily rain sample, collected in a standard rain gauge, gave the value of 5 180 = - 5.45%o. The hermetic-type collector was also the basis for a flash-flood sampler [6], which has a 3-litre container which is sealed hermetically by a float as soon as it fills (Fig. 3). Each instrument is buried in the wadi bed in a simply constructed chamber and covered by a succession of various sizes of clean gravel to filter the in-flow waters. The top does not protrude at all from the surrounding ground so as to escape detection and to prevent the sampler from being swept away by the flood. Different stages of the flood, except for the very start of flow, can be selected by placing the containers at different elevations above the flood plain. IAEA-AG-158/1 7

METALLIC FILTER

SAMPLE INLET- ... .o^r-Zrr. - : О o'. ••1 • ^ ■ ■CÜi ¿? ’¿>rf?¿r> C '

FLOAT COLLECTING FL ASK(3 Liter) INSTRUMENT SUPPORT

FIG.3. Collector for flood-waters.

2. PRECIPITATION

Daily rain samples were collected from the regular network stations of the Israel Meteorological Service, utilizing rain gauges which are emptied daily, thus minimizing evaporative water losses. The stable isotope data1 (Table I) represent the 1977—78 rainy season samples from Revivim, Sdeh-Boqer, Avdat and Mizpeh- Ramon, all situated in the Negev highlands, as well as from the Sinai sea-shore station at Sadot and also from Beer Sheva in the northern Negev (where rainfall is somewhat more abundant, the annual mean being 200 mm). The data are plotted on a conventional ó о vs 5 180 diagram (Fig. 4) with tie lines indicating the synchronous samples. On this diagram the d = 10%o Meteoric Water Line and the Mediterranean Precipitation Line are given as reference; the isotope data from the IAEA-WMO network station of Bet-Dagan, which is situated in the semi- arid coastal plain east of Tel Aviv where the mean annual precipitation is 535 mm, are given for comparison. The immense spread of these individual values, from + 3 % o to — 9 % o in 5 180 , is noted; however, such a spread is not outstanding for individual rain events, and almost as wide a scatter was reported by Gat and Dansgaard for showers at Ashdod [3]. The data obey a meteoric water relationship rather well and, with

1 Isotope analyses were performed mass-spectrometrically at the Isotope Laboratories, WIS, Rehovot (I. Mauravinsky, R. Silinikov and M. Feld). Data are given in 5 % o units relative to SMOW. Average reproducibility is ± 0 .12%o for 180 and ± 1.0% o for deuterium. 8 LEVIN et al.

TABLE I. PRECIPITATION IN THE NEGEV (MAJOR RAIN DAYS)

Date Rain amount 6180 T.U. (day-month-year) (mm) (%.) %ÍD Beer Sheva (300 m alt.) 14-12-77 13.9 -6.72 -30.5 21 21-12-77 2.4 -5.76 -30.6 27 22-12-77 11.2 -6.77 -29.3 37 23-12-77 4.4 -7.22 -25.6 39 2-01-78 1.2 -5.51 -13.2 - 23-02-78 19.2 -3.68 -18.1 _ 13-03-78 10.0 -5.17 - - 23-03-78 1.7 -1.83 - . weighted mean -5.42 - -

Sadoth 29-10-77 3.2 -0.76 + 9.3 32 (seashore) 2-11-77 3.9 -1.61 + 11.9 18 11-11-77 3.8 +0.42 -18.0 . 6/8-12-77 7.1 -2.43 - 6.8 8 12-12-77 5.1 -5.22 -31.4 _ 14-12-77 39.6 -6.94 -38.0 _ 21-12-77 0.6 -2.96 -17.8 - 22-12-77 6.1 -5.12 -23.1 27 23-12-77 9.6 -5.22 -17.3 24 2-01-78 4.6 -3.99 -16.3 15 3-01-78 2.8 -9.42 -58.9 _ 23-02-78 6.6 -3.68 -20.7 _ 12/13-02-78 U . 2 -4.84 - - 29-03-78 4.1 -5.37 - weighted mean (up to 23.02) -5.11 - 2.43

Kiryat Sde-Boqer 17-10-77 0.6 -2.26 - 1.5 . (S00 m alt.) 12-11-77 3.4 -1.78 + 5.6 28 6-12-77 0.6 -1.22 + 17.4 - 9-12-77 1.6 -1.03 + 8.3 18 14-12-77 4.9 -7.45 -37.9 22 22-12-77 16.8 -6.86 -33.1 30 23-12-77 6.6 -5.22 -14.5 30 2-01-78 1.4 -5.26 -22.1 _ 3-01-78 0.8 -6.16 -29.0 _ 17-01-78 0.2 +3.43 + 9.8 _ 19-02-78 1.0 -2.92 -36.9 _ 23-02-78 2.0 -3.90 -18.0 _ 12-03-78 1.1 -3.90 _ _ 13-03-78 2.4 -3.13 . _ 30-03-78 4.3 -5.45 -38.7 _ 24-04-78 1.2 -2.34 _ _ weighted mean -6.78

Revivim 11-11-77 10.0 -1.88 - 3.8 30 (300 m alt.) 9-12-77 1.2 -0.55 + 8.5 - 12-12-77 0.8 -0.46 + 1.0 - 14-12-77 23.0 -8.51 -46.8 14 15-12-77 0.5 -0.46 +18.3 - 21-12-77 4.1 -5.67 -28.6 19 22-12-77 15.1 -6.77 -25.8 31 23-12-77 9.4 -6.5b -24.5 21 2-01-78 1.6 -1.77 . _ 19-02-78 1.8 -3.77 . - 23-02-78 1.4 -3.12 - - 12-03-78 0.5 -4.54 - - 13-03-78 3.0 -4.54 _ _ 30-03-78 0.9 -5.13 - - w e ig h te d mean IAEA-AG-158/1 9

TABLE I. (cont.)

Date Rain amount 6 % T.U. (day-month-year) (mm) (%„)

Avdat 12-11-77 7.0 -2.36 + 2.4 20 (650 m alt.) 3-12-77 0.55 -7.98 -69.0 9-12-77 0.95 -1.36 + 4.4 14-12-77 3.0 -6.97 -37.4 36 15-12-77 1.0 -3.40 + 3.6 _ 22-12-77 15.4 -6.21 -31.1 4 7 23-12-77 8.0 -4.29 -28.0 22 19-02-78 1.1 -2.29 -40.0 _ 23-02-78 10.0 -5.34 -25.8 23 30-03-78 0.8 -2.82 . 24-04-78 3.0 + 2.46 _ _ weighted mean -4.50

Mizpeh-Ramon 21-10-77 1.15 -3.99 -16.3 (900 ra alt.) 11-11-77 2.6 -2.62 + 1.4 20 14-12-77 2.8 -9.42 -57.8 32 15-12-77 0.1 -3.05 - 5.7 22-12-77 7.2 -7.01 -31.0 37 23-12-77 8.0 -6.81 -22.5 34 3-01-78 0.3 _ 20-02-78 1.7 _ _ 23-02-78 8.6 -5.91 -25.1 29 24-04-78 7.4 -4.78 _ weighted mean -6.06

few exceptions, all data fall within the band of meteoric water lines (lines of slope of 8 in 5180 , Sp space) of 15%ó < d < 25 %o. One notes the close correlation between synchronous samples, with the position of data on the diagram apparently dictated first by the particular storm and less by the location of the station. Indeed there is no consistent order between the highland stations regarding either the daily data or even the monthly and annual seasonal averages; a one-year period is evidently insufficient for establishing a statistically significant mean. The data as a whole average around 5180 = — 6.5 %o, which is an unexpectedly low value. In Fig. 5 the data are plotted as a function of the daily rain amount. A rather weak “amount effect” can be seen — there are only isolated cases with 5180 < - 4%o in the group of samples which correspond to daily rain amounts of less than 2 mm, whereas, by contrast, the isotopic values of the rain events over 4 mm, fall in between - 4.5%c > 6180 > - 7.0%o (with few exceptions, notably the rain event of 14 December). A slight inland effect is recognized in that the data from the coastal stations (Sadot and Ashdod, the latter given for comparison purposes) are usually slightly enriched in 180 for any given rain intensity. The “amount effect” can be more clearly recognized on the öjj vs 8180 plot (Fig. 4). 10 LEVIN et al.

8 l 8 0 ( % o ) 0 -8 -6 -4 -2 0 +2

+ 2 0

0

o-° - 2 0 Q 60

- 4 0

- 6 0 ' x d = l07oo METEORIC WATER LINE J____ ° I_____I I I_____I_____I J_____I I 1

FIG.4. Stable, isotope data o f daily precipitation samples from the Negev during the 1977178 season. Samples collected simultaneously at different stations are identified by tie-lines. Mean values o f a ten-day rain collection at Bet Dagan in the Israel coastal area are shown fo r comparison purposes.

Data from rain-deficient days (amounts < 2 mm) can be seen to occupy predominantly the area of more enriched isotope data and also the area below the meteoric water line of d < 10%o. The “amount effect” is mainly the expression of the continuous depletion of an air mass by the preferential rainout of the heavy isotopic species. The larger the rain the greater the depletion - however, the process is one which obeys the meteoric water relationship of ASp/AS18 = 8. The effect is further enhanced by the enrichment of the heavy isotopic species as a result of evaporative water loss from droplets falling beneath the cloud base - an effect which is noted mainly in the case of slight rains which do not saturate the near-surface air layers. This process has the additional effect of moving the isotopic value away from the meteoric water line in the direction of smaller “d” values. The evaporation effect is usually the dominant part of the amount effect in the lowlands of the semi-arid zone. In the present set of data this effect seems unimportant (most enriched values lie within the band of meteoric IAEA-AG-158/1 11

+4 T Г

L e g e n d: + 2 • Negev Mountain Stations oS a d ot лВеег-Sheva x Ashdod (Jan. 1968)

»0 - 2 - % ^ X • XX л : OOP "4 Л » . О 60 «• о x I л Д .*

...•

/ Dec. 14

-10 X _L _L 10 15 20 25 30 35 PRECIPITATION (m m /day)

FIG.5. The “amount effect” in precipitation samples o f Negev stations, 1977/78 season. The abnormally depleted values o f 14 Dec. are marked by a tie-line.

water lines); possibly this is related to the low temperature on rain days at these high elevations, which would minimize any evaporative water loss. Whatever the reason, we note a distinction between precipitation in the desert highlands, compared with the lowland areas, with isotope composition of the latter dominated by the evaporation process, whereas in the highlands the isotope data are characterized by a high “d” value. The tritium content of the rain ranged from about 10—50 TU.

3. RUN-OFF FROM INDIVIDUAL STORM EVENTS

A number of the flash-flood samplers described above were distributed along three major watersheds of the central Negev. Our aim was to characterize the storm run-off in the wadis from the aspects of salinity and isotopes and to identify the sources of water and salinity. For this purpose we chose the Zin, Haro’a and Revivim watersheds (Fig. 6). The Zin drains into the Arava Valley, whereas the latter two are tributaries of Nahal-Besor, which finally drains into the Mediterranean. 12 LEVIN et al.

120 180 080-----

FIG. 6. Detailed map o f the Zn and Besor catchment area indicating the location o f the sampling stations.

Three of the instruments described above were distributed in each of these watersheds, and two more were located further downstream in Nahal-Besor. These samplers responded to the initial stages of any flood event; when the occurrence of a flood in the area was reported, the site was visited by one of us (M .L.) and hand-sampling of the late stages of the flood was carried out when possible. It must be appreciated that some areas, such as the pool areas of Wadi Zin, are practically inaccesible during a flood. IAEA-AG-158/1 13

The specific conductivity was measured soon after sampling at the Sdeh-Boqer- Institute. Stable isotope and chloride analysis were done on the samples later at R eh ovot.

3.1. The Wadi Zin drainage system

A schematic outline of the Wadi Zin system is shown in Fig. 6. The headwaters of the drainage area originate from the Avdat to Mizpeh-Ramon area, at an eleva­ tion of 600 to 900 m. Other tributaries draw their waters from lower elevations to the northeast. The main wadi enters the gorge of Ein Avdat, plunging in a series of spectacular waterfalls into pools, which are up to 7 m deep. These pools are perennial, being fed throughout the year by a number of rather saline springs and seepages. Further downstream the main wadi is joined by Wadi Havarim and other local tributaries and finally discharges into Nahal Arava and the Dead Sea. One flood sampler was located in Upper Wadi-Zin (upstream of the gorge and pools of Ein Avdat), another below the junction of Wadi-Zin and Nahal Havarim, and a third close to the discharge area near Mashash. Near the first and last sampler flood-level monitors of the Hydrological Service were also located. The perennial seepages of Ein Avdat were sampled (Section 4): their isotope content varies considerably from place to place but is rather steady in time, a mean value being 518 = - 5.9%;; = - 28 %o \ salinities range from 1200 to 1800 mg Cl/ltr. The pool water and the cascading water in the falls become enriched in the isotopic species and salinity, owing to the evaporative water loss, mainly during summer; typical values measured were 5180 = - 4%o; Sq = - 17%o.

3.1.1. The flood o f22/23 December 1977 (Fig. 7)

Widespread rain fell on 22 December 1977 and again on the 23rd. Based on daily rain sampling the isotopic compositions ranged from 6 180 = — l % o to - 5% o, the early rain (more intense ? ) being the more depleted in heavy isotopes. It is noteworthy that the rainfall on the second day was characterized by a marked deuterium excess, d = + 29%o. Flow started at the Upper Zin station late on 22 December, increasing slightly with the additional rain of the 23rd. Flow in the wadi continued well into 25 December. Downstream the flood was much larger, but began only at midnight of the 22nd, and in the hydrograph there was evidence of the arrival of a number of flash-flood fronts during the 23rd. Flow ceased rather suddenly. The first rush of waters as sampled by the installed flash-flood samplers was quite depleted (Fig. 7), more so than the value shown by the precipitation samples of the whole storm. The pick-up of the saline component during the first rush of water through the pool area was apparent. Assuming complete mixing of the pool’s waters, whose volume was estimated to be about 1000 m3, and based on MIZPE A V D AT RAIN GAUGE RAIN GAUGE UPPER-Z IN HYDROGRAPH -7 .0 14.6 — 22— -6.21 76.1

-6 .8 6.6 - 2 3 — -4.3 1 0.3 * 10.4*

+ 10.06 j UPPER Z 1 N

2 2 - -7.4 10

2 3 - - - DATE: 22/12/77 • 23/12/77 24/12/77 -6 .2 9.3 INTEGRATED ly ^ o c W 6.700m5 2.ÏOOm3 Legend: 2 4 - FLOW: 2 5 - -5.21 10.6 S A M P L IN G SITE

WADI HAWARIM

-5 .6 39.7 23- MASHASH-ZIN HYDROGRAPH

WADI Z 1 N MIDRASHA -6.9 522 * UTl -7 .0 267 «

LO W ER Z IN M ASH ASH DATE: 22/12/77 23/12/77 24/12/77 INTEGRATED 146.700 m3 2 2- -7.9 117 FLOW: - 53.300m*

FIG.7. 1B0 and salinity date for the flood in N ahal Zin, during 22—25 Dec. 1977. The dates (22, 23, 24, 25) are show n on one side o f the data. Sam ples are from the herm etic flood sam pler except when identified as manually sam pled by the letter H. IAEA-AG-158/1 15

MIZPE AVDAT RAIN GAUGE RAIN GAUGE -5.9 14.4 — 23 — -5.3 10.4

UPPER Z 1 N

2 3 - -6.2 42.3 24- 6.3 16.8

WADI Z 1 N MIDRASHA 2 3 - -5.3 83 4 24 -6.0 247

LOWER ZIN MASHASH 23- -5.6 152

FIG.8. Data fo r the flood in Nahal Zin o f 23 -2 4 Feb. 1978. Legend and symbols as in Fig. 7.

the hydrograph of the Upper-Zin station, the pool would be expected to flush out during the first day. Indeed, salinity was considerably reduced on the 23rd at the Zin-Midrasha station.

3.1.2. The flood o f 23 February

As shown in Fig. 8, the development of this flood more or less follows the pattern of the December event. The most notable fact is again the flushing out of the saline reservoir in the Ein-Avdat gorge.

3.2. The Besor-Revivim watershed

The schematic map of the area, Fig. 6, indicating sampling points, shows the following distinctive features. In the Haro’a branch, an earth dam was constructed to intercept flood-waters and only excess waters spill over into the Besor system: 16 LEVIN et al.

- 1 0

- 2 0

-30

-40

-50 ~0 . о О

L egend: о Roin Gou The Dec. I 4 event д Flood Sa • Rain Gau -70 The Dec.22 event{ ■ Flood Sa -80

-90

-14 -12 -10 - 8 - 6

8 1 8 0 (%o)

FIG .9. Stable isotope com position o f floods o f 14 Dec. and 22 Dec. 1977 in the Rivivim branch o f N ahal-Besor.

On the other hand, on the Revivim branch waste waters of the settlements in the Mashavei-Sadeh area, as well as storage pools, could possibly be contaminating the flood flow.

3.2.1. The flood o f 14 December 1977 (Fig. 9)

Following lighter rains on 12 December, a heavy storm on 14 December dumped an impressive 23 mm of rain on Revivim, with much smaller amounts measured at other precipitation stations. This rain triggered a flood in the Revivim branch of Nahal Besor, but not in the other tributaries. The day’s precipitation was characterized by extremely depleted isotopic values, averaging 6180 = - 8 %j . The resultant flood showed even more depleted isotopic values, with a value of 5180 = - 12.2%o being a record low for this part of thw world. The flood evidently did not pick up much salinity until far downstream at the Besor-Halutza Station. At this stage the isotope composition was quite a bit closer to that of the mean rain value, whether as a result of additional surface run-off or because of the admixture of water from a shallow aquifer in the stream bed at Bir-Ajluz (5 180 = - 4.4%o; §d = 18.0 %o\ S = 1 8 0 0 mg C l-ltr- 1 ), is still undetermined. IAEA-AG-158/1 17

3.2.2. The flood o f 22—23 December 1977

As already described in connection with the flood in Nahal-Zin, the rainfall on 22 and 23 December was widespread and rather uniform in composition over the whole central Negev. In the Revivim branch a flood was recorded on 22 December (Fig. 10). There is little that is noteworthy in this event except for the effect of the additional flow from the branch originating in the Boqer mountain. The rain of the 22nd caused flood flows in the Hazaz branch of this system (Fig. 10) and flood samplers in Wadi Hazaz and Haro’a station were filled, and showed rather similar composition (§ 180 = — 8.6%«). Apparently the flood front picked up a considerable amount of salinity, which reached 52 mg Cl/ltr in the downstream sampler. The flood ceased some hours later, enabling the resetting of the sampler in readiness for the next rain event, which occurred on the following day (the 23rd). Rain on the 23rd was more localized, and apparently flood occurred only in the Haro’a branch, filling both the upstream sampler of Wadi Haro’a and downstream sampler at Haro’a station. Here again the relative high salinity in the initial rush of waters was noted. Luckily the isotope composition of the rain of the 23rd was quite distinct from that of the previous day (Fig. 10), so that the admixture of water remnants can be clearly discerned from the previous day’s flood in Wadi Haro’a, resulting in the value of S180 = - 7.4%o at Haro’a station (the isotopic composition of rain and floods of the 23rd approximates 5 18 = — 5.9 %o). The continuing flood was then sampled manually, both in the tributaries (upstream) and the downstream section of the wadi, and a drastic reduction of salinity during the continuing flood and the flushing of the in-between storage was noted. The intercepted flood-waters near the earth dam were sampled the following day and give an integrated (mean) value of this flood event of 5 180 = - 7.79%o; S = 1 0 .3 mg/Ur which lies on a mixing line between characteristic values of the two days’ floods. Quite evidently the initial high salinity was diluted by the continuing influx of fresher flood flow, and the mean salinity of the whole event was not particularly high.

4. GROUNDWATERS OF THE NEGEV HIGHLANDS

The limited set of groundwater data given here (Table II) represents samples from the karst Eocene limestone which crops out over wide areas of the Negev as well as from shallow wells in the alluvium. In addition, data of few springs and seepages which occur in the area are included. Measurable tritium in many of these samples shows them to have been recharged recently. The shallow alluvial wells are, as a group, the freshest waters and evidently recharged directly from the surface flow. This group indeed has the highest 0 0

HAROA WADI HAZAZ TRIBUTARIES no flo o d — 22— -8 .7 12.4 - 5 .8 4 6 .6 — 23 morning - 5 .9 4 .7 — 23 afternoon

HYPOTHETICAL h - 1* HOLDUP Legend: SAMPLING SITE

Sl80(%oyn9^l/t -Date al. et LEVIN

HAROA STATION

22- -8 .5 51.7 u_ h - -7 .4 21 .2 L. -5 .5 2 2 . 0 *23affernoon

HAROA STORAGE L A K E \ -7.8. 10 -2 4 -9 - 8 ' 7 - 6 -5

Sl8 0 (%o)

FIG .10. Isotope and salinity data for the flood event o f2 2 -2 3 Dec, 1977 in the Wadi Hazaz and Haroa. Sam pling sites are indicated in Fig. 6. IAEA-AG-158/1 19

TABLE II. GROUNDWATERS

Name o f so u rc e : Type or Resource sa m p ling Cl 6l8 o Зн ÖD da te (m g/l) (%.) ( ' о ) (TU) B e 'e ro ta im w e ll in Eocene l’stone 2 0 -0 7 -7 7 475 -5 .9 3 -31.4 25±1 near river bed 30 -0 1 -7 8 566 -5 .6 6 23 -0 3 -7 8 561 -5.41

Ein-Kudeirat s p r in g 2 0 -0 7 -7 7 493 -6 .2 8 -3 1 .3 0.8 + 0 .6 2 4 -1 0 -7 7 497 -6 .2 3 -3 3 .8 0 . 5 ± 0 .3 2 3 -0 3 -7 8 504 -6 .0 5

Qseima w e ll Eocene i’stone 2 0 -0 7 -7 7 1474 -5 .4 8 -3 1 .3 14+3 and clay mantle 2 4 -1 0 -7 7 1302 -4 .78 -2 8 .6 1.Ш 2 3 -0 3 -7 8 504 -5 .1 0

B ir - M u ila j well in alluvium 2 0 -0 7 -7 7 1738 -5 .5 8 -2 8 .1 8±0.4 2 3 -0 3 -7 8 1838 -5 .5 3

Bir-Nitzana w e ll in Eocene I’stone 2 0 -0 7 -7 7 661 -5 .9 3 -2 6 .5 7±3 near river bed 2 4 -1 0 -7 7 653 -4 .7 8 -3 2 .2 19-01-78 646 -5 .3 6 2 3 -0 3 -7 8 659 -5 .6 7

B ir-M a in well in alluvium 2 4 -0 7 -7 7 176 -5 .6 8 -2 7 .1 23±3 2 1 -0 5 -7 8 238 -3 .8 4

Berot-Oded perched water table 2 4 -0 7 -7 7 115 -5 .3 8 -2 7 .7 32+4 1 9 -0 5 -7 8 116 -5 .1 1

Bir-Atam ed well in alluvium 2 5 -0 7 -7 7 175 -4 .7 8 -2 1 .4 19±5

B ir -B e id a well in alluvium 2 4 -0 7 -7 7 75 -5 .6 3 -2 8 .1 42.5+1

Ein -A q e v s p r in g 1 8 -0 9 -7 7 1061 -5 .2 4 -2 7 .7 12+0.3 1 9 -12-77 1049 -5 .4 1 -2 6 .7 26 -0 3 -7 8 730 -5 .8 6

E in -Z iq seepage 1 8 -0 9 -7 7 412 -6 .2 8 -2 9 .3 29+0.5

Ein-Avdat(l) seepage 2 6 -0 7 -7 7 -5 .8 8 -3 4 .6 4 .2 + 0 .5 2-0 9 -7 7 1249 -6 .2 8 -3 3 .7 1.3±0.7 9 -0 3 .7 8 1222 -5 .5 8

Ein-Avdat (2) seepage 2 6 -0 6 -7 7 1818 -5 .7 3 -2 2 .7 1.3 + 0 .6 2 -0 9 -7 7 1403 -6 .2 8 -2 5 .8

Ein-Avdat (3) seepage 26 -0 6 -7 7 1403 -5 .3 3 -2 2 .1 2.9 + 0 .6

B ir-A v d a t w e ll in Eocene l’stone 3 0 -08-77 863 -5 .9 4 -2 9 .6 4±4

E in -A v d a t big waterfall 26-0 7 -7 7 1079 -4 .5 0 -1 9 .4 9 . 5 ± 0 .4 small waterfall 2 6 -0 7 -7 7 1800 -3 .7 9 -1 6 .4 10.310.4

tritium values, approaching those of the atmospheric levels. The isotope composition is enriched (along evaporation lines) compared to the group of samples from the limestone wells in which the infiltration of rainwater occurs in a more direct way. These water sources from the limestone terrain form an intermediate group as far as salinity is concerned. There is quite a variety of tritium values, ranging from relatively high values in the wells situated close to 2 0 LEVIN et al.

Sl8 0(% o) -7 - 6 - 5 - 4

FIG. 11. Groundwaters o f the Negev Highlands - stable isotope data. A = Avdat area.

the Wadi (Beerotaim, Nitzana etc.) to some very low tritium values of less than 1 TU, which indicate quite a delay from the recharge site to the discharge area. The isotopic composition is somewhat less depleted in the heavy isotopic species than the mean precipitation of the 1977/78 season (and certainly less than the floods) (Fig. 11) but still quite a bit lighter in composition than the equivalent water sources of the Judean Mountains. Springs and seepages of the Avdat region (marked A on Fig.l 1) are quite varied in their stable isotopic composition, low in tritium and also quite saline. These are the water sources which contribute the salinity to the flood-waters in Nahal-Zin. It appears from these data that the tritium amount is a good indicator of direct flood-water contribution to a well, and that a lengthy underground route, under the arid conditions of the Negev, expresses itself immediately through a drastic reduction of the tritium content. Salinity remains a very individual matter, and no simple general rule emerges from these observations. IAEA-AG-158/1 21

5. SUMMARY DISCUSSION

Water sources of the Negev highlands were found to be surprisingly depleted in the heavy isotopic species and on the whole were characterized by the high value of the “d” parameter, which is typical of the Levant region. There is relative uniformity in the composition of individual storm events throughout the area and the “d” parameter is often a distinct property of a single day’s rain. This facilitates the use of stable isotope parameters for identifying components of the flood hydrograph. Invariably the flood-flow was found to be even more depleted in heavy isotopes than the whole storm’s average isotope composition. Assuming that the relationship of the amount effect holds also for parts of a single event (as indeed it did in the case of the storm analysed in detail in section 1) then these data point to the fact that run-off is activated selectively by the high-intensity rains. Since some of the extremely depleted floods (such as the one of 14 Dec. 1977) differ considerably in their isotopic composition from the commonly recognized Mediterranean Meteoric Waters, one should proceed warily in the identification of the origin of water sources solely on the evidence of the stable isotope composition. The data presented not only throw light on the role of the high-intensity rains in the inception of floods, but they also indicate quite clearly the salinity pattern of flood-flows in the desert environment: relatively high salinities (of the order of 50 mg Cl/ltr, compared with 5 — 10 mg/ltr in rain) accompany the initial rush of flood-water; salinity originates apparently from the surface accumulations of salts which are then rapidly flushed away. As exemplified by the flood of 22 Dec. in the Haro’a basin (Fig. 10) a continual high salinity throughout the duration of a flood may result from the successive mobilization of additional parts of the watershed later during the evolution of the flood. In many other cases the later stages of the flood consisted of very fresh waters indeed, pointing to the fact that the desert flood is a rather shallow surface event which does not involve the subsurface accumulations of salinity. However, higher salinities can result in areas, such as Ein Avdat, where seepage from saline groundwater sources accumulate large amounts of salinity over extended periods on the surface; these are then flushed and carried away by the flood-flow (Figs 7, 8). It is an encouraging fact that, despite the sporadic nature of the phenomenon, there is quite some regularity in the isotopic and salinity pattern of a desert flood, enabling a quantitative understanding of these important features of desert hydrology. From the rather limited set of groundwater data we learn that the water in the wells of the alluvial wadi fill, which undoubtedly are recharged through the intermediacy of flood-flows, are somewhat enriched in the heavy isotopes relative to the local precipitation, and therefore considerably so compared with the flood- flows (which, as described above, are generally depleted by a few per mill relative 22 LEVIN et al. to the mean isotopic composition of the precipitation). The limestone aquifers, which seem to be recharged by means of a more direct route, are closer in composition to the rain, but also in this case it would appear (Fig. 11) that some evaporative enrichment had accompanied the recharge process that the isotopic composition evolved, along low-slope evaporation lines. There appears at first sight a paradoxical situation where those groundwaters which are derived from the very depleted flood-flows are more enriched in the heavy isotopic species than others directly recharged by rain infiltration. Does this signify that the recharge from surface flows involves a delay of the waters near the surface before recharge? Our data are too scanty to answer this interesting point and further observation will be necessary to understand fully the relationship between groundwaters, the precipitation and the flood-waters.

ACKNOWLEDGEMENTS

We are grateful to Mr. Shavit of the Israel Meteorological Service for permission to use their rain stations. Special thanks are due to all those responsible for the rain stations. We also thank E. Adar, hydrologist of the Water Resources Unit (WRU) of the Institute for Desert Research (IDR) for valuable discussions on the location of the rain and flood stations and for help in the field work. The authors are indebted to L. Landsman, geologist of the WRU of the IDR, for his collaboration in the instrument installation and sample collection. We acknowledge gratefully the devoted analytical service by the staff of the Stable Isotope and Tritium Laboratory at Rehovot. Thanks are due to Uri Gat for chloride analysis.

REFERENCES

[1] GAT, J.R., CARMI, I., Evolution of the isotopic composition of atmospheric waters in the Mediterranean Sea area, J. Geophys. Res. 75 (1970) 3039. [2] TREWARTHA, G.T., The World’s Problem Climates, Univ. of Wisconsin Press, Madison (1961) 334 pp. [3] GAT, J.R., DANSGAARD, W., Stable isotope survey of the freshwater occurrences in Israel and the Jordan Rift Valley, J. Hydrol. 16 (1972) 177. [4] GAT, J.R., ISSAR, A., Desert isotope hydrology, water sources of the Sinai Desert, Geochim. Cosmochim. Acta 38 (1974) 1117. [5] ADAR, E., LEVIN, N., BARZILAI, A., “A multiple hermetic rain sample collector development”, in preparation. [6] LEVIN, M., ADAR, M.E., BARZILAI, A., “An hermetic sampler fo.' ephemeral Wadi floods”, in preparation. IAEA-AG-158/2

USE OF ENVIRONMENTAL ISOTOPES IN ARID-ZONE HYDROLOGY

T. D IN Ç ER Deli Huseyinpasa Cad. Ozyurt, Istanbul, Turkey

Abstract

USE OF ENVIRONMENTAL ISOTOPES IN ARID-ZONE HYDROLOGY. After aridity is defined some physical and hydrologie features common in arid lands are described. An attempt has been made to identify various groundwater recharge mechanisms in arid countries and to assess their relative importance. The influence of the arid climates on the isotopic composition of the precipitation is discussed. The use of environmental isotopes in run-off and precipitation infiltration and recharge is evaluated, and the importance of isotopic studies in investigating interrelations between aquifers is stressed. Finally, some examples of isotope applications in surface water and its relation to groundwater are given.

INTRODUCTION

Arid conditions are said to exist in a region when the potential évapotrans­ piration is larger than thè precipitation for the most part of the year. The difference between annual potential évapotranspiration and the annual precipita­ tion determines the degree of aridity in a given place. While in some very arid zones such as the Arabian Peninsula this difference exceeds 2 m annually, in semi-arid regions the difference is usually less than 1 m as in central and eastern Turkey. Long periods of aridity change the face of the land drastically, which in turn has a different hydrological response to the atmospheric inputs and to the incoming radiant energy. It is useful therefore to give a general description of the climatic and hydrological features of arid lands before discussing the use of isotope techniques in these areas. In very arid countries the precipitation is not high enough to sustain a per­ manent plant cover on soils. When topographic conditions are favourable to erosion the soil cover is removed and barren rock surfaces are exposed to the atmosphere. As a result the run-off from such areas increases, especially in response to short-duration heavy precipitation typical of arid climates. It would be a misconception to think that these intense rains are extremely rare phenomena — people living in arid countries are quite familiar with them. The run-off produced by these rains results in impressive flood waves which are absorbed by highly permeable wadi alluvia, or disappear in sink-holes in karst limestone with negligible evaporation losses. Often a flood wave completely disappears before reaching the

2 3 24 DINÇER terminal area of a wadi system consisting of a mud flat (sabkha, playa), sand-dunes or the sea. Thus, there is an intermittent but somewhat regular recharge to alluvial and karst aquifers and to aquifer formations in which alluvium has been deposited. As a result of soil erosion and the subsequent “sifting” of the sand by winds, the soils in arid zones tend to be sandy and in extreme cases consist of pure sand in sand-dune regions and in larger sand seas. The hydrological implication of this change are the higher infiltration rates and lower soil evaporation losses, both beneficial to the conservation of water. There are indications that even in very arid countries such as Saudi Arabia rain-water infiltrating in sand-dunes could contribute to the recharge of the aquifer formations underlying sand-dunes.

POSSIBLE GROUNDWATER RECHARGE MECHANISMS IN ARID ZONES

As one of the most promising uses of isotopes in arid zones is the study of the recharge to groundwater, an attempt has been made to identify recharge conditions most likely to exist in arid zones.

1. Run-off recharge: This type of recharge seems to be the most common in arid zones even under extremely arid conditions. Run-off recharge could be subdivided into:

1.1. Recharge to the wadi alluvium: Stream-flow measurements indicate that recharge to alluvium is one of the most important recharge mechanisms in arid zones. In many cases the aquifer formations (fractured crystalline rocks, sedi­ mentary formations, volcanic rocks) underlying or bordering on the alluvium are also recharged through the alluvium which is the first recipient of flood waters.

1.2. Recharge to karst limestone: It has been observed that when flood- waters flow over such formations large quantities of water disappear in caves and solution channels typical of karst limestone.

1.3. There are reasons to believe that when the mud flats, “sabkhas” , are flooded some water reaches the groundwater table by infiltration.

2. Groundwater recharge in volcanic rocks: This type of recharge occurs by the infiltration of precipitation into young basalts or into areas covered by pumice, which are devoid of run-off channels.

3. Recharge to groundwater through the infiltration of the precipitation in sand-dunes: The mean grain size of dune sand very often varies between 0.15 and 0.4 mm. Sand-dunes with coarser grains let the rainwater infiltrate at considerable IAEA-AG-158/2 2 5

depths owing to their low field capacity. Observations and experiments also show that evaporation from sand decreases as the mean or median grain size increases. Thus, even in regions with an annual precipitation less thah 100 mm there are appreciable quantities of soil moisture in dune sand profiles. Thus, areas covered by dunes should be considered as favourable areas for the recharge of ground­ w ater.

4. Recharge through perennial rivers and swamps: This type of recharge is observed rather frequently in semi-arid zones. But even in very arid countries it is possible to have perennial rivers such as the Nile in Egypt. This is also true for lakes and swamps. A good example is the Okavango swamp in Botswana, which regularly recharges the shallow aquifers in the region of the Okavango delta. Some of the recharge mechanisms described above could, be studied success­ fully with conventional techniques. The recharge to the alluvial aquifers from flood-flows, for example, can be estimated by stream-flow measurements, or by studying the width of the recession channel [1 ]. On the other hand, the study of the recharge through infiltration of the precipitation is much more difficult and requires the application of isotope techniques to reach quantitative estimates of recharge.

ISOTOPIC COMPOSITION OF THE PRECIPITATION IN THE ARID ZONES

In many arid countries precipitation is not only seasonal but short and heavy showers occur in a practically dry atmosphere. This influences the isotopic composition of the precipitation which clearly shows the effect of the evaporation in the atmosphere. The ratio of the standard deviation of the deuterium values to that of the oxygen-18 (monthly values) becomes less than the mean ratio (7.03 ±0.10) for 90 WMO-IAEA network stations. For six typical arid-zone stations the mean ratio is 6.31 ± 0.24. This departure from the world-wide mean is for isotope values weighted by precipitation and would certainly be larger if unweighted values would be used. It is perhaps useful to mention here that the slopes quoted here are not related to the slope of 8 obtained when time-averaged mean values are used. The comparison of the stable isotope content of the groundwater in alluvium in Saudi Arabia and of the precipitation shows that, as expected, only heavy precipitation is recharging these aquifers. This similarity can be used to estimate the mean isotopic composition of the precipitation by examining a few samples of shallow groundwater near the wadi beds. From the stable isotopic composition of old groundwater it is seen that the precipitation in some arid countries, such as Saudi Arabia, had a different isotopic composition in the past compared with modern precipitation. Not only is the 2 6 DINÇER

old groundwater isotopically lighter but it also has a deuterium excess significantly less than that of the local precipitation. The first point can be explained by a cooler climate, but it could well also be a result of the changing pattern of atmospheric circulation. Now that we have a satisfactory amount of isotope data on precipitation it can be seen that the so-called “temperature effect” is not, in nature, as strong as it should be, especially at low latitudes. This also applies to the deuterium excess which seems to indicate water vapour produced in more humid conditions over the oceans under past climatic conditions, but could as well be a result of changing atmospheric circulation patterns, and most probably a combination of many climatic factors. At present we do not have a satisfactory explanation for the discrepancy of the isotopic composition of modern and old precipitation, as inferred from groundwater data.

Study of the run-off recharge to the shallow and deep aquifers

Groundwater recharge through run-off is a visible and measurable pheno­ menon. Either stream-flow or channel geometry methods are of great use in estimating this type of recharge [ 1 ]. On the other hand, it is not possible to have a quantitative estimate of recharge using tritium concentration in shallow groundwater. Tritium and stable isotopes could, however, be useful in studying the groundwater movement beyond the wadi channel. In such studies one has to be careful in considering the fact that in arid countries irrigation is very often made without drainage, which results in recirculation and some change in the chemistry and isotopic composition of groundwater. Isotope techniques are extremely useful in studying interrelations between aquifer systems and in determining, together with the chemistry of water, groundwater mixture problems. There are numerous examples of such applica­ tions [2, 3] and there is no need to expand on this subject.

STUDY OF GROUNDWATER RECHARGE BY DIRECT INFILTRATION OF THE PRECIPITATION

It is rather surprising to observe that sand-dunes in very arid countries such as Saudi Arabia contain significant amounts of moisture despite low precipitation (between 50 and 100 mm/a on the average). This is also true for sandy desert flats such as the Kalahari desert, which shows that precipitation could be a direct source of recharge to groundwater. The presence of moisture in sand layers depends a great deal on the climate and on the size distribution of the sand. While no moisture can be found in sand- dunes with a mean grain diameter of 0.2 mm in Saudi Arabia — except during the rainy days and the week following — sand with a similar grain size in the IAEA-AG-158/2 2 7

FIG .l. Tritium content o f the sand m oisture at 114 km on the Riyadh-Dhahran highway.

(From R ef.[4\j

northern Kalahari desert has, on an average, 7% moisture by volume. This is the result of much higher precipitation in northern Kalahari (500 mm/a) compared with 70 mm/a in central Saudi Arabia. The sampling of sand-dunes in Saudi Arabia has shown that in the dunes where the mean grain size is about 0.3 mm, the whole sand profile down to a depth of 7 m contains moisture which can be as high as 7% by volume, but in general is lower than the “field capacity”. It is interesting, of course, to study the movement of this moisture which could eventually find its way to the saturated zone. Measurement of the tritium content of the sand moisture made in 1972 in Dahna sand-dunes has shown that the tritium peak of 1963—1964 had moved down to 4 m (Fig.l) [4]. Recent measure­ ments given in another paper [5] at this meeting show that the peak is no longer there; in addition, the moisture content of the sand at the same location was measured to be 1.73% by volume compared with 4.55% in 1972. The low moisture content of the sand prevented a good sampling owing to the auger hole caving in, and this may account for the uniform isotopic composition of the sand moisture, at least partly. It is hoped that with more advanced sampling techniques developed by the Heidelberg Environmental Physics group such problems will no 2 8 DINÇER longer plague experimental results. In any case, the study of the tritium move­ ment in the sand moisture, either naturally or artificially injected, seems to be one of the most promising applications of isotope techniques in arid zone hydrology, especially when it is used to calibrate evaporation models from the sand. In the sand-moisture movement study in Saudi Arabia another observation of interest was the extensive stable isotope enrichment of the sand moisture as given in Ref.[4]. This is interesting because in the case where the moisture could reach the saturated zone it would give an opportunity for identifying water originating from sand-dunes. This applies not only to modern water but also to. old water and could explain the lower deuterium excess of the old groundwater in the Sahara and Saudi Arabia. The slope of the deuterium/180 line of the sand moisture is unusually low at about 2. This is a startling result as it seems to be much lower than any evaporation slope measured by different isotope workers. A plausible explanation is that the slope of the evaporation line is determined not only by the relative humidity of the atmosphere but also by the isotopic compo­ sition of the atmospheric moisture, which may be of local (in oceans and in large lakes and swamps) or of non-local origin (small lakes, evaporation pans and sand dunes). Evaporation from sand in arid countries cannot produce enough water vapour and the sand moisture, during evaporation, must have molecular exchange with an alien water vapour. If one assumes an isotopic composition for the atmospheric moisture in isotopic equilibrium with the local precipitation, it is possible to have evaporation slopes of 2 and even lower. One should also consider the fact that evaporation from sand does not take place exactly like the evapora­ tion from open water — in sand we are dealing with a thick diffusion layer (20 cm of dry sand layer at the surface is equivalent to 1 m of diffusion layer in the air) and also a very large variation of temperature in that layer (Fig.2). As a result, evaporation from sand is very inhibited and amounts only to 10 mm from May to October when the daily atmospheric temperatures are above 30°C.

STUDY OF THE INTERRELATIONS BETWEEN AQUIFERS USING STABLE ISOTOPES

A limited number of stable isotope measurements usually gives a good idea of the typical isotopic composition of a groundwater body. In large aquifers, such as Umm-Er-Radhuma in the Arabian Peninsula, there is also significant isotopic variability within the aquifer which may be of great use in studying groundwater recharge and flow conditions. The most promising application is perhaps the study of the interrelations between aquifers, leakage problems and water source in springs. Such a study was made in Saudi Arabia in relation to the Al-Hasa springs with an estimated discharge of 12 m3/s. The results have shown that the spring IAEA-AG-158/2 2 9

20 30 40 50 60 TEMPERATURE OF THE SAND °C

~ 0600 /0715 time „'1)850 „ - "1020 1400

V I , ^ , 4 ' " '' < / -10

E

x f- 1 1-20 , I

-30 ------D R Y -M O IS T SAND INTERFACE

FIG.2. Diurnal temperature variation in the surface sand layers in July. (From R ef[4\)

water consists mainly of old water originating from the Umm-Er-Radhuma forma­ tion [2]. Recent studies have indicated that there is also a minor component of old water [3]. As the isotopic composition of modem recharge waters, inferred from samples collected from alluvium, differs significantly from the isotopic composition of old waters in some arid countries, this difference could be used in assessing the presence of modem recharge in the outcrops of the aquifer formations which essentially store old water. In practice such studies are unfortunately handicapped by the absence of sampling points at desired locations. Hydrogeologists should therefore plan the drilling of exploratory wells by taking into account the benefits to be gained from the isotopic measurements in evaluating the recharge including measurements of 14C and tritium.

RECHARGE OF AQUIFERS BY SURFACE WATER

As mentioned earlier, large perennial rivers do flow in very arid lands. In addition to the Nile in Egypt one can mention the Niger in its middle reaches and the Boteti River flowing in the Kalahari desert. Some of these rivers have 3 0 DINÇER their waters naturally labelled with stable isotopes as they originate from large lakes and swamps (Victoria Lake in the case of the Nile, Timbuctu swamps in Niger and the Okavango swamp for the Boteti River). It is possible in such situations to study the relation of these surface waters with the groundwater. In the study of the Okavango swamp for example the aquifers recharged from the swamp were very easily identified by comparing the isotopic composition of swamp waters with that of the groundwater [6]. The natural labelling of the Niger River as a result of the evaporation in the Timbuctu swamps was used to study mixing in the Kainji Reservoir in Nigeria [7]. Finally, the degree of isotopic enrichment in three lakes in southern Turkey was used to evaluate their water balance and to determine the origin of water in large karst springs on the Mediterranean coast of Turkey [8]. Although these studies are not specific to arid regions the isotopic enrichment of surface waters in arid zones is in general more extensive than the humid regions and allow better comparisons to be made between the isotopic composition of surface and groundwaters.

REFERENCES

[1] MOORE, O.M. (United States Geol. Team), Unpublished work, Ministry of Agriculture and Water, Saudi Arabia, Riyadh (1977). [2] DINÇER, T., NOORY, M., JAVED, A.R.K., NUTI, S., TONGIORGI, E., “Study of groundwater recharge and movement in shallow and deep aquifers in Saudi Arabia with stable isotopes and salinity data”, Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp. Vienna, 1974), IAEA, Vienna (1974). [3] Quaternary Period in Saudi Arabia (SAAD, S., AL-SAYARI, and ZÖTL, J.G., Eds), Springer-Verlag, Vienna-New York (1978). [4] DINÇER, T., Al-MUGRIN, A., ZIMMERMANN, V., Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium, J. Hydrol. 23 (1974). [5] SONNTAG, G., THOMA, G., MÜNNICH, K.O., DINÇER, T., KLITZSCH, E., “Environmental isotopes in North African groundwaters — The Dahna sand-dune study, Saudi Arabia” , IAEA-AG-158/6, these Proceedings. [6] DINÇER, T., HUTTON, L.G., KHUPE, B.B.J., “Study, using stable isotopes, of flow distribution, surface-groundwater relations in the Okavango Swamp, Botswana”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) I, IAEA, Vienna (1979) 3-26. [7] Advisory Group Meeting on the Application of Nuclear Techniques to the Study of Lake Dynamics, Vienna, 29 August - 2 September 1977. [ 8] DINÇER, T., PAYNE, В.R., An environmental isotope study of the south-western karst region of Turkey”, J. Hydrol. 14(1970). IAEA-AG-158/3

INTERPRETATION OF ENVIRONMENTAL ISOTOPIC GROUNDWATER DATA Arid and semi-arid zones

M .A. G E Y H Niedersächsisches Landesamt für Bodenforschung, Hanover, Federal Republic of Germany

Abstract

INTERPRETATION OF ENVIRONMENTAL ISOTOPE GROUNDWATER DATA: ARID AND SEMI-ARID ZONES. Various hydrodynamic aspects are discussed in order to show their implication for the hydrogeological interpretation of environmental isotope and hydrochemical groundwater data. Special attention is drawn to radiocarbon and tritium studies carried out in arid and semi-arid zones. An exponential model has been utilized to determine the mean residence time of the long-term water from springs in karst and crystalline regions. Hydrogeological parameters such as the porosity can be checked by this result. In addition, the exponential model offers the possibility of determining the initial 14C content of spring water, which is sensitively dependent on the soil of the recharge area. A base-flow model has been introduced to interpret the 14C and 3H data of groundwater samples from older karst regions. Differences between pumped and drawn samples exist with respect to the groundwater budget. Owing to pumping, the old base flow is accelerated and becomes enriched in pumped groundwater in comparison to the short-term water. Radiocarbon ages of groundwater in alluvium may be dubious because of isotope exchange with the C 0 2 in the root zone along the river bank. Under confined conditions I4C groundwater ages are diminished if the hydraulic head of the confined aquifer is lower than that of the shallow one. This is due to the radiocarbon downwards transport by convection of shallow groundwater. The same effect occurs, though much faster, if the groundwater table is depleted by groundwater withdrawal. The decrease of the radiocarbon groundwater ages in time can be used to determine the hydraulic transmissibility coefficient of the aquitarde. According to the practical and theoretic results obtained the hydrodynamic aspects require at least the same attention for the interpretation of environmental isotope and hydrochemical data of groundwater as do hydrochemical and isotope fractionation processes.

INTRODUCTION

Environmental isotopes have been increasingly used in hydrological studies. Increasingly complicated hydrochemical and isotope models have been introduced to interpret the measured data (1 -3 ). Recently, first attempts were made to take into account hydrodynamic aspects in the interpretation of environmental isotope data [4, 5] and to reach quantitative conclusions.

31 3 2 GEYH

rrïH

FIG .l. R un-off o f the Bardagué River at Bardai [6].

In arid zones, fossil groundwater resources and groundwater recharge are restricted. Thus, the determination of the groundwater budget based on hydro­ chemical or environmental isotope data is of interest since the common hydraulic methods are often too expensive and time-consuming. However, even simple methods often cannot be applied so far because hydraulic data have seldom been recorded. In this paper we discuss again the radiocarbon and tritium data of a case study in the arid zone of the Tibesti Mountains [6]. Obviously, the hydrodynamic situation, rather than the hydrochemical history of the groundwater, seems to have determined the environmental isotopic and hydrochemical composition.

1. HYDROGEOLOGICAL CASE STUDY: VALLEY OF THE BARD AGUE RIVER

The largest town of the Tibesti region in Chad is Bardai (elevation: 1020 m above sea level). The annual rainfall, which occurs as occasional storms, amounts to 15—200 mm, increasing with altitude. The potential evaporation is estimated to be 4000—6000 mm [6]. Groundwater is only recharged by run-off in the IAEA-AG-158/3 3 3

FIG.2. Sampling sites in the Bardagué Valley at Bardai [6].

alluvium of the Bardagué River and its tributaries. A seepage of 6 m3/m of river bed was estimated for a peak discharge of 30 m3/s, which may occur at intervals of several years (Fig. 1). Sandstone overlain by volcanic rock predominates in this region. The high mountain platform above 3000 m above sea level serves as the recharge area of the tributaries of the Bardagué River, which have eroded steep canyons into the Bardagué Valley. According to the isotope results for samples drawn from wells situated along the river bed (Fig. 2), the groundwater is several thousand years old [6]. Present-day recharge occurs at rare intervals. Since interpretation methods of isotope data have been improved during the last decade, it is worth while reconsidering the previously measured isotope concentrations, although a quantitative evaluation is still not possible owing to the lack of supplementary hydrochemical and hydraulic data.

2. METHODS FOR INTERPRETING HYDROCHEMICAL AND RADIOACTIVE ENVIRONMENTAL ISOTOPE DATA

2.1. Hydrochemical model for 14C groundwater dating

The only method applied in practice for dating groundwater is based on its 14C content. According to the Münnich model [7], rain-water seeping through the top soil dissolves C 0 2 formed in the root zone and soil carbonate, which is usually assumed to be fossil and of marine origin. The C 02 of the root zone should have the 14C content of the atmospheric carbon dioxide. Hence, the 14C 34 GEYH content of recently recharged groundwater, designated as initial 14C content, is determined by the part of admixed soil carbonate and that 14C content. Changes in the 14C content in the groundwater in a confined aquifer are due only to radioactive decay. The most-discussed problems in I4C groundwater dating are still

(a) How to determine the initial 14C content [8]; and (b) Whether secondary processes occur which change the 14C content in the groundwater besides the radioactive decay.

There are various methods for theoretically estimating the initial 14C content [1—3]. Their applicability is questioned since the isotopic content of soil lime and soil C 02 varies greatly [9-11 ]. An empirical method based on the application of an exponential model to radiocarbon and tritium measurements [12] seems to give better estimates [8]. According to experience, the initial 14C content seems to be determined mainly by the geological and pedological situation in the recharge area [13] rather than the climate. Even barely distinguishable soil covers are reflected in this value [ 14]. Secondary processes that change the 14C content may be isotope exchange or hydrochemical reactions. Münnich [7] estimated that isotopic exchange between the bicarbonate and the lime in the rock is not likely to play a predominant role in the 14C age determination of groundwater. If it did, the 14C data would be too large by a small but constant factor. Wigley considered open and closed systems [2] and a steady-state process of precipitation and re-dissolution of carbonates [15]. According to these studies,' large age shifts are to be expected and should sometimes be reflected by anomalous hydrochemical properties of the groundwater. To check whether secondary processes should be taken into account in practice, we re-evaluated the 14C data for a sandstone aquifer in the United Kingdom [16]. Bath et al. found that corrected and uncorrected groundwater ages differed either by 3700 years (= 63%-modern) or 5600 years (= 50%-modem) from each other independently of the actual groundwater age. A hydrochemical explanation for these two values could not be given. The initial 14C content (referring to sample 9), determined empirically by applying the UK 3H input curve, yielded 65%-modem, in very good agreement with the above-mentioned lower value. This shows us that, at least during the last 30 000 years, secondary processes could not have changed the environmental isotopic and hydrochemical composition of the groundwater in this case. Hence, we may summarize that the initial 14C content of groundwater can be estimated reliably, and secondary processes do not need to be taken into account if a normal hydrochemical situation exists.

2.2. Exponential model in hard rock hydrology

Spring water and shallow groundwater from hard rock regions consist of at least two components: short-term and long-term groundwater. The flow velocity IAEA-AG-158/3 3 5 of short-term groundwater can be measured by dye experiments. The amount of this short-term groundwater is almost always greater than that of thé long-term groundwater. Long-term groundwater is a mixture of components of different ages. The proportion of these older components in the long-term groundwater decreases exponentially with increasing age, because broad fissures are flow paths of large quantities of young groundwater and the smaller ones of smaller amounts of older groundwater. Under steady-state conditions between recharge and discharge, the isotope concentration Bn of long-term groundwater of mean residence time (MRT) is described by

j (n -1950) A. Bn 1 - ( MRT MRX 1 00

n

( 1)

1951

The values for aj are the isotope concentrations of the rain-water of the years j after AD 1950 (input curve). The initial 14C content of the groundwater ranges from 100 to 50%-modem [7, 13]. In the case of tritium, Aj is set at 100. The radiocarbon input curve can be taken from Nydal (17). Regionally valid tritium input values are obtainable from the IAEA, Vienna. In our case study of the Bardai region, two groundwater samples gave 14C and 3H contents which should be evaluated independently by the exponential model (Table I). Initial 14C contents of 90%-modem and more have been found for groundwater recharged in catchment areas covered with sediments poor in carbonates [13, 18]. This is the case in the Tibesti Mountains, where the conventional 14C ages exceed the actual water ages by less than 850 years. The reliability of the MRT estimated by the exponential model of both the radiocarbon and tritium contents has been tested in various ways:

(a) The mean residence time of the long-term water of perennial springs has been found to be a function of the recharge area F (km2), the porosity of the aquifer P(%), the thickness of the water-bearing formation H(m), and the mean annual discharge of the long-term groundwater MQ (m 3/s).

P * H * F (2) R 3 X 15 * MQ

The recharge rate R is given in % of the annual rainfall. 3 6 GEYH

TABLE I. 14C AND 3H DATA OF TWO SPRING-WATER SAMPLES FROM THE BARDAI REGION AND THE CORRESPONDING MRT VALUES DETERMINED BY THE EXPONENTIAL MODEL [6]

Sampling site Guelta (No. 9) Guelta (No. 17)

Lab. number H v4 3 3 0 Hv 4327

14C content (%-modern) 101 89

3H content (TU ) 70 28

M RT (a) 35 5 0 - 1 0 0

Ai (%-modern) 90 85

1СГ ppm юн MIGMATITE INSELBERG

WEI

7 -

Z 6 ш > - GN8SS4 у saitwater Z ¡ »rçüûûppm 8 sh < . шa 4 Z

3 -

2 -

10 20 30 40 50 60 70 80 90 yrs. MEAN RESIDENCE TIME

FIG.3. Mineral contents (ppm) and MR T o f joint groundwater from the hard rock peneplain in NE Brazil [18]. IAEA-AG-158/3 37

TABLE II. DIFFERENCES IN THE YIELD OF WELLS IN NE BRAZIL SITED ACCORDING TO TWO DIFFERENT CONCEPTS [19]

Former concept New concept Wells in peneplain Wells at inselbergs

Number o f wells 67 32

Yield (m3/h) 1.5 3

Depth (m) 78 55

Mineral content (ppm) 36 0 0 500

The validity of Eq. (2) has been tested many times [14] and has been utilized for estimating porosity in unexplored, arid, karst regions from the 14C and 3H contents of sometimes only a single spring-water sample.

(b) The MRT of the subsurface flow in the semi-arid peneplain of NE Brazil (annual precipitation < 400 mm) was found to be a function of the mineral content of the groundwater [18]. This result (Fig. 3) revealed evidence for present-day groundwater recharge, forcing the geologists to modify their former concept for siting water wells [19], which are now drilled at the foot of the inselbergs instead within the peneplain. As a result, the qualitative and quantitative yields of the wells have noticeably improved (Table II). The hydrogeological estimated groundwater recharge rate, R, of 3% corresponds to the observed MRT of 25—40 years (Eq. (2)).

2.3. Base-flow model

MRT values for holokarst regions, i.e. old karst regions, estimated indepen­ dently from the radiocarbon and tritium contents of conduit spring water [20], were frequently found to disagree with each other. The reason is that such groundwater is better described as a mixture of a young component, for which the environmental isotope content can be estimated by the exponential model, and an old component (base flow) of an age of several centuries or millennia, and free of tritium. The 14C and 3H contents of the young components ( 14CeXp and 3Hexp) were first estimated using an assumed or measured MRT. This 3Hexp was then used to calculate the proportion of the base-flow component of the samples q.

. 3HSpi (3) ^exp 3 8 GEYH

FIG.4. Content o f the base flow and its flow direction according to pumped samples from wells in the recharge area o f the mineral springs at Bad Canstatt, Fed. Rep. o f Germany [21 ].

t

The 14C content of the base-flow component is given by

. ( 14Cspi — 14Cexp) 14c bf = ------C exp (4 )

Attempts to apply the base-flow model in our case study did not succeed because isotope exchange with soil C 02 occurred (see Section 2.4.). However, the application of the base-flow model has given so much insight to the origin of the environmental isotope and hydrochemical composition of well-water samples that the instructive case study of the mineral springs at Bad Canstatt, Federal Republic of Germany, should be discussed [21 ]. The karst recharge area of these mineral springs is partly covered with a limey and loamy sediment. The youngest well-water should have hydrogeologically acceptable MRT values between 5 and 40 years. Although isochrons could not be delineated, we have assumed an increase in the MRT of 5 years in the SW to 40 years in the NE (Fig. 4). This follows from the hydrogeological conception. The calculated flow velocity of 350 to 400 m/a for this young component fits the known discharge of the mineral springs. On the other hand, according IAEA-AG-158/3 3 9

Q=1 (Us)

FIG .5. Schem atic presentation o f the disturbance o f the flow velocities o f both the young com ponent and the base flow owing to pumping.

to the 14C data the base flow has a velocity between 0 .7 -4 .7 m/a whose direction (derived from the hydroisochrons) corresponds to the piezometric lines. However, in apparent contrast to the above-mentioned mass balance, the base- flow component of the water samples was determined to be 20—60% instead of the 2 —5 %o that was expected from the ratio of the flow velocity of the base flow to that of the young component. We reached the following conclusion (Fig. 5): Groundwater is pumped off more or less from the whole aquifer (here approx. 15 m) with pumping rates > 1 ltr/s. Hence, the flow velocity at the surface of a cylinder of 5 m diameter and a rock porosity of approximately 3% already amounts to vart > 220 m/a. As a result, the base flow becomes accelerated. In this case, the ratio of the base flow to the young component is approximated by the quotient of Hbf and Hexp (Fig . 5 ). This result has implications for the application of environmental isotopes and hydrochemical data from pumped water samples for determining the water budget. According to the vertical mixture model [22], the widespread impression is that the 14C groundwater age reflects the young components more than the older ones. However, the reverse is true for water-budget considerations. As the old water generally has a very small flow velocity, its part in the mass balance is over-emphasized by the 14C age of pumped water samples [20]. 40 GEYH

TABLE III. 3H CONTENT OF WATER SAMPLES FROM THE WELL OF THE RESEARCH STATION BARDAI BEFORE AND DURING PUMPING [6]

Date State 3H content (TU)

2 April 1968 Unpumped [23] 322

7 September 1968 Start of pumping [23] 2-3

5 January 1971 Continuous pumping [6] < 2

TABLE IV. EXAGGERATED 14C CONTENT IN WATER SAMPLES DRAWN FROM WELLS SITUATED IN THE ALLUVIUM OF THE BARDAGUÉ RIVER UPSTREAM AND DOWNSTREAM FROM A NATURAL CANYON BARRIER IN THE RIVER BED

Sample 14C content 3H content Location No. Hv (%-modern) (TU)

8 4823 114 13 Upstream

10 48 2 4 111 35 Upstream

13 4825 96 12 Upstream 7 4 8 2 0 115 5 Downstream

15 4827 120 < 2 Downstream

TABLE V. VARYING 14C GROUNDWATER AGES WITH INCREASING DISTANCE FROM BARDAI IN THE ALLUVIUM OF THE BARDAGUÉ RIVER

Distance (km) 0 2 23 63 80 87 113

14С age (b .p .)a 27 0 0 37 0 0 9 5 0 0 67 0 0 59 0 0 4 0 0 0 8000

Well No. 6 18 - - - - -

a b.p. = before present. IAEA-AG-158/3 41

Differences must be expected between these and drawn samples. These differences may be used to explain differing results between radioactive and stable environ­ mental isotope data. It follows that the measured, rapid disappearance of tritium after the commencement of pumping of the well at the research station in Bardai (Table III) may not be interpreted as indicating a lack of groundwater recharge as has been done in the past [6].

2.4. Isotope exchange between bicarbonate of groundwater and soil C 02

Water samples taken from wells situated in alluvium sometimes have a high 14C content and a low content, or an absence, of tritium [6]. There are many examples of this from the Bardai case study (Table IV). In these cases we always found that groundwater rises to the surface in the root zone throughout the year. The high 14C and C 0 2 contents in the root zone favour isotopic exchange with the bicarbonate and the C 02 in the groundwater. At the same time, atmospheric oxygen is dissolved [23] and salt crusts are formed by evaporation of groundwater. As a result of this isotope exchange, the 14C age of the groundwater in the river valley does not increase steadily in direction of the river flow, as seen from our results of the Bardai case study (Table V). No samples contained tritium. After two years without rainfall, the 14C ground­ water ages increased by 3000 years. As the run-off recharge is the most important one in arid zones [24], neither radiocarbon nor stable isotope data for the groundwater in alluvium allow quantitative hydrological interpretations.

2.5. Hydraulic (convection) model

At the end of the alluvium, the run-off groundwater may become confined. We would like to consider an aquifer of thickness H2 and porosity P2 (Fig. 6). It should be overlain by a poorly permeable layer (aquitarde) of a thickness H b a pore-water content P, and a coefficient of hydraulic permeability K. Owing to assumed continuous recharge, the shallow groundwater of the upper aquifer should have a high and constant content of radioactive environmental isotopes in comparison with that of the confined groundwater. Apart from radioactive decay, two further physical processes may change the isotopic composition of the confined groundwater. These are:

(i) Diffusion: According to Fick’s law, there must be a mass transport of radioactive isotopes from the shallow groundwater with its high concentration to the confined one with its low concentration [25]. As a result, the latter is increased and the radiometric age of the confined groundwater appears to be too 42 GEYH

^OHWENCE UA-UPPERAQUFER

FIG. 6. Scheme o f an aquifer system consisting o f a shallow unconfined aquifer separated from a deeper confined aquifer by an aquitarde.

low. This case has to be considered if the shallow aquifer does not exist but sporadic groundwater recharge occurs in the region of investigation. Then, the pore water of the uppermost part of the poorly permeable layer may have a high radioactive isotope content but there is no hydrostatic difference between this and the confined groundwater.

(ii) Convection: The radioactive isotope transport by diffusion becomes comparably negligible if there is a hydrostatic pressure difference Д between the shallow and the confined groundwater [5]. Provided that the hydraulic head of the confined groundwater is higher than that of the shallow groundwater, an upwards movement occurs through the aquitarde. A hydraulic pressure difference of a few decimetres is sufficient to cancel completely the diffusion effect to the isotope composition. In this case, the radioactive isotope content of the confined groundwater may be a function of its actual age, secondary processes excluded. In the contrary case, the downwards movement of the shallow groundwater with its high radioactive isotope content increases the isotope content of the confined groundwater by several orders of magnitude faster than diffusion (Fig. 7). The radiometric age approaches a maximum value, which is a function of the hydraulic pressure difference Л and the geometric and hydraulic properties of the whole aquifer system. Even thick and very poorly permeable layers do not protect the confined aquifer against a noticeable input of shallow groundwater over periods of millennia. Hence, the hydrochemical and stable isotope composition of the confined groundwater also becomes noticeably influenced by the admixture of shallow groundwater. Its content slowly IAEA-AG-158/3 43

FIG. 7. Relationship between apparent14 С ages and actual ages o f groundwater due to the convection fo r a special case study.

approaches 100%-modem. Of course, exchange processes may also change the hydrochemical and stable isotope composition of the shallow groundwater while passing through the aquitarde. The general observation that the mineral content of confined groundwater increases with its age could be a result of the continuous admixture of shallow groundwater. According to Bögli [26] a mixing of groundwater with different temperatures, C 0 2, or bicarbonate contents, distorts the hydrochemical equilibrium and results in a chemical attack on the rock in the aquifer. The hydraulic (convection) model must be utilized if I4C data of old ground­ water taken from wells situated in the Sahara desert is to be interpreted paleo- hydrogeologically. In many cases, not the paleohydrogeology but the hydro- dynamic situation during the past may be reflected.

2.6. Mining model

Confined groundwater resources are being increasingly exploited in arid zones for irrigation water. At the beginning of the pumping of a well, a ground­ water cone is formed which quickly lowers the hydraulic head. Later, depletion rates of 0.5 m/a and more are common. According to the convection model, 44 GEYH

------>► GROUNDWATER WITHDRAWALS]

FIG.8. Changes o f the 14C water ages in a confined aquifer owing to groundw ater mining as a function o f the duration o f groundw ater withdrawal and different depletion rates o f the hydrostatic pressure considered for a special situation.

water from the shallow aquifer will commence to penetrate the aquitarde and eventually enter the confined aquifer [5]. At this moment, the radioactive isotope content will rise and accordingly the groundwater ages will decrease with increasing pumping time (Fig. 8). The fall of the confined groundwater age is a function of the depletion rate of the hydraulic head and the geometric and hydraulic properties of the whole aquifer system (Fig. 6). Therefore, the change in the confined groundwater age with time may be used to check the assumptions on the hydraulic parameters of the aquifer system.

3. CONCLUSION

Various hydrodynamic aspects have been discussed in order to show their implications for the hydrogeological interpretation of the composition of radio­ active and stable environmental isotopes in groundwater. It became obvious that these aspects have a similar if not greater importance than hydrochemical processes, isotope exchange and isotope fractionation. As the radiometric ages IAEA-AG-158/3 45 of old groundwater are particularly affected, the hydrodynamic aspects require special attention to environmental isotope and hydrochemical groundwater studies in arid and semi-arid zones.

ACKNOWLEDGEMENT

I am grateful to Dr. A.H. Bath for providing the United Kingdom tritium input data.

REFERENCES

WENDT, I., STAHL, W., GEYH, M., FAUTH, F., “Model experiments for 14C water-age determinations”, Isotopes in Hydrology, (Proc. Symp. Vienna, 1966), IAEA, Vienna (1967) 321-37. [2 WIGLEY, T.M.L., Carbon-14 dating of groundwater from closed and open systems, Water Resour. Res. 11 (1975) 324-28. [3 MOOK, W.G., “The dissolution-exchange model for dating groundwater with 14C”, Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology (Proc. Panel Vienna, 1975), IAEA, Vienna(1976) 213—25. [4 WALLIK, E.I., TOTH, J., “Methods of regional groundwater flow analyses with suggestions for the use of environmental isotopes”, ibid., pp. 37—64. [5 GEYH, M.A., BACKHAUS, G., “Hydrodynamic aspects of 14C groundwater dating”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) 2, IAEA, Vienna (1978) 631—43. [6 GEYH, M.A., OBENAUF, K.-Р., Zur Frage der Neubildung von Grundwasser unter ariden Bedingungen, Pressedienst Wissenschaft, Freie-Universität-Berlin 5 (1974) 70—91. V MÜNNICH, K.O., Isotopen-Datierung von Grundwasser, Naturwissenschaften 34 (1968) 32-33. [8 TAMERS, M.A., Validity of radiocarbon dates on groundwater, Geophys. Surv. 2 (1975) 217-39. [9 GEYH, M.A., “Carbon-14 concentration of lime in soils and aspects of the carbon-14 dating of groundwater”, Isotope Hydrology 1970 (Proc. Symp. Vienna, 1970), IAEA, Vienna (1970) 215-23. [10 SALOMONS, W., MOOK, W.G., Isotope geochemistry of carbonate dissolution and reprecipitation in soils, Soil Sei. 122 (1) (1976) 15—24. [11 RIGHTMIRE, C.T., Seasonal variations in P^o and 13C content of soil atmosphere, Water Resour. Res. 14 (4) (1978) 691—92. [12 GEYH, M.A., “On the determination of the initial I4C content in groundwater”, Proc.VIII Int. Conf. on Radiocarbon Dating, Wellington ( 1972) D 58-69. [13 GEYH, M.A., Basic studies in hydrology and C-14 and H-3 measurements, Proc. XXIV Int. Geol. Congr., Montreal (1972) Section 11, 227—34. [14 GEYH, M.A., GROSCHOPF, P., Isotopenphysikalische Studie zur Karsthydrologie der Schwäbischen Alb, Jh. Geol. Landesamt Baden-Württemberg 8 (1978) 7—58. [15 WIGLEY, T.M.L., Effect of mineral precipitation on isotopic composition and 14C dating of groundwater, Nature (Lond.) 263 (1976) 219—21. 46 GEYH

[16] BATH, A.H., EDMUNDS, W.M., ANDREWS, J.N., “Palaeoclimatic trends deduced from the hydrochemistry of the Triassic sandstone aquifer, United Kingdom”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) 2, IAEA, Vienna (1978) 545—68. [17] NYDAL, R., personal communication. . [18] GEYH, M.A., KREYSING, K., Sobre a idade das aguas subterráneas no polígono des secas do Nordeste Brasiliero, Rev. Bras. Geoscie. 3 (1973) 53—59. [19] KREYSING, K., LENZ, R., MÜLLER, W., Problems of groundwater exploration in metamorphic rocks for domestic water supplies in northeastern Brazil, Proc. XXIV Int. Geol. Congr., Montreal (1972) Section 1 1, 73—79. [20] SHUSTER, E.T., WHITE, W.B., Seasonal fluctuations in the chemistry of limestone springs: A possible means for characterizing carbonate aquifers, J. Hydrol. 14 (1971) 93-128. [21] GEYH, M.A., KÖHLE, H., In preparation. [22] VOGEL, J.C., “Carbon-14 dating of groundwater”, Isotope Hydrology 1970 (Proc. Symp. Vienna, 1970), IAEA, Vienna ( 1970) 225—37. [23] SIEGENTHALER, U., SCHOTTERER, U., OESCHGER, H., MESSERLI, B., Tritium- messungen and Wasserproben aus der Tibesti-Region, Hochgebirgsforsch. (High Mountain Res.) 2 (1972) 153-59. [24] DINÇER, T., “The use of environmental isotopes in arid zone hydrology”, these Proceedings. [25] KLITZSCH, E., SONNTAG, C., WEISTROFFER, K., EL SHAZLY, E.M., Grundwasser der Zentralsahara: Fossile Vorräte, Geol. Rundsch. 65 (1976) 264. [26] BÖGLI, A., Mischungskorrosion — ein Beitrag zum Verkarstungsproblem, Erdkunde 18 (1964) 83-92. IAEA-AG-158/4

A GEOCHEMICAL AND ISOTOPIC APPROACH TO RECHARGE EVALUATION IN SEMI-ARID ZONES Past and present

W.M. EDMUNDS, N.R.G. WALTON Institute of Geological Sciences, Wallingford, Oxfordshire, United Kingdom

Abstract

A GEOCHEMICAL AND ISOTOPIC APPROACH TO RECHARGE EVALUATION IN SEMI- ARID ZONES: PAST AND PRESENT. The magnitude of any recharge to aquifers in semi-arid and arid zones is the principal uncertainty in estimating a water balance. Recent studies in Cyprus and Libyan Arab Jamahiriya are currently being used to demonstrate the application of geochemical and isotopic techniques, to the determination of both current and palaeo-recharge. In Cyprus, solute profiles of the unsaturated zone have been interpreted to provide estimates of the direct recharge component using a steady-state, mass-balance approach; results from the chloride profiles compare well with recharge estimates using tritium. In addition, it is found that some solute peaks, notably for specific electrical conductance, give a reasonably accurate record of the rainfall history during the period 1950—1975. The solute profile method is relatively unsophisticated and' could be more widely applied to recharge estimation in other semi-arid areas of the world. In Libya, a clear distinction can be made using the combined iso topic, hydrological and geochemical results between regional groundwaters recharged to the upper, unconfined aquifer of the Sirte Basin before 13 000 years BP and younger waters recharged locally during the period 5000—7800 years BP. A well-defined fresh-water channel, superimposed upon the regional water quality pattern, can be traced within the aquifer for some 130 km and represents d i r e c t evidence of recharge during the Holocene. Some shallow groundwaters of similar composition to the fresh-water channel are also considered to represent recent, if intermittent, recharge which took place during historical times. It is concluded that geochemical and iso topic studies of both the unsaturated zone and of shallow groundwaters in semi-arid regions, can be used to determine not only the present-day direct recharge component, but also a recharge chronology of immediate historic times, which may be important in the estimation of long-term water resources.

INTRODUCTION

' Natural replenishment of groundwater reservoirs in arid zones can take place by two mechanisms - (i) direct infiltration of rainwater via the soil and unsaturated zone and (ii) local recharge of surface runoff via permeable wadi beds or drainage systems. The flashy and unpredictable nature of rain­ fall events in semi-arid and arid zones makes the accurate determination of rainfall amounts and of surface runoff almost impossible. The problem of

4 7 48 EDMUNDS and WALTON

determining aquifer recharge is further compounded by the difficulty in measuring évapotranspiration with any accuracy. Add to this the infinite variety in soils, topography, geology and land use and the problems of determining recharge in semi-arid zones are placed in perspective.

The magnitude of the natural recharge component is generally the largest uncertainty in water balance calculations in the arid zones. Many major groundwater development schemes are proceeding with the uncertainty as to whether natural replenishment is taking place at all; other schemes are being developed on the concept of 'groundwater mining', which is pro­ bably the only safe basis of exploitation in the absence of good recharge f i g u r e s .

Conventional methods of calculating the direct recharge component (Penman, [l-2] , Monteith, [3] ) have limitations imposed upon them when applied to arid and semi-arid zones, not only by the problem of the accuracy of the measured parameters, but possibly even by the method itself. Lysimeters have been used successfully to measure direct recharge (Kitching et al., [4]). In addition to the logistics and cost of instal­ lation in remote areas, they present problems of disturbing the terrain during construction and although integration over a sizeable area can be achieved, typically up to 9 m 2, problems of representativeness of the re­ sults over a wide catchment area still remain. Indeed, this last point is probably the most difficult factor in any recharge calculation.

Tritium profile methods developed by Zimmerman et al.[5], Miinnich et al. [б] , Smith et al. [7] , Andersen and Sevel, ["g] , among others, have been found applicable to recharge estimation in various temperate zone environments. Their principal advantage is the integration of in­ formation from a large time sequence, (as represented by the vertical interval) and areal extent, through repeated profiles made over a wide geographical area. Several studies have now demonstrated that tritium profiles can be used in semi-arid zones including India (Sukhija and S h a h , [9 ])/ Australia (Allison and Hughes, [ l o j ) an<3 Saudi Arabia (Dinçer et al., [ i l ] ) .

Whilst tritium has been successful in current recharge evaluation, other isotopic methods, notably the conjunctive use of carbon, oxygen and hydrogen stable isotope ratios,together with carbon-14, have proved valuable in demonstrating the existence of fossil groundwater and, in favourable circumstances, in providing an approximate groundwater age. Such studies, usually showing that significant recharge has not occurred in recent times, have been particularly useful in arid zone water resource applications. However, most isotope studies in arid zones have'tended to deal with regional rather than local relationships. In such studies, pumped samples are likely to represent '.age mixtures ' and at shallow depths ambiguous results, often with high % modern carbon, are particularly difficult to interpret.

It is likely however, that recharge in present day arid zones has been continuing, although intermittent, process during the Holocene and that careful field study, combined with isotopic and geochemical analysis, can be used to investigate this. Evidence of significant, if isolated, recharg during the past 500 or 1,000 years from catastrophic rainfall events would be of considerable significance in evaluating the longer term resources of arid areas. IAEA-AG-158/4 49

The purpose of this paper is firstly,to investigate isotopic and hydrogeological evidence that recharge has occurred at successive intervals ' during the Holocene, using examples from Libya; and secondly, to discuss the application of geochemical techniques for the estimation of current recharge using results from a continuing study in Cyprus. Furthermore,it is intended to examine the possibility of linking these methods of inves­ tigating current and palaeo-recharge with a view to determining the historic recharge which is important for longer term water resource estimations.

1. EVIDENCE FROM LIBYA OF SUCCESSIVE RECHARGE EVENTS DURING THE LATE PLEISTOCENE AND HOLOCENE.

Detailed hydrogeological studies in the Sirte and Kufra basins in Libya have provided an opportunity to carry out isotopic and geochemical investigations of groundwaters,both on a regional and on a local scale ( W r i g h t e t a l . , [l2] , E d m u n d s a n d W r i g h t , [l 3] , B e n f i e l d a n d W r i g h t , [l4]). Geochemical studies have been used to define limits of potable water, groundwater source areas and the connection between individual aquifers and basins as well as investigating the recharge history. Vast reserves of fresh groundwater have been proved in aquifers within the two basins which are stored in clastic sediments of Cretaceous and Tertiary age.

The overall groundwater mineralisation in the Post Middle Miocene (PMM) aquifer of the Sirte Basin is represented in Figure 1 in which isochlors, drawn from over 200 data points, define the average water quality distribution in this upper, unconfined aquifer system. A distinct body of non-saline water extends throughout the area,in which the total mineralisation increases, although without any change in ionic ratios, from 1,000-2,000 mg/1 south to north along the flow line. Superimposed upon this general pattern there exists a significant fresh water lens (Cl < 50 mg/1) which is at least 70 m thick and which transects the re­ gional water quality contours SE of Jalu oasis. This feature can be traced as a broad channel about 10 km wide for some 130 km and is con­ sidered to be a superimposed feature created by recharge from a former wadi, which is illustrated in cross-section in Figure 2. It is apparent from the rather limited number of shallow wells existing in the region, that low salinity water also exists extensively as a thin layer (< 10 m) at the water table; this less saline water is also considered to represent recharge from one or more late-stage events.

To investigate these anomalies in the basic water quality pattern in more detail, isotopic analyses were made including ^H, 62H, 613C and 6^**C for which representative results are given in Table 1 (the others being given in full in Edmunds and Wright, [l3]). Tritium results for the shallow wells are 0±2 TU and indicate that current recharge is improbable, although, as shown by the results from Cyprus,discussed below, any recent rainfall may well be stored within the unsaturated zone.

The carbon isotope results fall into two groups:-

(a) those with low ^ C activity ratios (below 4% modern) and with slightly enriched 61 ^C contents S- 7 to -11 ° / „ ) , corresponding to the chemically- similar regional, non-saline groundwaters

(b) those with modern li(c activity ratios (between 35 and 55% modern) and with considerably enriched 6^C (-3 to - 5 ° / J which correspond to the. low salinity groundwaters within the freshwater channel and at shallow depths, represented by the first two groups in Table 1. 50 EDMUNDS and WALTON

FIG .l. Water quality map o f the central Sirte Basin and Kufra, Libyan Arab Jam ahiriya , show ing localities referred to in the text.

Two separate models were necessarily considered for treatment of this basic division of the data to attempt a derivation of age. The first is based on a classical approach to carbonate dissolution,whilst the second takes into account some of the limitations imposed by the semi-arid zone environment.

Isotopic results from the local environment were first used where practicable to provide a realistic control on the interpretation of the carbon acquisition by the groundwater. The PMM aquifer is predominantly fluviatile, and marine carbonate is considered to be virtually absent; IAEA-AG-158/4 51

FIG .2. H ydrological m odel to illustrate the probable developm ent o f the freshw ater channel, (i) M ajor recharge period - corresponds to regional, late Pleistocene accum ulation o f ground­ water; (ii) Arid phase - characterized by low rainfall, little or no recharge and calcrete accum ulation; (iii) Late-stage w et phase - significant recharge takes place represented by groundwater having Cl~ < 250 m g/l; (iv) Present day - a little or no recharge; situation com parable to (ii). Lf\ to

TABLE 1 . STABLE ISOTOPE AND RADIOCARBON RESULTS FOR GROUNDWATERS AROUND JA L U , L IB Y A , THE LOCATIONS OF WHICH ARE INDICATED IN FIGURE 1 .

SITE APPROX SAMPLE S IC CALCITE FIELD С UNCORRECTED- CORRECTED NO. DEPTH BELOW SATURATION 61 3C PH HCO3 (% MODERN) AGE AGE I (MAP) WATER TABLE (XoSMCW) (m g/I) INDEX

GROUNDWATER IN FRESHWATER CHANNEL

1 El-105 1/73 - 8.6 - 6 7 7 . 6 8 2 8 4 + 0 .0 8 - 3 . 6 3 7 . 6 7 8 5 0 NO FIT

SHALLOW GROUNDWATER FROM JALU AREA WALTON and EDMUNDS

2 JALU (0. FADEL) 12/73 2 - 4 - 6.8 - 6 0 7 . 9 98 + 0 .6 1 - 3 . 2 5 1 . 2 5 3 8 0 NO FIT 3 JALU (A. REJAB) 12/73 2 - 4 - 7 . 1 - 7 0 7 . 9 9 8 + 0 .2 5 - 5 . 2 3 9 . 1 7 5 4 3 NO FIT 4 GATMIR 1 2 / 7 3 2 - 4 - 0 . 5 - 3 4 8.1 3 9 5 +0. 75 - 4 . 9 5 3 . 6 5 0 1 0 NO FIT

REGIONAL GROUNDWATERS MAINLY FROM INTERMEDIATE DEPTH

5 D -1 0 2 1/ 7 3 1 7 - 2 1 - 8.1 - 7 2 7 . 5 3 1 2 5 + 0 .3 3 - 6.2 5 . 4 2 3 4 0 0 9 3 5 0 6 JA-P 3/ 73 2 5 - 7 3 - 6.8 - 7 6 7 . 6 5 2 5 4 + 0 .3 9 - 7 . 2 1.2 3 5 7 6 0 2 4 1 0 0 7 W 81-103A 1 / 7 3 6 3 - 1 0 3 - 8.6 - 7 4 7 . 3 3 1 9 5 +0.10 - 7 . 1 - 1.6 3 3 3 0 0 2 1 5 0 0 8 W 52-103D 3 / 7 2 6 6 - 1 0 3 - 9 . 0 - 7 4 7 . 29 1 3 5 - 0 . 0 9 - 6.0 1 . 6 3 3 2 2 0 1 8 5 0 0

9 JD-P 1 2 / 7 2 3 2 - 8 2 - 8 . 3 - 7 3 7 . 4 1 1 8 6 + 0 .2 2 - 6 . 4 3 . 4 2 7 2 1 0 1 3 8 5 0

1 0 JE-P 1 / 7 3 2 3 - 7 3 - 8 . 5 - 7 0 7 . 6 7 1 9 2 + 0 .4 0 - 5 . 9 4 . 8 2 4 3 5 0 9 1 0 0

11 W 1 5 3 -5 9 E 8 / 7 1 5 0 - 9 4 - 8 . 7 - 7 4 7 . 6 8 1 1 7 - 0 . 3 3 - 7 . 0 2 . 8 2 8 6 3 0 1 6 4 0 0

14 T / F F I- 6 5 P 1 0 / 7 3 7 8 - 1 2 9 - 9 . 5 - 7 8 7 . 4 3 ’ 2 8 9 + 0 .2 8 - 6 . 8 0 . 7 3 9 8 5 0 2 7 7 0 0

15 T / U I-6 5 P 2 9 / 7 3 5 9 - 1 1 9 - 9 . 6 - 7 9 7 . 6 3 3 5 1 + 0 .3 9 - 1 0 . 7 0 . 7 > 4 0 3 6 0 3 3 8 0 0

1 6 T / T 2 -6 5 P 1 1 / 7 3 6 3 - 1 1 3 - 1 0 . 5 - 8 0 7 . 3 1 2 7 -Ю .11 - 1 1 . 7 0 . 7 > 4 0 3 6 0 3 4 3 6 0 IAEA-AG-158/4 5 3

the carbonate carbon source is likely to be soil carbonate or calcrete which had 6I3C = -2.6 to -4.1%0. A meaningful value for biogenic or soil CO 2 was much more difficult to obtain in view of the problem of extrapola­ tion to former vegetation conditions; in the calculation, a value of 6 = -21.5%0 was used - this was an average value,measured during the study, for modern vegetation and wood preserved in soil horizons (range

Stable oxygen and hydrogen isotope results (quoted in detail in Sdmunds and Wright,,[13]) show a consistent regional trend within the 3irte and Kufra basins, becoming generally enriched in *®0 and 2H from north to south. All the results are related by an evaporative line (Figure 3) with a slope 62H = 4.5 <5lsO-35 which intercepts the meteoric line at

Ь'в0

FIG.3. Oxygen and hydrogen stable isotope ratios for Sirte and Kufra basin groundwaters, linked by an ‘evaporative line - 8D = 4.58lsO -35. Current recharge (light isotopic lim its only) are shown for stations at Tunis (1); Faya Largeau, Fort Lam y and Kano (2) and Bam ako (3). All results are relative to SMOW and based on data in IAEA (Vienna) Technical R eports Series N os 96, 117, 129, 147, 165. FIG .4. Groundw ater ‘ages’ in the Sirte and Kufra basins com pared against indirect palaeoclim atic evidence for the Sahara region for the late Pleistocene and H olocene, sources o f which are given in Edm unds and Wright [ 13]. 5 6 EDMUNDS and WALTON

The results have a significance in relation to possible historical recharge events in the Sahara and its margins. The groundwater radio­ carbon results are summarised in Figure 4 in relation to other palaeo­ climatic data for the Saharan region during the late Pleistocene and Holocene. From the various studies cited it has been established that significant humid episodes probably occurred at least twice during the Holocene. The isotopic results from Libya, substantiated by the hydro- geological evidence, provide direct evidence that groundwater recharge occurred during this period, which can be clearly distinguished from water recharged during earlier episode (s). The replenishment is con­ sidered to have taken place both by direct recharge and by surface run­ off. It is likely that a quite major river system was still active in eastern Libya up until ca. 5000BP and possibly later. Pachur [2l] records elephant bones with an age of ca.3500BP in fluviatile sediments in the Wadi Behar Belama (Figure 1) - a short distance south-west of the known groundwater channel - which is a likely extension of the same hydrological system. The shallow, low salinity groundwaters in the area with consider­ able contents give corrected ages around 5,000-7,000 BP and are considered to represent Holocene recharge. Whilst these samples may correspond solely to the same pluvial events that caused the freshwater channel, there is the possibility that they represent accumulations of direct recharge much younger than 5,000-7,000 years mixed with much older water during pumping.

Further study is clearly required in Libya and elsewhere of the water at the water-table and immediately below, since this zone is often ignored during water well drilling. It is therefore likely that some recharge may have occurred during the past 500-2,000 years, signigicant from a water resources viewpoint,yet which is not detectable without a careful evaluation technique.

2. GEOCHEMICAL METHODS FOR ESTIMATING THE CURRENT RECHARGE COMPONENT - CYPRUS.

Environmental tritium profiles of unsaturated zone moisture are now fairly widely established as a technique for investigating recharge rates and the mechanism of water movement to temperate zone aquifers (Zimmerman et al. , [5], Münnich et al., '[б ]» Smith et al. £ 7] Andersen, [s] ) • There have also been several successful attempts at the use of tritium profiles in semi-arid and arid zones, e.g. India (Sukhija and Shah, [ э]), Australia (Allison and Hughes, [lo]), and Saudi Arabia (Dinçer et al., [il] ) ■ Whilst it is recognised that tritium offers a valuable and successful method of recharge estimation, especially when compared with the other available tech­ niques, it must also be recognised that the method has several limitations.

(a) the relatively short tritium half life of 12.3 years limits the long term usefulness of the method

(b) the method is vulnerable to contamination uuring sampling, and sub­ sequent processing, a factor that is enhanced in remote areas and at low total moisture levels

(c) analysis is highly specialised and relatively costly, making detailed profiles rather expensive

(d) although the 1963/4 tritium peak can be expected to be resolved,at least in the northern hemisphere, annual increments of moisture cannot be identified. IAEA-AG-158/4 5 7

Tritium studies have usually been carried out without any supporting data on the geochemistry of the dissolved solutes. The aim of the current research programme was therefore to investigate a wider possible range of geochemical methods that might be applicable to the estimation of recharge in semi-arid and arid zone environments. Since estimates of recharge are urgently required in remote areas where sophisticated methods are in­ appropriate, the target was set that any technique had to be logistically simple and inexpensive, as well as reliable.

It was considered that certain constituents, notably Cl, SO^ and pos­ s i b l y NO3 , might be conservative within the unsaturated zone environment, thereby retaining a chronology of recharge events within the profile record that could be used at least to supplement the information obtained from tritium, if not to provide an alternative means of assessment. The idea of using chloride for recharge estimation is not new - it has been used for example by Eriksson [22] in India, and Allison and Hughes [23] in Australia. In both these investigations the objective was to compare the saturated zone chloride values with atmospheric inputs to derive a mass balance. However, in using water-table samples, it is clearly dif­ ficult to discriminate between the contribution from the aquifer lithology and that from atmospheric input. The value of using the unsaturated zone profiles described in the present study, is that it is then possible to readily distinguish between these' two components and to have a far greater control over the recharge interpretation. Two main approaches, analagous to tritium profile interpretation, are therefore being adopted:

(a) Mass Balance : The unsaturated zone profiles should contain a solute loading which is composed of two inputs (i) rainfall, dry deposition and aerosols, (ii) aquifer lithology. These two components should be clearly distinguishable within the profile record. The value of (i) can then be used to calculate the recharge, assuming also that soil moisture, rainfall amount and rainfall composition over a number of years are known.

(b) Solute Peaks : The working hypothesis is comparable to tritium inter­ pretation, that in areas where a finite amount of annual recharge occurs, solute increments are transmitted annually by piston displacement through the unsaturated zone. The amount of solute transmitted will be dependent upon a number of factors, particularly the degree of evaporative concentration and the intensity and amount of the following seasons' rainfall. In a homogeneous medium therefore, a series of peaks should be generated corresponding to annual recharge events, whilst variations in the solute profile may also be apparent on a wider secular basis following groups of dry or wet years. An inverse correla­ tion between mean annual rainfall and profile record over a number of years could then be used to calibrate the profile on a time basis and enable a recharge calculation analogous to the use of tritium peaks. It is stressed that this is primarily an empirical approach designed to by-pass, at least initially, the complexity of soil physics theory, which,in the case of water itself,is made more problematical by the likelihood of 2-phase movement. This idealised model might be com­ plicated where recharge was not an annual event, or where percolation was non-homogeneous due to fracture or pipe flow for example, or to variable lithology or sedimentology. Some of the limitations that apply to tritium profile interpretation would also therefore apply to solute profiles. Although tritium levels vary considerably between summer and winter rain in most parts of the world, there is no corre­ sponding contrast in the percolating water, since only the heaviest 5 8 EDMUNDS and WALTON

rains (with seasonally constant 3H) are significant as recharge; it may be that solute profiles can therefore record annual events where tritium cannot.

The project was initiated in Cyprus for logistic as well as scientific reasons. This was partly because the geochemical results could then eventu­ ally be compared both with simultaneous lysimeter measurements of recharge and also with recharge estimates from local water resource studies. The Akrotiri peninsula in the extreme south of the island was chosen initially for geochemical profile work on account of its lithology, which comprises relatively homogeneous unconsolidated Holocene/Quaternary dune sands and be­ cause a water balance study of the minor aquifer of the peninsula had already been made (Fink, [24]). An unsaturated zone up to 50 m thick exists, and land use varies from agricultural to undeveloped areas of state forest. In addition, a principal meteorological station was located within the research area at Akrotiri, which could be used as a control together with a local rainfall station on site. The mean annual rainfall at Akrotiri (1965-76) was 420 mm and these records could be matched over a 60 year period with those at Limassol, 5 km to the north.

Experimental Methods

The problem of obtaining uncontaminated samples for chemical and isotopic investigation to a depth of 30 m presents severe practical dif­ ficulties, not least in semi-arid or arid zones where low soil moisture contents are found. Drilling during the current project was carried out using a Pilcon Wayfarer 1500 portable rig. Various attempts at dry drilling by percussion and air flush rotary were made at the start of the project, including U4 coring, but without success. However,a simple per­ cussion technique was eventually developed which will be described in full elsewhere .. (Edmunds et al., Г 25j ). This method avoided contamina­ tion and overheating in unconsolidated material did not occur; a continuous sampling record every 20 cm was obtained at a rate of 4-7 m/working day. The method was found to be simple, cheap and effective and probably widely applicable in semi-arid and arid terrains.

It was initially hoped that direct extraction of soil moisture by centrifuge (Edmunds and Bath, [2б ]) might be possible, but this was totally impracticable at the low (4 ml/100 g) water contents found. Immiscible fluid displacement (Patterson et al., [27]) was also tried, but was not successful for the grain size/moisture content of the Akrotiri formation. An élutriation technique was subsequently adopted, whereby 30 ml distilled/ deionised water was added to each 50 g sand sample, which was stirred occasionally before removing the supernatant solution after 1 hour. This was then filtered through 0.45 цт filters and the specific electrical con­ ductance measured in the field. Approx. 15 ml of clear elutriate was obtained from each 20 cm profile sample. This was sealed in polycarbonate tubes and transported to the OK for microchemical analysis. Soil moisture was determined within 1 hour of sampling and 5 kg double-wrapped sand samples were airfreighted to UK for analysis by AERE Harwell.

This method has obvious limitations with regard to a full geochemical interpretation of the solute, notably the carbonate system, but probably represents the only method of obtaining solute data in semi-arid or arid zones. However, in the present application, it is arguably the most effective technique since it is unsophisticated and requires only the mini­ mum of field equipment. Furthermore, an on-the-spot assessment of the IAEA-AG-158/4 5 9 SPECIFIC ELECTRICAL CONDUCTANCE Cl (mg/100 g) log Cl (mg/ litre) uScm-1 at 25‘C-ELUTRIATE SOIL MOISTURE

N03 (mg/100g) SOIL MOISTURE ELUTRIATE (mg/100g)

FIG .5. Solute, soil m oisture and partial tritium profiles for borehole AK2, A krotiri, Cyprus. Specific electrical conductance peaks are num bered for reference purposes; corresponding peaks in Cl and N O 3 are also indicated. The m ean value for Cl used in recharge calculations is indicated in the log Cl (soil m oisture) profile. 6 0 EDMUNDS and WALTON salinity of the profile can be made from the conductance logs, which gives an immediate feed-back for judging the success of the drilling and in planning the subsequent programme.

PRELIMINARY RESULTS

Partial, preliminary and only semi-quantitative results from the first two of three field seasons (1977-8) are presented here as a means of illus­ trating the type of solute profiles obtained and of deriving an initial comparison with tritium profiles. So far eight boreholes have been drilled, but complete results are only available for the first two. The first drilled borehole, AK2 will be taken as a model for the study. This was drilled on horizontal ground in sandy soil which was at the boundary of the agricultural land of the village; variable calcrete (Kafkalla) was found everywhere below the surface. The profiles for soil moisture, tritium, specific electrical conductance corrected to 25 С (SEC), chloride and nitrate are given in Figure 5 from which it can be seen that:

1. Distinct peaks are evident for SEC, Cl and NO 3, which indicate that homogenisation by dispersion has not taken place. Most of the peaks correlate between all three parameters and there is an apparent periodicity of rather less than 1 m.

2. SEC and Cl oscillate about mean values, implying a steady state per­ colation through the profile without any uptake from the lithology. Nitrate, on the other hand, increases down the profile and this is considered to represent a former, different land use.

3. Cl profiles, plotted as weight/unit volume and as equivalent pore water concentration (corrected using soil moisture), both retain peaks. This demonstrates that the variations in the elutriate сощ- positions are not a function of variations in the soil moisture and imply that the water and solutes are moving downwards at approximately the same rate.

4. A fairly well defined tritium peak occurs around 10 m and there is apparently little dispersion or leakage to the deeper profile, although tritium mass balance calculations have yet to be made.

The equivalent profiles for the second borehole drilled, AK3, (Figure 6) represent a complete section of the unsaturated zone. This borehole was drilled within 5 metres of AK2 on horizontal ground where the only essential difference was the extent of calcrete development at 0.80-2.00 m below the surface. The differences or similarities with AK2 may be summarised as f o l l o w s :

1. Solute peaks are not well developed in the upper 16 m and it is con­ sidered that a degree of homogenisation has occurred. An exception would be at the SEC peak between 11 m and 15 m which broadly reflects the form of the AK2 profile.

2. The Cl profile can be ascribed a mean value of log Cl = 2.08 (120 mg/l) over the interval 3.0-16. 5m. At this point a mixed sand and gravel horizon was penetrated and the increase in Cl illustrated on the plot of soil moisture composition reflects a lithological input which affects the profile below this point. The profile above 3.0 m,which is still subject to soil moisture flux, is ignored. DEPTH 6. and tritium profiles n r e h t u o s , i r i t o r k A , 3 K A e l o h e r o b r o f s e l i f o r p m u i t i r t d n a e r u t s i o m l i o s , e t u l o S . 6 . G I F The mean value n recharge calculations i n the log Cl (soil i o s ( l C g o l e h t in d e t a c i d n i is s n o i t a l u c l a c e g r a h c e r in d e s u l L C W S r o f . e e l u i l f a o v r p n a ) e e r m u t e s h i o T m . s u r p y C SPECIFIC ELECTRICAL CONDUCTANCE (mg/100g) Cl 0 30 0 500 400 300 200 4iScm'1 at25'C-ELUTRIATE = . l e v e l r e t a w c i t a t s IAEA-AG-158/4 2 6 4 2 0 . 1.0 0.5 S O I LM O I S T U R E (mg 100g)

logCl (mg/ litre) 2 5 7 100 75 50 25 0 OL MOISTURESOIL TRITIUM (TU)

61 6 2 EDMUNDS and WALTON

TABLE 2. VALUES OF MEAN ANNUAL RAINFALL, RAINFALL CHLORIDE AND SOIL MOISTURE COMPOSITION USED TO CALCULATE RECHARGE AT AKROTIRI SITE.

Mean annual rainfall Akrotiri Met. Station (1963-1976) = 420 mm Annual rainfall Akrotiri Met. Station (1977-8) = 456.2 Annual rainfall Akrotiri Village site (1977-8) = 470.8

Mean rainfall chloride - Akrotiri Met. Station 1977-8 = 12.3 mg/1 Cl_ Cp Mean rainfall chloride - Akrotiri Village site 1977-8 16.3 mg/1 Cl Average value used in calculations = 14.3 mg/1 Cl

Mean soil moisture composition AK2 = 120 mg/1 Mean soil moisture composition AK3 = 120 mg/1

Recharge at AK2 = 50 mm Rd Recharge at AK3 = 50 mm

3. The tritium profile contains a peak corresponding to the 1963/4 atmospheric maximum, and the very low values in the lower half of the profile and 0.0 TU at the water' table, suggest that by-pass flow has not been significant.

This rather striking difference between two adjacent profiles was un- . expected but may possibly be a good indication of the problems to be expected in semi-arid terrains, the explanation is thought to lie either in the variability of calcrete development, or in local flux due to root trans­ piration. Nevertheless,it is considered that both Cl profiles can be used to derive recharge estimates assuming steady state conditions and that the AK2 profile might also be used to derive recharge estimates using the solute peaks.

(i) C h l o r i d e M a s s B a l a n c e

The volume of the direct recharge component (R^) can be expressed as f o l l o w s :

R d = P - Ë - S where P = mean annual rainfall (mm), E = mean évapotranspiration and S = mean surface runoff. If only those sites where surface runoff is negligible are selected for profile drilling, then S =0. Since évapotranspiration represents the greatest uncertainty in the calculation, this term can be replaced by a term for the solute mass balance if it can be demonstrated (or assumed) that the total atmospheric solute loading for any conservative element for a given site is eventually transmitted through the soil in a steady state process. Then: IAEA-AG-158/4 6 3 where Cp is the solute .concentration in the rainfall (P) and Cs is the soil - moisture composition - in this case that of Cl-. Thus it is possible to solve this equation by simply knowing these three parameters.

The mean annual rainfall at the Akrotiri site is known for the 14 year period 1963-76 (Table 2), which corresponds roughly to the time scale of the profile. The rainfall chloride levels measured at the two stations for the first year differ by 4 mg/l; for preliminary calculation, the average value has been used. The soil moisture chloride concentrations (Cs) have been derived using a mean value for soil moisture for both profiles of 4.8 mg/100 g, with mean chloride levels per 100 g derived from Figures 5 and 6 , and a formation bulk density value of 1.50. For both profiles, identical recharge values of 50 mm/annum are thus calculated, despite the apparent difference in the form of the profiles.

(ii) Solute Peaks

The AK2 profile contains several resolved peaks with an apparent periodicity of rather less than 1 m for SEC, Cl and NO 3 . In order to check whether some or all of these correspond to annual inputs to the profile, tritium was used to calibrate the likely recharge rate. Using the AK3 profile a definite maximum of 99 TU occurs at ca. 9.5 m which corresponds with the AK2 maximum of 105 TU at a comparable depth. Identifying this as the 1963/4 recharge peak then permits a downward percolation velocity of 0.66 m/yr to be determined. An independent recharge estimate of 48 mm/yr using tritium can then be obtained using a soil moisture value of 4.8 mg/100 g and a bulk density of 1.50,assuming no by-pass flow. This is remarkably close to the chloride mass balance result given above; however at this stage, it has not been possible to fully interpret the tritium profile without knowing the up-to-date rainfall tritium inputs for stations in Cyprus.

Using this recharge rate the mean annual rainfall data for Akrotiri has been plotted at an equivalent scale against the SEC and Cl profiles of AK2 (Figure 7). Meteorological data for Limassol have been used to provide an extension of the calibration before 1965. It is found that an inverse correlation exists between rainfall amount and total solute con­ centration as expressed by SEC. The two dry periods of 197 2-4 and 1955-60 both correlate with a broader zone of higher SEC and certain individual peaks, e.g. 5 and 8 could liberally be interpreted to coincide with the dry years of 1967 and 1963 respectively. The chloride profile does not reflect the periodic climatic characteristics in the same way as SEC. The reason for this is not entirely clear without additional chemical results for the other major ions. Only 8 or 9 recognisable peaks are found in the 1964-1977 interval, and an exact annual correlation does not seem possible without more detailed analysis of the data. However, it is likely that some peaks would be less distinct in years of similar annual rainfall and con­ versely, that the most significant peaks would appear in a dry year(s) followed by a wet year (e.g. 1960/1). In addition, embryo peaks within the top few metres will not be fully developed until they have passed below the zone of evaporative flux; thus at least three peaks can probably be accounted for within this upper zone. On balance,therefore,it would seem unlikely that the full quota of peaks would ever be developed within any p r o f i l e .

In the case of the AK2 profile therefore it is concluded that an in­ verse correlation between mean annual rainfall and the SEC profile can be made and that this can be used to calibrate the profile record over at 6 4 EDMUNDS and WALTON

ANNUAL RAINFALL mm

0 200 400 600

FIG .7. Annual rainfall for A krotiri area for period 1938-1976 with SEC (specific electrical conductance) and Cl profiles (AK2) drawn at a vertical (tim e) scale, derived from the position o f the tritium peak in Figs 5 and 6. Concentration scales are given in Fig. 5.

least a 25 year period. At this stage it is pointed out that the correla­ tion is not independent of tritium and further results would be required to see whether the solute profiles alone, calibrated approximately using the Cl mass balance results, reflect the preceding rainfall record. However, the SEC and tritium peaks taken together strengthen the conclusion that the downward percolation is 0.66 m/yr,representing a mean recharge of 4 8 m m / y r .

The foregoing results provide encouraging evidence that solute profiles may be as useful, if not more useful, in recharge estimation than tritium pro­ files alone, and that both techniques used together could provide the most effective method for determining direct recharge. In comparison with con­ ventional methods of recharge estimation, the solute profile approach is totally independent of soil physics theory. One of its main advantages IAEA-AG-158/4 6 5 is the ability to investigate the variability of recharge over a wide geographic area; profiles of only 10-15 m are required for interpretation, representing little more than 3 days drilling per site, and a preliminary interpretation of the results can be made in the field.

There are limitations to the use of solute profiles, as indeed there are for other methods of recharge estimation. The method is limited by the depth of the unsaturated zone and is favoured in unconsolidated lithologies; rotary air flush drilling, once successfully developed,might offer a satisfactory if logistically more difficult and expensive method for con­ solidated lithologies. The greatest source of error is likely to be in the rainfall chloride results and ideally, several years consecutive chemical analysis should be carefully obtained from stations within the area of recharge investigation.

Further studies are being carried out in Cyprus and elsewhere on the application of the solute profiling technique and it is intended to publish a full account of the work at the end of the three year study. It is planned to compare data from some 15 boreholes at the same site during the project for which the relative behaviour of other solutes is also being studied. The results given in this paper therefore should be regarded as provisional and must await a fuller comparison with other recharge estimates, particu­ larly frcm a lysimeter which is being constructed on the same site at A k r o t i r i .

CONCLUDING DISCUSSION

The results from Cyprus demonstrate that current, direct recharge estimation in semi-arid zone terrains, may be possible using the infor­ mation stored in the solute profiles of the unsaturated zone. It is argued that the use of solutes such as chloride or the specific electrical conductance of the soil moisture, offer a more straight-forward means of recharge assessment than the study of the soil moisture itself,since the mass transport of the solutes is less complex than that of the soil moisture. The solute profile method is also, in principle, well suited for application to remote areas due to its relative lack of sophistication. In addition, the method is probably applicable to wide geographical areas of the semi-arid and arid regions of the world, where thick unsaturated zones within arenaceous lithologies are fairly widespread.

Further field trials are needed to demonstrate the wider applicability of the method however, and, at this stage, several possible limitations should be mentioned. It is considered that reproducibility of the solute profiles is likely to be the exception rather than the rule; variation in the solute distribution may be due to the following!(a) land use (b) the influence of phreatophytic vegetation (c) the extent and nature of calcretes or similar accretionary deposits s (d) geological factors including sedi­ mentary features, pipes and fissures. It may be difficult to find sites where it is known that land use has remained constant for 10-50 years, although it is considered likely that a change in land use during this time may be identifiable by use of nitrate, as may be the explanation in the case of the AK2 profile from Cyprus (Figure 5). Further studies are being carried out in Cyprus to examine the effects of deep rooted vegetation on the method,since clearly, in any recharge study, the relative importance of vegetation types must be taken into account. The effects of calcrete appear to have a profound influence on percolation and it is considered that calcrete development may be one of the most significant factors 6 6 EDMUNDS and WALTON affecting both the development of the solute p ro files, and the amount of direct recharge. This conclusion has also been reached by Foster e t a l . [28] • in recharge estimation studies in Botswana, during which, solute profiles were also determined; chloride profiles, taken together with other evidence indicate very low recharge rates through the Kalahari Beds.

Results from the Cyprus profiles show that there is the possibility of a method to determine not only the recharge amount, but also the recharge history. It has been noted in the case of Libya, that it is possible to distinguish isotopically and geochemically between major recharge events with a time difference of the order of 2000-10000 years. There is , there­ fore, a likelihood that, by careful examination of the solute, as well as the isotopic record of the unsaturated zone and the immediate water-table, that a chronology of recharge events throughout the historical period could be established. During periods of climatic oscillation in semi-arid zones, there is likely to be a change from steady-state percolation, to a period of solute accumulation when recharge has ceased. However, on a longer time scale, of several decades or several hundred years for example, the accumulated solutes will eventually be flushed towards the w ater-table. In the thick ('vlOOm) unsaturated zones that exist in many parts of the semi-arid regions of the world, therefore, there may be a record of medium term climatic changes, and corresponding changes in recharge history, that can be investigated by geochemical profile studies, by methods analogous to those developed in Cyprus. From both the preceding examples - Cyprus and Libya - two general re­ quirements for further research emerge. F irs tly , i t is considered that greater attention must be paid to the geochemical processes taking place within the unsaturated zone. This is not only because of the immediate need for understanding the development of solute p rofiles for recharge estimation, but also because the accumulation of calcretes and similar deposits directly affect recharge to aquifers; conversely, the dissolution of accreted carbonates and soluble salts affects, amongst other problems, the interpretation of radiocarbon ages. Furthermore, it is likely that the investigation of stable isotope profiles of carbon, as well as hydro­ gen and oxygen, will also increasingly assist the understanding of the record contained in the water and solutes of the unsaturated zone.

Secondly, i t is considered worthwhile to stress the almost complete lack of rainfall chemical data currently available for most parts of the world, and for semi-arid and arid zones in particular. If recharge studies using solute mass balance calculations are to continue, then it is impprtant that chloride at least of the input constituents, be measured on a global basis, in addition to tritium and stable isotopes, possibly by the IAEA or a similar agency - using the established network, which could then be augmented by detailed rainfall studies in the area of interest to recharge assessment.

ACKNOWLEDGEMENTS

The work in Libya was carried out during a hydrogeological study of the Sirte Basin and the cooperation of staff of the Kufra-Sarir Authority is gratefully acknowledged. The radiocarbon analyses were performed by Dr D Harkness of the Scottish Research Reactor Centre, East Kilbride.

The results from Cyprus have been obtained with the cooperation of the Water Development Department; Mr J Jacovides and the sta ff of the IAEA-AG-158/4 6 7

Limassol office are thanked for their assistance. The boreholes were drilled by Mr M Howard assisted by Mr G Hadjifilactou. Analyses for major ions were carried out in the UK by Mr D L Miles, Ms J M Cook and Ms J Trafford. Tritium determinations were made by AERE Harwell. John Barker, IGS is thanked for his help in the computer production of the solute p ro files. Our colleagues in the Hydrogeology Department, In stitu te of Geological Sciences, in particu lar Drs A H Bath, R Herbert, R Kitching and E P Wright have helped considerably in the discussion of the results. The Cyprus studies fora part of the Semi Arid Zone Recharge project which is being funded by the Ministry of Overseas Development, London. This paper is published by permission of the Director, Institute of Geological Sciences, London.

REFERENCES

[lj PENMAN, H.L., Proc. Roy. Soc. A 193 (1948) 120

[2] PENMAN, H .L., Neth. J . Agrie. S c i .,£ (1956) 9

[3] MONTE I TH, J . L . , Quart. J . Roy. Met. Soc., 87_ (1961) 171

[4] KITCHING, R., et al., J. Hydrol., 33 (1977) 217

[5] ZIMMERMAN, U., et a l ., Isotopes in Hydrology (Proc. Symp. Vienna 1966) IAEA, Vienna (1967) 584

[6] MUNNICH, K.O., et a l .. Isotopes in Hydrology (Proc. Symp. Vienna 1966) IAEA, Vienna (1967) 305

[7] SMITH, D.B., et a l ., Isotope Hydrology (Proc. Symp. Vienna 1970) IAEA, Vienna (1970) 73

[8] ANDERSEN, L .J . , SEVEL, T ., Isotope Techniques in Groundwater Hydrology (Proc. Symp. Vienna 1974) IAEA, Vienna, 1 (1974) 3

[9] SUKHIJA, B.S., SHAH, C.R,, J. Hydrol., 30 (1975) 167

[10] ALLISON, G.B., HUGHES, M.W., Isotope Techniques in Groundwater Hydrology (Proc. Symp. Vienna 1974) IAEA, Vienna, 1 (1974) 57

[11] DINCER, T., et al., J. Hydrol., 23 (1974) 79

[1 2 ] WRIGHT, E .P ., et a l ., Jalu-Tazerbo P roject: Phase I. Final Report to Kufra Sarir Authority, Libya. (1974) unpublished.

[13 ] EDMUNDS, W.M., WRIGHT, E .P ., J . Hydrol., 40 (1979) 215

[14 ] BENFIELD, A.C., WRIGHT, E .P ., Proc. Second Symposium on Geology of Libya, Tripoli (1978) in press

[15 ] REARDON, E .J ., FRITZ, P ., J . Hydrol., 36_ (1978) 201

[16] DEINES, P ., et a l ., Geochim. Cosmochim. Acta, 3£ (1974) 1147

[1 7 ] SONNTAG, C ., et a l ., Geol. Rundsch-, 6^7 (1978) 413 EDMUNDS and WALTON

SONNTAG, C ., e t a l ., Isotope Hydrology 1978 (Proc. Symp. Neurerberg) IAEA, Vienna, 2(1979) 569

MALEY, J . , Nature, 269 (1977) 573

ROGNON, P ., WILLIAMS, M.A.J., Palaeogeogr., Palaeoclimatol., Palaeoecol., 2l_ (1977) 285

PACHUR, H-J., Erde, 106 (1975) 21

ERIKSSON, E ., Interpretation of Environmental Isotope and Hydro- chemical Data in Groundwater Hydrology, IAEA, Vienna (1976) 171

ALLISON, G.В ., HUGHES, M.W., Aust. J . Soil Res., 16 (1978) 181

FINK, M., Preliminary Report on the Hydrogeology of the Akrotiri Peninsula. Tahal Ltd. Isreal (1965) unpublished.

EDMUNDS, W.M., et a l ., in preparation

EDMUNDS, W.M., BATH, A.H., Environ. Sei. Techol., 10 (1976) 467

PATTERSON, R.J., et al., Canad. J. Earth Sei., (197 )

FOSTER, S.S.D., in preparation IAEA-AG-158/5

AN EXAMINATION OF RECHARGE MOUND DECAY AND FOSSIL GRADIENTS IN ARID REGIONAL SEDIMENTARY BASINS

J.W. LLOYD Hydrogeological Section, Department of Geological Sciences, University of Birmingham, United Kingdom

Abstract

AN EXAMINATION OF RECHARGE MOUND DECAY AND FOSSIL GRADIENTS IN ARID REGIONAL SEDIMENTARY BASINS. In many of the vast arid sedimentary basins of the world, groundwater gradients exist that appear to be anomalous in the context of the probable modem recharge potential. The possibility that such gradients are in fact remnant fossil conditions representing the decay of ancient recharge mounds is examined. An example of decay condition is represented using a resistor-network analogue model in which the time control is based on 14C ages. The decay hypothesis is found to be plausible w ith realistic aquifer characteristics but a non-homogeneous flo w is indicated from the 14C data.

INTRODUCTION

In spite of the relatively isolated nature of many of the groundwater studies carried out in the vast sedimentary basins of North Africa and Arabia a general interpretation is emerging of groundwater equi-potential surfaces. These surfaces appear normally to slope away from the aquifer outcrop areas in the direction of the predominant geological dip. As such they represent classical groundwater flow patterns consistent with those found in temperate and other areas. A major difference exists, however, in that it is difficult in some cases to account for the gradients and the apparently large volumes of flow moving through the systems on the basis of the present-day recharge.

HYDROGEOLOGICAL CONDITIONS

The average annual rainfall over aquifer outcrop areas in the basins is low (usually < 50 mm), and frequently no rainfall occurs for long periods. Although limited comprehensive precipitation and recharge control is available, it is obvious

6 9 7 0 LLOYD that only minor direct recharge can occur. In certain areas, such as southern Jordan and parts of Saudi Arabia.it is probable that important indirect recharge occurs to the basin system through run-off, from for example the Precambrian Basement mountains adjacent to the sedimentary outcrop areas. Undoubtedly due to the arid nature of the climate, major storm and run-off events produce intermittent indirect recharge in all the basins. Unfortunately such instances are not well documented, but they must be few in number and the resultant recharge can only equate to a small proportion of the throughput volume in many areas. Environmental isotope data support the presence of some modern recharge at outcrop. Away from outcrop, however, radiocarbon data throughout most of Arabia and North Africa indicate that the bulk of the groundwater is old, up to and exceeding 35 000 years (the reasonable limit of radiocarbon dating). Even in the outcrop areas local gradient control solely by modern recharge may be questionable as, for example, in Jordan where the well hydrographs in the confined areas close to outcrop do not always show evidence of seasonal water-level fluctuation as would be expected with such recharge (Lloyd, 1970) [l],1 a similar lack of fluctuations is reported from central Saudi Arabia (Clarke, 1977) [2]. Burdon (1977) [3] in an excellent discussion has postulated a variety of mechanisms other than modem recharge to account for the groundwater gradients found in these arid basins. Some of the suggestions, though possible, are unlikely to be significant. However, the hypothesis that the existing gradients can be attributed to the creation of recharge mounds in the pluvial Pleistocene periods and subsequent long-term head decay under distant gradual discharge appears plausible and merits further consideration.

INDICATIVE MODEL

Unfortunately, to study the hypothesis only broad physical dimensions can be considered at present owing to limited regional data. The hypothesis, however, can be tested by flow model techniques using reasonable parameter assumptions as detailed by Burdon in combination with control data from a particular basin. For this purpose, in the present study Burdon’s data have been combined'with data from the Western Desert of Egypt and certain groundwater control based on the Bahiriya Oasis (Farag, 1978) [4] to provide a representative basin. To model the basin a resistance network analogue has been used representing a section of the Lower Bahariya Formation aquifer extending north-eastwards from the Sudan, Libya and Egyptian border intersection through Bahariya Oasis to the Mediterranean. The aquifer is a Cenomanian sandstone with an average thickness of 500 m. The data for the existing groundwater gradient is based on work by Farag (Fig. 1) and it is assumed that leakage occurs from the system north IAEA-AG-158/5 71

FIG.l. Regional groundwater piezometric surface for the lower Bahariya Formation aquifer. 2 7

Piezometric Head (m] E 150 E £ О S É"O

200 100

FIG.2. f I J f ♦ Bto f f r ife u q a of Bottom ♦ r ife u q Ja I r ife u q a me- el i p at i sis a O a riy a h a B t a ip h s n tio la re d a e -h e im T ---- Details o f gradient decay in an unconfined area. unconfined inan decay f gradient o Details Top I Bottom ~ .. .. ~ Bottom I Top 2 0 ¿0 60 80 100 100 80 60 ¿0 0 2 1—* dts --p f f r ife u q a 0f -j-0p * dates C U • ♦ • AB A В ----- •—r ie 10 y s ) rs a ye (1000 Time ------i 100 er ) years 11000 e Tim LLOYD 1 ------= dy =m h m/day = К 1------1 ------.. . IAEA-AG-158/5 7 3

900-

800 -

- 700 - S £ э

Q 6 0 0 - Assumed piezometric level 10.000 years before present * 500 - x» a> Z a» £3О Д00Ч a *o o a» .c 0 3 0 0 -

01 Piezometric levels E o after Oiab and Farag a

Piezometric level 100- obtained from model using aquifer characteristics as shown on Figure 3 * \

100 200 300 400 '500 600 700 800 900 Distance (km)

FIG.3. Simulated piezometric surface through a southwest to northeast section o f the Western Desert o f Egypt.

of the Mediterranean coast. It will be noted that the head in the aquifer, although it is confined, is at Mediterranean datum at the coast. Pumping test data from the Bahariya Oasis give a permeability range of 6 to 0.4 m/d while Burdon uses a permeability of ! .7 and 0.8 m/d for his conceptual basin. In the present study these values have been taken as a guide. In the absence of reliable measured specific yield data reasonable estimates have been made. The control of the Bahariya Oasis is important to the model since, apart from head control, hydrochemical data distinguish a calcium-sodium chloride water at depth. These data suggest that the groundwater at depth has traversed 7 4 LLOYD longer flow paths in the aquifer than the shallower water. This supposition is supported by radiocarbon data indicating an age of 25 000 years for the upper water and approximately 35 000 or a little older for the deep water. The model has been constructed assuming steady-state Laplace conditions with the time variance of the groundwater flow being approximated to a series of steady-state situations (Herbert and Rushton, 1966) [5]. For initial conditions in the model a head of 9 00 m has been assumed in the outcrop area with 0 m at the discharge point. To simulate head decay in the recharge mound the release of groundwater from storage with decreasing head has been determined by the following equation:

Ah, sz = (A t/S y)K — * (1) Az

where sz is the decline in head in the vertical (z) direction At is the time interval Sy is the specific yield for the outcrop area К is the permeability AhL is the head difference between two vertical nodes Az is the vertical distance between two nodes

To represent realistic conditions an analogue was constructed using a decreasing permeability distribution with depth, as this parameter is considered to be the most likely control of the differential flow considered to be present at Bahariya. On the basis of past climatic records (McBurney, 1967) [6] it was assumed that maximum head (9 00 m) was operative in the outcrop until 10 000 years ago. From 10 000 years to the present time, head decay was assumed to operate (Eq. ( 1 )) and was measured over a series of decline steps. The rate of decline in the outcrop area is shown in Fig. 2 for various permeability and specific yield values. For control the flow velocities from the model were calculated at Bahariya and converted to ages. It will be seen that the В combina­ tion provides a reasonable correlation with the Bahariya control data and that the aquifer characteristics are of a good acceptable order. Further, as can be seen in Fig. 3 the simulated decayed gradient using the В parameters shows a good approximation to that believed to extend through the Egyptian Western Desert today.

CONCLUSIONS

It is considered that the study supports the view that present-day gradients in the regional arid basins of Saudi Arabia and North Africa can in part be attributed to head decay of recharge mounds created in the Pleistocene. IAEA-AG-158/5 7 5

The model study presented is not claimed to be definitive for any particular basin as similar results may be possible from a variety of parameter permutations. However, the simulation obtained using reasonable basin dimensions, aquifer parameters and climatic assumptions, clearly shows that a gradient can persist for a long period and that fossil gradients and their implications should be sensibly considered in proposals for long-term groundwater developments in the basins under discussion.

REFERENCES

[1] LLOYD, J.W., Hydrogeological procedures in consolidated sediments of underdeveloped arid areas, Ph. D. Thesis, University of Bristol, England (1970). [2] CLARKE, L., (In discussion of BURDON, D.J., 1977), Flow of fossil groundwater, QJ. Eng. Geol. 10(1977) 97. [3] BURDON, D.J., ibid. [4] FARAG, M.H., The geology and hydrogeology of the Bahariya Oasis Western Desert, Egypt, Ph. D. Thesis, University of Birmingham, England (1978). [5] HERBERT, R., RUSHTON, K.R., Groundwater flow studies by resistance networks, Geotechnique 16(1966) 53. [6] McBURNEY, C.B.M., The Haua Fteah, Cyrenaice and the Stone Age of the South-East Mediterranean, Cambridge University Press (1967) 387pp.

IAEA-AG-158/6

ENVIRONMENTAL ISOTOPES IN NORTH AFRICAN GROUNDWATERS; AND THE DAHNA SAND-DUNE STUDY, SAUDI ARABIA

C. SONNTAG, G. THOMA, K.O. MÜNNICH Institut für Umweltphysik der Universität Heidelberg, Heidelberg, Federal Republic of Germany

T. DINÇER Food and Agriculture Organization of the United Nations, Rome

E. KLITZSCH Technical University, D-1000 Berlin

Abstract

ENVIRONMENTAL ISOTOPES IN NORTH AFRICAN GROUNDWATERS; AND THE DAHNA SAND-DUNE STUDY, SAUDI ARABIA. I. North Saharian palaeowaters were mainly formed during a long humid period between 50 000 and 20 000 years BP., which was followed by a cool dry period from 20 000 to 14 000 years BP. These palaeowaters show a significant west-east decrease in deuterium and 180 because of past groundwater formation by local rainfall from the western drift. Sahel zone groundwaters seem to show meridional variation of deuterium and lsO due to a tropical convective influence. II. A computer model estimate of the alternate play between rainwater infiltration and evaporation in the Dahna sand-dune (near Riyadh, Saudi Arabia) yields a mean annual ground­ water recharge of 20 mm annually which agrees with that obtained from bomb tritium vertical profiles of the sand moisture. The model also describes the deuterium and 180 profiles.

I. ISOTOPE DATING OF SAHARIAN GROUNDWATERS

The west-east decrease of the heavy stable isotopes (D and 180 ) in the groundwaters from the central and eastern Sahara suggests that these groundwaters were mainly formed by local rainfall during humid phases of the past [1 ,2 ]. Carbon-14 data give no indication of long-range movement, with increasing 14C age in the direction of flow as suggested by Ambroggi [3]. This means that the inner Saharian groundwater storage is not recharged under present climatic

7 7 7 8 SONNTAG et al. conditions. There may exist groundwater recharge in some places on a regional scale, particularly near mountainous terrain with sufficient rainfall. An extension of our isotope dating to the western and the southern Sahara, including the Sahel Zone, is under way. Ulf Thorweihe (geologist, Berlin) and Jochen Rudolph (physicist, Heidelberg) are now on their way from Algier to Lake Tchad, collecting groundwater samples for isotope and noble gas analyses. It is hoped that the noble gas data will provide information on the ground-level temperature at the time and place of groundwater formation [4]. Up to now we have no available data. Our data collection now contains environmental isotope data, hydrological and hydrochemical data, of several hundred groundwaters from northern Africa. The data are of various origins and cover Algeria, Tunisia, Libya, Egypt, Sudan, Chad, Nigeria, and Senegal. The coverage of the western Sahara, and the region south of the Sahara, however, has been relatively poor up to now. A tendency of meridional variation of D and 180 is to be seen in the southern area groundwaters indicating a tropical convective origin. Part of the deep groundwater ( 14C age > 20 000 years) [5] in the Bara Basin (Kordofan/Sudan) is considerably lower in D and 180 than modern groundwater at the same place. This may be due to a different water vapour origin in the past. The groundwater in northern Senegal, an area which is surrounded by the rivers Senegal and Gambia, shows uniform isotopic composition in both aquifer systems, the Maestrichtian Sands, and the Continental Terminal [6, 7]. The D and 180 appear to be low (5D s - 4 0 %o, 50-18 = - 6.2%c) if compared with local rainfall; presumably it is derived from the rivers mentioned. The 14C and tritium data indeed show groundwater flow towards the centre of the basin indicating recharge by river-water infiltration. This conclusion holds for the deep groundwater showing high 14C ages as well as for the modern groundwater containing tritium. Hydrochemical data support this by an increasing salt content towards the basin centre as well as by a typical development, from a bicarbonate type at the periphery to sulphate and, eventually, chloride type in the centre.

II. ENVIRONMENTAL ISOTOPES IN THE DAHNA SAND-DUNE

At the beginning of 1978 T. Dinçer resumed his 1972/73 isotope study at the Dahna sand-dune, Saudi Arabia [8]. The vertical profiles of soil moisture, tritium, D and 180 as observed in 1972 and in 1978, are presented in Fig. 1. The D and 180 profiles are fairly similar. This is more apparent in the 5D vs. 5180 diagram (Fig. 2) where all data points fall'on nearly the same evaporation Une. The low slope is due to kinetic fractionation by molecular diffusion of the water vapour through the stagnant air in the sand. The evaporation line cuts the meteoric water line at a point which approximately represents local groundwater. We intend to check this concept experimentally with sand samples to be taken IAEA-AG-158/6 7 9

FIG.l. Vertical distribution o f soil moisture and environmental isotopes in the Dahna sand- dune, Saudi Arabia. The 1972 data are taken from Dinçer et al. [8].

6 D [•/..]

FIG.2. SD versus 180 diagram o f the Dahna sand-dune moisture. The 1972 data come from Ref. [8]. Lower part shows the laboratory column sand evaporation experiment. 8 0 SONNTAG et al.

[ w e ig h t °U]

0 2 4 6 0 2 í, 6

MARCH '70

FIG.3. Model estimate of the moisture development in the Dahna sand-dune, starting with a uniform moisture distribution below field capacity.

on our current North Africa expedition mentioned earlier. The data points on the other evaporation line in the lower part of Fig. 2 come from an evaporation experiment with a sand column in the laboratory. This line, naturally, cuts the meteoric water line at the point representing Heidelberg groundwater since the column had originally been loaded with tap water. The 1972 tritium profile shows (Fig. 1) a broad maximum in 4 to 6 m depth, and the 1978 profile is at constant tritium level all the way down to 6 m. With Dinçer’s previous [8] recharge estimate (23 mm/a) and a 5 vol.% field capacity, the 1972 tritium peak is expected to have moved down by about two metres. The infiltration/evaporation model described in the following section yields about the same groundwater recharge rate as estimated by Dinçer [8]. This model further predicts that the sand should essentially be at field capacity below about one metre deep (Fig. 3). This, however, is not confirmed by observation. The moisture content of the 1978 profile is considerably lower throughout. This finding presents a problem.

EVAPORATION MODEL

Our model (Fig. 4) describes the alternate play of rain-water infiltration and evaporation from the sand with its effect on the moisture profile and its isotopic IAEA-AG-158/6 81

box 1 BOX 2 SPHERE 1

h 1 2 H 2 1. . N N N-1

VAPOR PRESSURE = ELECTR. POT. MOISTURE = ELECTR LOAD

r2 u2 R1 «3 rn

=02_ VAPOR /TRANSPORT RESISTANCE LIQUID

FIG.4. Desk computer evaporation model and electric analogue. Moisture transfer between boxes is by molecular diffusion of vapour, depending on saturation pressure difference, and by capillary flow. The vapour pressure decreases at very low moisture content (lower part of the Figure).

composition. We take the unsaturated zone as a series of N boxes with moisture content Qi, temperature Ti, and vapour pressure p¡. Starting with the highest box number the vapour pressure in each box is compared with that of the two adjacent boxes. If a pressure difference exists, vapour is transported to the lower pressure box by molecular diffusion through the soil air (with different diffusion constants for the different isotopic species of water) during the time interval selected, and the programme loop starts anew. The model includes liquid water flow by capillary suction. At high moisture content the saturation pressure depends exclusively on temperature; below the permanent wilting point, however, the pressure decreases with moisture content. This causes the boundary between dry and moist sand to cease being steplike, but rather to be of the error-function type. The moisture development of the Dahna sand-dune as given by the model for the period 1964 to 1972 is shown in Fig. 3. To illustrate the downward movement of moisture we started with an arbitrary uniform moisture content below field capacity (which is estimated to be about 6 wt.%). It is evident in this case that infiltration exceeds evaporation loss. Calculations further show that a single heavy annual rainfall each 2 0 years or each 100 years must exceed 35, 77, and 196 mm, respectively, to compensate for evaporation under these 8 2 SONNTAG et al.

DAHNA SAND DÜNE

О 200 [TU]

2 3- k

5-

6

7 D E P T H [m l ■♦■— DINCER ETAL... ■» 11972 9 7 2 ------*.- -.»— .-.*.1978

\ E V A P O R A T IO N M O D E L , R A IN : RIYADH 1972 ♦ ♦♦♦♦ 1978 TRITIUM: BAHRAIN

FIG.5. Observed bomb tritium profiles (1972, Ref. [8], and 1978) together with model estimates of the vertical tritium distribution in the Dahna sand-dune. Time period 1964-1972: rainfall from Riyadh, and tritium rain data from Bahrain (taken from Ref. [8 ]j. Time period 1972-1978: rainfall extrapolated from the previous data; tritium estimated from European rain data.

conditions. These lower limits then correspond to mean annual evaporation rates of 35, 3.8, and 2.0 mm, respectively. The tritium profiles of the Dahna dune as estimated by the model are presented in Fig. 5, together with the observed profiles of 1972 and 1978. Since the rain data after 1972 were not available to us when the calculation was done, we assumed constant annual rainfall as extrapolated from the 1964—1972 data.

CONCLUSION AND RECOMMENDATIONS

Isotope studies in dune-sand areas should be intensified. With our new suction technique [9] samples can be taken from depths down to 30 m. Such studies can yield information on infiltration, evaporation and ablation in sand sea areas. The technique is relatively simple and cheap. A specific point in groundwater balance is the question of evaporation loss through the unsaturated zone. As an example Fig. 6 shows an estimate of the drawdown of the ground­ water surface under a sand sea. It is assumed that at the end of the last humid phase the water table was close to the ground surface. During the following hyper-arid phase (no rainfall at all) the water table then decreases with the square root of time (age), the evaporation rate decreases correspondingly with the increasing thickness of the diffusion diaphragm. In this simplified model the IAEA-AG-158/6 8 3

depth groundwater surface z Q peter?|

FIG. 6. Time (age) dependence of drawdown of the groundwater surface owing to evaporation through a completely dry sand cover. A t T = 0 (end o f the last humid phase) the water-table is assumed to have been close to the ground surface.

unsaturated zone is assumed to be completely dry. After a 4000-year dry period, for instance, the drawdown is about 10 m, the remaining annual evaporation rate only about 1 mm. As in the case of the diffusion/advection experiment of Zimmermann [10] one must expect isotope fractionation effects in this case which, however, do not reach a steady state. A stepwise calculation is now under way.

REFERENCES

[1] SONNTAG, C., KLITZSCH, E., EL SHAZLY, E.M., KALINKE, Chr., MÜNNICH, K.O., “Paläoklimatische Information im Isotopengehalt C-14 datierter Saharawässer: Kontinentaleffekt inDundO-18”, Geol. Rdsch. 67 Heft 2 (1978) 413-24. [2] SONNTAG, C., et al., “Paleoclimatic information from deuterium in and oxygen-18 in carbon- 14-dated North Saharian groundwaters: Groundwater formation in the past”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978), IAEA, Vienna (1978) 569—81. [3] AMBROGGI, R.P., Water under the Sahara, Sei. Am. 214 (1966) 21-29. [4] MAZOR, E., “Paleotemperatures and other hydrological parameters deduced from noble gases in groundwaters; Jordan Rift Valley, Israel”, Geochim. Cosmochim. Acta 36 (1972) 1321-36. [5] MABROOK, B., ABDEL SHAFI, M.Sh., “Hydrological and environmental isotope studies of Bara Basin, central Sudan”, Symp. on Trace Elements in Drinking Water, Agriculture SONNTAG et al. and Human Life, Middle Eastern Radioisotope Centre and Goethe Institut, Cairo, Jan. 10-13, 1977. CASTANY, G., et al., “ Etude par les isotopes du milieu du régime des eaux souterraines dans les aquifers de grandes dimensions” , Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp.Vienna, 1974) 1, IAEA, Vienna (1974) 243. KLUBMANN, P., SONNTAG, C., (unpublished work). DINÇER, T., AL-MUGRIN, A., ZIMMERMANN, U., “ Study of the infiltration and recharge through the sand dunes in arid zones with special reference to the stable isotopes and thermonuclear tritium ” , J. Hydrol. 23 (1974) 79— 109. THOMA, G., et al., “ New technique of in-situ soil-moisture sampling for environmental isotope analysis applied at Pilat Sand Dune near Bordeaux: HETP modelling of bomb tritium propagation in the unsaturated and saturated zones” , Isotope Hydrology 1978 (Proc. Symp. Neuherberg, 1978) 2, IAEA, Vienna (1978) 753— 68. ZIMMERMANN, U., EHHALT, D., MÜNNICH, К.О., “ Soil-water movement and évapotranspiration: changes in the isotopic composition of the water” . Isotopes in Hydrology (Proc. Symp.Vienna, 1966), IAEA, Vienna (1967) 567. IAEA-AG-158/7

AN INJECTED GAMMA-TRACER METHOD FOR SOIL-MOISTURE MOVEMENT INVESTIGATIONS IN ARID ZONES

A.R. NAIR, S.V. NAVADA, S.M. RAO Isotope Division, Bhabha Atomic Research Centre, Bombay, India

Abstract

AN INJECTED GAMMA-TRACER METHOD FOR SOIL-MOISTURE MOVEMENT INVESTIGATIONS IN ARID ZONES. A method for the in-situ determination of soil-moisture transport rates using K 360Co(CN)6 is discussed. The tracer compares well with tritiated water in laboratory investigations and the results obtained in limited field studies are very encouraging. The method promises to be of specific interest in arid-zone investigations where the soil-moisture fluxes in liquid and vapour phases could cause com plications fo r tritiu m tracer data interpretation.

INTRODUCTION

In regions where the rainfall is low only a small fraction of the precipitation contributes to groundwater recharge, a major portion being lost by evaporation from the surface layers. Estimation of the direct recharge to the groundwater body is of special importance in such regions where often the only source of water supply is sub-surface. Tracer methods either involving the study of natural tritium profiles in the soil [1] or the injection of artificial tritium [2] are now well established and extensively applied by many investigators. The main disadvantage of the tritium method is that it is not amenable to in-situ and non-disturbing detection. The procedure involves collection of soil samples by augering, extraction of moisture by vacuum distillation in the laboratory and finally liquid scintillation counting for tritium activity. The advantage that the tritiated water is an “ideal” tracer for water transport could sometimes “paradoxically” be a disadvantage. For example, in low moisture areas such as arid zones, the tracer could get into vapour phase during the day and may condense at points above the injection point during the night and thus cause complications in tracer profile interpretation. The authors had this experience while using the tritium tracer in soils with 1% moisture content in an arid zone in Rajasthan.

85 NAIR et al.

COBALT-60 METHOD TRACER - Kj^CoiCNle COMPLEX OR 6°Co-EDTA COMPLEX • Labelling of water layer around a bore hole with tracer at desired depth • In-situ detection of tracer position with scintillation detector

FIG.l. Soil-moisture movement study with radiotracers.

2. Experimental set-up for soil-moisture tracing with K3Co(CN)6 and НТО. IAEA-AG-158/7 8 7

2000 6000 6000 8000 Ю000 12000 —*■ Counts/min o- Scintillation Counting £ Ю- INJECTIONi U réór *20- НТО, Kj[6ôCo(CN)^] WATER SPRAYED'71 mm I—I 30i CL

10000 20Ó00 — -»• Counts/min r 0- Scintillation Counting Ю- 12 days after injection 20* WATER 5PRAYED‘86mm 30-

40- Liquid Scintillation Counting 5 0 0 1000 ------Counts/min 500 0 Ю000 » Counts/min Ю Scintillation Counting 20 26 days after injection 30' WATER SPRAY ED : 1A6 m m 40-

50- Liquid Scintillation Counting 1500 — Counts/min loo’o 20'00 3000 4000 5000 C o u n ts /m in 20- Scintillation Counting

30- 56 days after injection 4 0 - WATER 5PRAVE0-. 157mm 50-

6 0 - Liquid Scintillation Counting 1000 -■ Counts / min

FIG.3. Comparison between К 3 [^CofCNjf,] and НТО.

Also Bath [3], while discussing the possibility of assessing modern recharge using natural tritium profiles, reported on the absence of peaks in the unsaturated zone in the Kalahari desert. Sonntag et al. also reported [4] on the disappearance of tritium peak which was observed in 1972 in the unsaturated zone of the Dahna sand-dune (Saudi Arabia).

GAMMA-TRACER METHOD

The gamma-tracer method (Fig. 1) being used by us both in laboratory models as well as in the field studies is as follows:

The selected radiotracer is injected around an access hole at several points on a circle at a suitable depth; DEPTH IN 8 8 FIG.4. Laboratory comparison o f K 3 [^’CoiCN)^] and НТО movement in clayey soil. clayey in movement НТО and 3[^’CoiCN)^] K f o comparison Laboratory FIG.4. h rcri alwdt ifs adfr arn-ielyraon te access the around layer ring-like a form and diffuse to allowed is tracer The logging the hole with a scintillation detector. scintillation a with hole the logging oe and hole; The position of the tracer peak is determined at regular intervals by simply by intervals regular at determined is peak tracer the of position The The same borehole or access hole can be used for soil moisture and bulk and moisture soil for used be can hole access or borehole same The Higher accuracy in positioning the tracer patch. tracer the positioning in accuracy Higher to disturbance no hence and samples soil extract to augering repeated No are: method tritium the over method tracer gamma the of advantages The No elaborate laboratory support is needed; is support laboratory elaborate No density measurement using a neutron/gamma gauge; and gauge; neutron/gamma a using measurement density conditions; soil NAIR et al. et NAIR

IAEA-AG-158/7 8 9 OEPTH IN

FIG.5. Comparison between 6aCo-EDTA and K 3 [^Co/CN)^] in clayey soil.

« НТО X * « Injection s ite

FIG. 6. K 3 \^°Co(CN)6] and НТО injections at Chikli. DEPTH IN cm SITE: CHIKL1 VILLAGE . BHUSAVAL DIST: MAHARASHTRA JALGAON. FIG.

7. S o i l - m o i s t u r e m o v e m e n t s t u d y w i t h Н Т О a n d K * [ C o ( C N ) 6 \ OL SURFACESOIL

DEPTH IN cm Z \o о IAEA-AG-158/7 91

LABORATORY EVALUATION OF THE METHOD

Initial column studies carried out with K 3Co(CN)6 labelled with shorter- lived S8Co and reported elsewhere [5] showed that the tracer maintained its position integrity in a dry soil column for over four months and it was possible to follow its movement with intermittent spraying at the surface of the column. To compare the transport characteristics of K3 Co(CN)6 with those of tritiated water, a special laboratory model has been built (Fig. 2), consisting of a large drum filled with soil and having a bottom outlet for water. Rigid plastic tubes with nearly 30 to 40% perforations are placed around a central access hole. The cobalt-cyanide tracer labelled with 60Co and tritiated water are injected together at points between the perforated tubes at a specified depth. The position of the gamma-emitting tracer is located by logging the access hole with a scintillation probe. The position of the tritium tracer is detected by extracting one of the plastic tubes, cutting it into sections, extracting the moisture and assaying for tritium by liquid scintillation counting. Figure 3 compares the two tracers in sandy soil. Similar studies were carried out on clayey soils. Figure 4 gives an example of such a comparison. ' Laboratory studies carried out using 60Co-EDTA indicate that this tracer was retarded with respect to the cyanide tracer (Fig. 5).

FIELD STUDY

Tracer studies are being carried out to estimate direct recharge to the groundwater reservoir in the Tapi alluvium between the Satpura hills and the Tapi river, in western India. The alluvial deposits consisting of clays, silt, sand, gravels and pebbles vary in thickness from 0 to 3000 m and have a number of water-bearing sand formations with interspersed clay lenses. The main problem in the region is the rapid depletion of the water-table, particularly in the case of the dug wells nearer to the Satpuras. Besides the usual artificial tritium method, the cobalt method is also being tried to examine its feasibility under field conditions. The cobalt-cyanide tracer was injected around an unused 20-cm bore-well in Chikli village in the region in September 1976. Figure 6 gives details of cobalt and tritium injections at this site. The position of the gamma tracer is determined by logging the bore-well with a gamma-scintillation detector. The tritium profile is obtained by the usual core sampling and tritium analysis procedures. Figure 7 shows the results obtained at Chikli with the cobalt and tritium tracers for the period December 1976 to April 1978. Below is the tabulated recharge data obtained by the two methods: 9 2 NAIR et al.

Tracer Tracer Moisture Rainfall Recharge % of displacement content rainfall (cm) (cm) (cm)

60Co 20(C.G .) 6.1 74 8

НТО 35 (peak) 10.6 74 14

DISCUSSION AND CONCLUSION

The reason for the difference between the transport rates of the cobalt complex and the tritiated water is not readily discernible in view of their similar behaviour in laboratory experiments carried out on the same soil. It should, however, be noted that the error in peak positioning in the НТО method is at least ± 5 cm. Considering the extremely slow movement of the soil moisture and the possibility of differences in transport rates from one point to another in the same site, the results obtained in the field study are quite encouraging. However, the study has not been undertaken on a larger scale with cobalt and tritium injections in five different sites in the region. The field and laboratory experience with the cobalt complex method indicates the feasibility of its use in arid regions in understanding the mechanisms by which moisture is transported downwards through the unsaturated zone. By using a double-tracer method involving the cobalt complex and tritiated water it should be possible to differentiate between the liquid and vapour phase movements of soil moisture in the arid zones.

REFERENCES

[1] MÜNNICH, К.О., ROETHER, W., THILO, L., “ Dating of groundwater with tritium and 14C” , Isotopes in Hydrology (Proc. Symp.Vienna, 1966), IAEA, Vienna (1968) 3 0 5 -2 0 . [2] ZIMMERMANN, U., MÜNNICH, K.O., ROETHER, W., “ Downward movement of soil moisture traced by means of hydrogen isotopes” , Isotope Techniques in Hydrological Cycle, Proc. Symp. University of Illinois, 10-12 November ( 1965) 28— 36. [3] BATH, A.H., Private communication. [4] SONNTAG, C., THOMA, G., MÜNNICH, K.O., DINÇER, T., KLITZSCH, E., “ Environ- mental isotopes in North African groundwaters, and the Dahna sand-dune study, Saudi Arabia” , IAEA-AG-158/6, these Proceedings. [5] RAO, S.M., Use of injected radioactive tracers in groundwater hydrology, Indo-German Workshop on Approaches and Methodologies for Development of Groundwater Resources, Hyderabad,India,26— 30 May, 1975. IAEA-AG-158/8

ASPECTS OF THE ISOTOPE HYDROLOGY OF TWO SANDSTONE AQUIFERS IN ARID AUSTRALIA

P.L. AIREY, G.E. CALF, P.E. HARTLEY, D. ROMAN Isotope Division, Australian Atomic Energy Commission, Research Establishment, Lucas Heights, NSW, Australia

Abstract

ASPECTS OF THE ISOTOPE HYDROLOGY OF TWO SANDSTONE AQUIFERS IN ARID AUSTRALIA. Much of Australia’s arid zone lies in a high-temperature region where potential evaporation rates can exceed the mean annual rainfall by more than an order of magnitude. Underground water therefore tends to be a biased sample of precipitation. The effect of this biasing on the stable isotopic composition and the deuterium excess of water from the Mereenie Sandstone aquifer and the western extremities of the Great Artesian Basin is discussed. An explanation is offered for an observed systematic decrease in the standard deviation of stable isotope ratios with increasing residence time. An analytical solution of a simplified version of the ground­ water transport equation is presented which allows a clear mathematical distinction to be made between a ‘flowing’ and a ‘non-flowing’ aquifer system.' This distinction can be useful in arid zones where the isotopic data may not be inherently predictable from the modern conditions at input because of substantial differences in recharge rates over the accessible time scales. The mathematical expressions also provide a basis fo r assessing when 14C data are more logically presented as absolute activities than as the conventional 14C (per cent modern carbon) ratios.

INTRODUCTION

Australian arid zones have high potential evaporation rates which can exceed mean annual rain fall by more than an order of magnitude. Thus, if precipitation rates were constant there would be no accumulation of groundwater due to local recharge; however groundwater reserves exist because the rain fall distribution is far from uniform. Net recharge occurs during prolonged periods of higher than average in ten sities. The underground water is , therefore, a very biased sample of precipitation and this is reflected in its isotopic composition.

The boundaries of arid A ustralia, as defined for a major symposium held at Canberra in 1969 [1] are shown in Figure 1. The total area of the zone is approximately 5.0 x 106 km2. "By

9 3 VO 4^

TABLE I

STABLE ISOTOPE RATIOS

Area Sample age (a) <5d fi^O d (b) SD (ÔD) (с) grouping (per mille) (per mille) (per mille) (per mille) Newer Basalts aquifer [4] upper aquifer groundwater 1 -32.8 -5.64 12.3 4.3 lower aquifer и " 1 -30.8 -5.08 9.8 5.6

Melbourne rain -31.2 -5.31 11.3 18 al. etAIREY Mereenie Sandstone groundwater 2 -61.5 -8.9 9.8 1.87 aquifer [3] Alice Springs rain -27.3 -3.7 22(d) 30(d) Great Artesian Basin - western extremiti es groundwater 3 -48.3 -7.03 8.0 1.33 - Jurassic aquifer Queensland [10] .. 4 -42.4 -6.78 11.8 1.10

(a) Residence times increase in the order 1 < 2 < 3 < 4 (b) Deuterium excess d = ÔD - 8 6180 (c) Standard deviation of the population (d) Standard deviation of values exceeding the 90 percentile level

\ IAEA-AG-158/8 9 5

© LAKE EYRE © COOPERS CREEK © DIAMANTINA RIVER

BRISBANE

PERTH SYDNEY

URNE

FIG.l. Location map. The dotted line delineates arid Australia as defined in Ref. [1]. The Great Artesian Basin is shown; the region o f the basin discussed in this paper is cross- hatched. far the greatest part of th is, nearly 3.8 x 10® km^ is an area of internal drainage, where streams are ephemeral or non-existent, most have no defined destination, and those that do, such as the Cooper and Diamantina systems term inating in Lake Eyre, reach it only on rare occasions of the largest floods.. The unreliability of runoff and the high rate of evaporation prevalent throughout the area enforce a widespread dependence on groundwater for pastoral and domestic purposes" [2].

In the present paper, the interpretation of hydrogen, oxygen and carbon isotope data is discussed. Reference is made to published data from the Mereenie Sandstone aquifer south of A lice Springs, Northern Territory [3] and to unpublished in­ form ation . from the western extrem ities of the Great A rtesian basin (Figure 1). The results from the arid zone are compared with those from a study of the Newer Basalts aquifer near Melbourne, V ictoria [4].

STABLE ISOTOPE DATA

The stable isotope data from the aquifers of interest, and from monthly samples of precipitation in the recharge area are summarised in Table 1. 9 6 AIREY et al.

In the discussions it is shown that

"I Q (i) the ÔD and 6 0 values provide excellent evidence that groundwater in arid zones is a biased sample of the precipitation in the recharge area;

(ii) in low rainfall areas, great care needs to be exer­ cised interpreting differences in the magnitude of deuterium excess d(= ÔD - 8 ô^®0) from region to region ; and

(iii) there is some evidence that the standard deviations SD (6D) decrease with increasing age.

V ariability of ¿D and б-^О

A histogram comparing the frequency distribution of 6D values in monthly rain fall samples from Melbourne and A lice Springs is shown in Figure 2 [5]. The same scale has been used to emphasise the greater variability at the arid zone station. The data for months with rain fall exceeding the 70 and 90 per­ centile values are distinguished [6]. At Alice Springs, extended rain fall events tend to be associated with depleted rain fall. This has been shown quantitatively by Hartley [7] who has recent­ ly analysed in d etail the <5D levels in monthly rain fall samples from 14 stations throughout A ustralia for the years 1973-1975. He has calculated the correlation coefficients with the parameters tem perature, rain fall and dewpoint which have a ll been weighted by rain fall. The results for Alice Springs and Melbourne are re­ spectively: temperature (-0.08, + 0.40); rainfall (-0.62, -0.35) dewpoint (+0.44, + 0.16); rain fall and dewpoint (0.66, 0.51),. It is relevant that for Alice Springs there is a significant corre­ lation with dewpoint and rain fall, but not with temperature.

The mean 6D values for the groundwater and the weighted mean values for the precipitation are listed in Table 1 and shown in the histogram (Figure 2). It is significant that although the stable isotope ratios from the Newer Basalts aquifer [4] water are close to the precipitation mean, those for the Mereenie Sandstone aquifer samples are highly depleted (Figure 2, lower inserts). In the latter case, the stable isotope data imply that significant recharge occurs only if the monthly rain fall exceeds the 90 percentile value.

The average <$D and 6^®0 values for 19 w ells sampled in the region of the Great Artesian Basin (cross-hatched in Figure 1) are -48.13 ± 1.33 per m ille and 7.03 ± 0.6 per m ille. The stand­ ard deviations are of the populations. The ratios are significant IAEA-AG-158/8 9 7

GROUNDWATER J

I A )

MELBOURNE 15

J FMAMJ J ASONO MONTH MONTHLY RAINFALL EXCEEDING I 90 PERCENTILE

S-20 70 PERCENTILE 0 30 50 70 90 P

№

( В

ALICE SPRINGS

10 MONTHLY RAINFALL EXCEEDING I 90 PERCENTILE

70 PERCENTILE

-1 5 0 - 1 0 0 - 5 0

6 d per m ille FIG.2. The frequency distribution ofbD values for monthly samples o f Melbourne (Part A ) and Alice Springs (Part B) rainfall. The mean ÔD fo r groundwater sampled in the Newer Basalts aquifer (A) and the Mereenie Sandstone aquifer (B) are shown. The insert (upper) are the mean monthly rainfall for the periods 1856 to 1965 (Melbourne) and 1874-1965 (Alice Springs) [6]. The inserts (lower) are the mean 5D values for monthly rainfall exceeding the percentile value P. 9 8 AIREY et al. ly d ifferent from samples taken from w ells tapping the principal Jurassic aquifer in Queensland (6D = -41.79 ± 1.10, ô^O = -6.6 ± 0.9 which is good supporting evidence for recharge near the western extrem ities of the basin [8,9]. The stable isotope ratios [10] are substantially less depleted than those from the Mereenie aquifer. This could be due to a number of factors resulting from the geographical separation including the follow ing:

(i) The relative contributions of different vapour sources to the rain bearing clouds could be significantly different, and this could affect the isotopic composition of the precipitation.

(ii) The threshold rainfall level for significant recharge might vary appreciably in the two recharge areas. In general lower thresholds would be associated with groundwater that is less de­ pleted in ÔD and 6180 than the weighted mean of precipitation in the recharge area.

Deuterium excess

The ÔD and 6^®0 values for m eteoric waters frequently satis­ fy the linear relationship

The observed values of deuterium excess are listed in Table 1. It is seen that groundwater from the Newer Basalts aquifer has an average value (+11 per m ille) which is expected, whereas that sampled from the western extrem ities of the Great Artesian Basin is approximately 8.0 m ille, which is typical of Saharan groundwater. It is surprising that the deuterium excess for Mereenie Sandstone groundwater is close to 10 per m ille; however paradoxically this is a further consequence of groundwater being a biased sample of precipitation. IAEA-AG-158/8 9 9

GROUNDWATER ( A )

18 MELBOURNE 16 14 12 LdCL 10 æ 1» z 6 i

JUL

LÜ CD 5

DEUTERIUM EXCESS d ( - 6D-8 61a0)

FIG.3. The frequency distribution o f the values o f the deuterium excess d (= ÔD - 8 5 lsO) for monthly samples o f Melbourne (Part A) and Alice Springs (Part B) rainfall. Those months exceeding the 70 percentile and 90 percentile levels are distinguished by cross-hatching and full shade respectively. The mean values for groundwater samples from the Newer Basalts aquifer (A) and the Mereenie Sandstone aquifer (BJ are shown.

A frequency distribution of deuterium excess values for rain­ fa ll at A lice Springs and Melbourne is shown in Figure 3. The data for rainfall above the 70 and 90 percentile value are dis­ tinguished. Clearly, in the Alice Springs region, relatively high monthly rain falls are associated with large deuterium excesses. The weighted mean value for months exceeding the nominated recharge threshold (90 percentile) is +22 per m ille. This is clearly con­ sisten t with a groundwater value of +10 per m ille for the deuterium excess when allowance is made for surface evaporative effects.

The absolute magnitude of the deuterium excess sometimes observed in precipitation at A lice Springs requires comment as it can exceed all values observed at coastal stations for which both ÔD a n d 6 1 8 0 data are available. The normal deuterium excess is attributed to kinetic isotope effects associated with marine evap­ oration under non-equilibrium conditions. As condensation usually occurs by an equilibrium process, it does not lead to a change 1 0 0 AIREY et al. in the value of the parameter. If during the passage of vapour across the continent, water removed by condensation is in part returned by non-equilibrium evaporation of either the rain drops, or the surface water, the deuterium .excess associated with the water in the cloud w ill be increased. Subsequent condensation w ill reflect the enhanced deuterium excess provided that the effect is not masked by evaporation accompanying the second event. Large deuterium excesses are correlated with high rain fall be­ cause under these conditions, relative losses due to evaporation are sm aller. It is relevant to note that a D/H rainfall model developed from basic principles by Hartley [7] which assîm es that all vapour is of oceanic origin but is otherwise quite flex­ ible, can be made consistent with all Australian data except those recorded at A lice Springs for two months in which the ÔD values are more depleted than -100 per m ille. The clear im pli­ cation is that vapour originating from re-evaporation processes was involved.

The variability of 6D v a l u e s

It is commonly observed that'the standard deviation of the groundwater D/H ratios is substantially less than that of the weighted values of the stable isotope ratios of monthly rain­ fall samples in the recharge area. Examples are listed in Table 1. For Alice Springs, only those months in which the amount of rain fall exceeded the threshold value for recharge were included. A combination of hydraulic considerations and carbon-14 dating have shown that the groundwater ages increase from com paratively modern to about 350 000 a in the order shown in Table 1. The standard deviations decrease in the same order. No attempt has been made to subtract the contribution due to measurement error, which is estim ated to be 0.6 per m ille, as the same mass spectrometer was used in each case. The effect would not be expected if the spread of ÔD and <5-*-®0 values were due prim arily to the mixing of groundwater from different source a r e a s .

In assessing this result, account must be taken of the time over which the stable isotope ratios of the precipitation is averaged to determine the standard deviations. It is significant that the standard deviation of the isotope ratios of samples from the Newer Basalts Aquifer is comparable to that associated with . monthly rain fall measurements but the standard deviations of samples from aquifers with larger residence times are substan­ tially less (Table 1). Smoothing associated with time averaging may well be the dominating effect, especially for very old ground­ water. The time over which the input data should be averaged is comparable to the resolution of the groundwater residence tim es. IAEA-AG-158/8 101

CARBON ISO TO PE DATA

M athematical form ulation

The following analytical treatment is presented, not because it can be directly used to describe the geochemistry of many real systems, but because it forms a basis for discussion of the treatm ent of data in arid zones where selection processes may be important. Some sim plifying assumptions are therefore made:

(i) the steady state 'piston flow' approximation adequately describes groundwater transport i.e . dispersion terms are neglected;

(ii) Cartesian co-ordinates (x,y,z) may be transformed into a time coordinate system (s,t) defined by the movement of water particles along groundwater stream lines [12,13]; and

(iii) the mean particle velocity Vs is independent of the position s along the flow line.

The third assumption im plies that either the flow lines are parallel and the potentiom etric surface is stable, or the ground­ water velocities are essentially zero.

With these sim plifications, the generalised dispersion-con­ vection equation may be sim plified to

C(S,t)/3t + U(9C(S,t)/9S = Q (2)

where C(S,t) is the species concentration at (s,t);

U is the mean particle velocity; and

Q = Q'/p is the ratio of the source term Q ' to the groundwater density, p.

The solution of equation (2) depends on the nature of the source/ sink term Q which is a function of the time scales involved.

In the study of any hydrogeochemical system s, at least three time scales need to be considered:

(a) the time for the establishm ent of chemical equilibria with accessible surface m inerals;

(b) the time for the transport of water through a flowing aquifer system; and 1 0 2 A IR EY et al.

(c) the time for changes to occur in the composition of surface m inerals.

It w ill be argued below that in many settings, these time scales are separable.

(a) Chemical time scales The rate of change of concentra­ tions of chemical species is directly related to the magnitude of the displacem ent from thermodynamic equilibrium . However, chemical reactions associated with the establishm ent of bicarbonate equilib­ ria are normally rapid compared with the l^C decay rate. Concentra­ tions are therefore determined not by chemical kinetic effects but by the surface free energy of m inerals accessible to the groundwater Under these conditions, the source term Q takes the form

Q (S,t) = Q0 (S) - k'C(S,t) (3) where Qo(S) is determined by changes in the free energy d iffer­ ences between the mineral and groundwater near the point S, and the first order rate constant k 1 includes the radioactive decay c o n s t a n t .

Complications which can arise are well illustrated by the transport of groundwater through the Jurassic sandstone aquifer of the Great Artesian Basin [10]. The measured rate of carbonate solution in low temperature bores (T< 60°C) is consistent with a cumulative uptake of carbonate since the mid Tertiary of 0.1 per cent of the aquifer mass which is more than an order of magnitude less than the carbonate mineral cement in the sandstone pores. One conclusion is that the bulk of the water flows along preferred paths in which solution is virtually complete. The rate deter­ mining step would then be the transport of carbonate from "stag­ nant" regions. Other factors being equal, the higher the concen­ tration of carbonate in the flowing groundwater, the lower w ill be the concentration gradient and the lower the reaction rate. The first order constant k' (equation (3)) should thus take the form

к' = X + к (4) where A is the radioactive decay constant and к represents all the other first order processes.

(b) Hydrodynamic time scale In any study involving ground­ water dating, the hydrodynamic time scale is comparable to the decay constant of the isotope. In the case of tritium it is of the order of decades ; for ^C it is in the range of 10^ t o 10^ years. As the time scale for the establishm ent of carbonate equilibria is very rapid compared with the hydrodynamic time scale the two are separable. However, processes associated with the IAEA-AG-158/8 1 0 3 transport of carbonate species from "stagnant" pores to the flow­ ing regions of the aquifer may occur with a rate comparable to d e c a y .

(c) Geological time scale Over a long period the continuous input of active solutes at recharge w ill result in system atic changes to the surface composition of accesible m inerals. These effects cannot normally be directly observed as the residence time of the water is inevitably short compared with the geological age of the aquifer.

A special case has been identified in the unsaturated zone associated with aquifers in low rain fall areas. Under these con­ ditions, cycles of surface water infiltration followed by evapora­ tive concentration have led to the accumulation of modern carbon­ ate mineral in the permeable m atrix. This effect has been pre­ dicted by Ingerson and Pearson [14]; has been observed directly by W allick [15] ; and has been inferred from the results of Calf [3] and Verhagen et al [16] but not from those of Pearson and Swarzenki [17].

Analytical solutions of the transport equation

The transport equations may be solved subject to the follow­ ing boundary conditions:

C(0,t-S/U) = A (a constant) (5 ) i.e . the concentration of bicarbonate at input (S = O, time = t - S/U) is constant; and

C(S;0) = В (a constant) (6) i.e. the in itial concentration at position S is constant. The general solutions of equations ( 2) t o (6) which can be verified by substitution are

C(S,t) = A exp (-(к + X) S/U) H (t - S/U)

+ В exp (-(к + X) t) + (Q0/(k + X) ) (1 - езф - (к + X) t )

- В exp (-(к + X) t) H (t - S/U) (7 )

(Qo/(k + X)) [exp (-(к + X) S/U) - exp - (k + X)t] H (t - S/U)

Where H is the Heavyside function

H (t - S/U) =1 t > S/U ( 8 )

= 0 t í S /U 1 0 4 AI RE Y et al.

E q u a t i o n ( 8) allows a useful mathematical distinction to be made between 'flow ing 1 and 'non-flowing' aquifers. An aquifer can be said to be flowing if the time t, since a definable zero point (t=0) is greater than time S/U for a particle of water to be transported from the recharge area to the observation point. In this mathematical sense it is non flowing if t < S/U.

Both the flowing and non flowing solutions take the follow­ i n g fo r m

С (S,t) = (D - Q 0/ (k + X)T exp (-(к + X) T) + Q0/Ck + X)(9a)

= D + Q0T (if к + X = 0) (9b)

Where T = S/U (flowing)

= t (non flowing)

and D = A (flowing)

= В (non flowing)

The distinction between flowing and non-flowing systems is arbitrary unless the point of zero time corresponds to a discon­ tinuity (or rapid change) in the hydrodynamics or geochemistry. It is likely to be of special value in low rainfall areas where recharge occurs only after periods higher than average precipita­ tion. As pointed out by Pearson and Swarzenki [17] and confirmed by Calf [3] net recharge to aquifers in arid and sem i-arid areas is not a steady process. Periods of higher than average in filtra­ tion which could be considered as points of zero time in non flowing systems can be separated by well defined time intervals.

Equation (9) can be used to assess factors involved in the presentation of carbon isotope data. Since the expression applies only to the saturated zone in which the aquifer carbonate is " d e a d " t o 14c, some sim plifications are possible.

12c = (D - Q0/k) exp (- kT) + Q0/k (10)

13C = (13Ir D - 13Ia Q0/ak) exp (- ak T) + 13Ia Q0/ak (11)

1 4 1 4 С = Ir D e'xp (-X T) (12)

w h e r e 14 , 13I r , 14 , 13Ia are the 14C / 1 2 C a n d 13C/12C ratios as the in filtratin g water enters the saturated zone, and in the ground­ water sample respectively; and a is the 13k/12k ratio. IAEA-AG-158/8 1 0 5 6D 6D (SMOW )per m ille

FIG.4. Spatial distribution of: A, absolute 14C activity (mBq ltr~l: 1 Bq = 1 dis/s); В, 14C, %mc; C, S13C (per mille PDB), and SD (per mille, SMOW); D, [carbonate] and [Cl~] (mmol Itr“V; E, temperature ( C). The data is related to the appropriate water well on the sketch map (part F) by the vertical dotted lines. The location o f the Mereenie Sandstone aquifer [19] and the Roe Creek is shown. The water-well classification [3] from the A14С - [HCO3'1 ] relationship, i.e. from the absolute 14 С activity (see Part A j is shown. 1 0 6 A IREY et al.

Aspects of the interpretation of carbon isotope data from arid zone sandstone

Specific reference is now made to data from the portion of the Mereenie Sandstone aquifer shown in Figure 4F. A possible source of recharge is the Roe Creek floodout. The aquifer has been classified into regions on the basis of the slope of dis­ crete linear ^C (% m.c.) - [HCOg]-! relationships i.e . the absolute [Н-^СОз]. Two particular features of the results w ill be discussed:

(i) the interpretation of differences in the absolute 14c concentration in water, and the l^C (% m.c.) variability; a n d (ii) the tendency for the frequency distribution of carbon-14 levels to be multi-modal.

(a) The presentation of carbon-14 data

The variation of the absolute -^C activity with distance is shown in Figure 4A. As seen from equation (12) the data are independent of the concentration of dead carbonate dissolved within the aquifer, but is strongly dependent on the isotope activity at recharge Ir which is determined by a number of factors [18] including

(i) the concentration of dissolved atmospheric ^CC^;

(ii) the input of l^C carbonate by recharge from Roe Creek during periods of high rain fall;

( i i i ) the uptake of 14C02 from soil air; and t ( i v ) concentration effects due to sub-surface evapora­ tion and transpiration.

In the absence of evaporative effects, the concentrations of 14C w ill be determined principally by the partial pressure of CC>2 in the soil air, the chemistry of the infiltrating water and the availability of calcite for the reaction H ,0 + + C a C 0 3 + C 0 2 i C a + 2 HCO3- (1 3 )

The effect of the relatively low specific activity of ^C in soil calcite on the groundwater would be comparatively sm all. A factor which is potentially much more important is the effect of concentration due to sub-surface evaporation and transpiration. The absolute ^C concentration at input would respond directly to this effect. IAEA-AG-158/8 1 0 7

Despite the high potential evaporation rates relative to mean annual rain fall, the influence of évapotranspiration on the absolute concentration in the groundwater is thought not to be important for the following reasons :

(i) the [H^CO^] decreases in a regular stepwise manner from the recognised recharge area near the Roe Creek floodout (Figure 4A and 4F); and

(ii) the chloride levels (Figure 4D) are not particularly high. Extensive évapotranspiration would lead to concentration.

Both results suggest that effective recharge occurs only during periods of higher than average rain fall when sub-surface losses may not be greater than those in less extreme clim ates.

In contrast to the absolute -*-4C concentrations, the ^4C (% m .c.) ratios are independent of dilution effects but are sensitive to the solution of inactive carbonate. That the spatial variability of both parameters is sim ilar (Figures 4A and 4B) suggests that the solution of carbonate within the saturated zone is a minor process. This is supported by the general constancy of the ó-^C values and the good correla­ tion between the groundwater chloride and bicarbonate concentra­ t i o n s .

(b) The distribution of carbon—14 abundances in arid zones

Assuming a uniform, spatial distribution of sampling w ells, a constant transport rate and a one dimensional geometry, equal numbers of samples should b e i n e a c h ag e interval. The numbers would increase monotonically if the flow lines radiated from the recharge area. However, there is increasing evidence that the distribution of residence times in arid zones is multi-modal i.e . the probab­ ility of recharge is greater in some periods of time than in others. Examples can be sited from North-Africa [11], Kenya [17] and Central A ustralia [3]. Explanations are sometimes sought in terms of the clim ate changes in recharge areas throughout the accessible tim e-scale. However, it has been recently pointed out that the sub surface mixing of modern and old water can lead to a lower variability in the 1^C (% m .c.) levels than predicted from the age distribution of the older samples [ 20].

Nevertheless the observations are consistent with the general hypothesis that effective recharge occurs only when the rain fall intensity exceeds a threshold value. If the threshold value is high small changes in the mean or the standard deviation of the rain fall distribution w ill lead to much larger changes in the 1 0 8 AI RE Y et al. recharge rate. Statistical formulations developed for flood re­ currence analysis are applicable. Of the various procedures available, use w ill be made of the distribution of extreme values following Gumbel [21]. If X^, X2 .... Xn are extreme values of, say, monthly rain fall amounts in n samples of equal size N and if X (the threshold value for recharge, say) is an unlimited expon­ entially distributed variable, then the cumulative probability P that any of the n extremes w ill exceed the value X approaches the e x p r e s s i o n

P = 1 - exp [- (exp(-y))]

where y = a (X- X + bôx)

X is the mean monthly rain fall;

ÔX is the standard deviation; and

a and b are constants.

A sm all decrease in the value of the reduced variable y could have a very much larger effect on P. The decrease would result either from an increase in the mean monthly rain fall, or an in­ crease in <5x i.e. in the rainfall variability. For instance, if P were of the order 0.1, a reduction of 20 per cent in the value of y would lead to a 50 per cent increase in P; again, if P were 0.01 the same proportional decrease in Y would lead to a 2.5 fold increase in the probability that the threshold value would be exceeded.

Thus, in arid zones, modest changes in conditions in the input areas can lead to much larger changes in the probability of recharge'. To understand the im plications of this fully is one of the challenges of arid zone hydrology.

SUMMARY AND CONCLUSIONS

1 ft 1. The stable isotope ratios ÔD and 6 0 in the Mereenie Sandstone aquifer, Central A ustralia are substantially depleted compared with the weighted mean values of modern rain fall, indi­ cating that recharge in this region of the arid zones is a biased sample of the precipitation.

2. Care must be exercised in interpreting values of the deuterium excess in arid zone groundwaters. As with many other m eteorological param eters, values of the deuterium excess in low rainfall areas are very variable. At Alice Springs they tend to increase with increasing rain fall. The observed values in ground­ water samples are the net result of an increase due to biasing and a decrease due to surface evaporation. IAEA-AG-158/8 1 0 9

3. The standard deviations associated with the ÔD values de­ crease with .increasing residence tim e.

4. A sim plified version of the generalised transport equation was solved analytically. The solution allowed a clear mathematical distinction to be made between a flowing and non­ flowing aquifer system. A "non flowing" system is one in which the isotopic composition of the water is inherently unpredictable from modern conditions at recharge. This concept may be useful in arid zones where the rate of recharge may vary substantially over the time scale of interest.

5. The relative advantage of presenting С data as per cent modern carbon, or as absolute concentrations is discussed. 14C (% m.c.) values are sensitive to variations in extent of solution of inactive carbonate; absolute concentrations respond to factors such as évapotranspiration which lead to concentration changes in the unsaturated zone.

6 . The stable isotope results suggest that effective re­ charge to the Mereenie Sandstone aquifer occur when the monthly rain fall exceeds the 90 percentile value. Under these conditions, small changes to the mean or the standard deviation of the rain­ fall distribution lead to proportionally larger changes in the probability of recharge.

ACKNOWLEDGMENTS

The invaluable assistance of Dr. M. A. Habermehl of the Bureau of M ineral Resources, Geology and Geophysics, Canberra, in those aspects of the work involving the Great Artesian Basin, is gratefully acknowledged. Discussions with Mr. Wall and Mrs. Komarower on the Newer B asalts Aquifer were appreciated.

REFERENCES

[1] SLATYER, R.O ., PERRY, R.A ., Arid Lands of A ustralia, Australian National U niversity, Canberra, (1969).

[2] FISHER, N.H., Water resources, ibid chap.4, p .55.

[3] CALF, G.E., "Isotope hydrology of the Mereenie Sandstone Aquifer, Alice Springs, A ustralia. J. Hydrology, 38^ (1978) 3 4 3 .

[4 ] KOMAROWER, P . , W ALL, V . S . , "T h e c h e m i c a l e v a l u a t i o n o f groundwaters in the tertiary basalts of V ictoria", Proc. AWRC Groundwater Pollution Conference, Perth (1979). 1 1 0 A IR EY et al.

[5] (a) INTERNATIONAL ATOMIC ENERGY AGENCY, 'Environm ental Isotope Data N o.l: World Survey of Isotope Concentration in Precipitation (1953-1963), Technical Report Series No.96, IAEA, Vienna (1969).

(b) Environmental Isotope Data No.2: World Survey of Isotope Concentration in Precipitation (1964-1965), Technical Report Series No. 117, IAEA, Vienna (1970).

(c) Environmental Isotope Data No.3: World Survey of Isotope Concentration in Precipitation (1966-1967) . Tech- nicla Report Series No.129, IAEA Vienna (1971).

(d) Environmental Isotope Data No.5: World Survey of Isotope Concentration in Precipitation (1970-1971). Tech­ nical Report Series No.147 IAEA Vienna (1973).

[6] AUSTRALIAN WATER RESOURCES COUNCIL, Review of A u stralia's Water Resources; Monthly R ainfall and Evaporation. Part 1, Data Tabulations. Bureau of Meteorology, Melbourne, 1968.

[7] HARTLEY, P.E. Deuterium hydrogen ratios in Australian rain and groundwater, M.Sc. Thesis, University of New South Wales (1978).

[8 ] HABERMEHL, M .P., Hydrogeology of the Great A rtesian Basin, A ustralia. J. Bureau of Mineral Resources, A ust., in p r e p .

[9] SEIDEL, G.E., Gabhyd digital computer model of the Great Artesian Basin, A ustralia. J. Bur. Min. Resour. Aust., in p r e p .

[10] AIREY, P.L ., CALF, G .E., CAMPBELL, B .L ., HARTLEY, P .E ., ROMAN, D. /‘Aspects of the isotope hydrology of the Great A rtesian Basin, A ustralia”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg 1978) 1, IAEA, Vienna (1979) 205-19.

[11] SONNTAG, C ., KLITSCH, E ., LOEHNERT, P ., MÜNNICH, K.O. , EL SHAZLY, E.M ., KALINKE, G ., THORWEIHE, U ., WEISTR0FFER, K ., SWAYLEM, F.M. “ Paleoclim ate inform ation from deuterium and oxygen-18 in carbon-14 dated North Saharan groundwaters; groundwater form ation in the past”, ibid. 2, pp. 569—81. • -

[12] MERCADO, A., BILLINGS, G .K., Kinetics of mineral dissolution in carbonate aquifers as a tool for hydraulic investigations. I concentration - time relationships. J. Hydrology 2£ (1975) 3 0 3 . IAEA-AG-158/8 111

[13] SHAMIN, U.Y., HARLEMAN, D .R .F., Numerical dispersions for solutions in porous media, Water Resources Research J3 (1967) 5 5 7 .

[14] INGERSON, E ., PEARSON, F .J., Jr. Estimation of age and rate of motion of groundwater by the -^C method, Recent Research in the Fields of Hydrosphere, Atmosphere and Nuclear Geochemistry, Maruzen Co., Tokyo (1964) 263.

[15] WALLICK, E .I. Isotopic and chemical considerations in radiocarbon dating of groundwater within the sem i-arid Tuxon Basin, Arizona. Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology (Proc. Advisory Group M eeting, Vienna) IAEA, Vienna, 1976.

[16] VERHAGEN, B. Th., SMITH, P .E ., McGEORGE, I ., DZIEMBOWSKI,Z., “Tritium profiles in Kalahari sands as a measure of rain-water recharge”, Isotope Hydrology 1978 (Proc. Symp. Neuherberg 1978) 2, IAEA, Vienna (1979) 7 3 3 -5 1 .

[17] PEARSON, F .J. Jr ., SWARZENKI, W.V., 14C evidence for the origin of arid region groundwater. Northeastern Province, Kenya, Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp. Vienna, 1974) 1^ IAEA, Vienna (1974) 95-108.

[18] MOOK, W.G. The dissolution - exchange model for dating groundwater with Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology (Proc. Advisory Group Meeting, Vienna) IAEA, Vienna,1976.

[19] WOOLEY, D., Geohydrology of the Emily and Brewer Plains Area, Alice Springs, N.T. Water Resources Branch, Department of Northern Territory 1966.

[20] GEYH, M.A. Interpretation problems of groundwater isotope data from arid and sem i-arid zones (1978) IAEA Advisory Group Meeting on the Application of Isotope Techniques to Arid Zones Hydrology, paper 8.

[21] LINSLEY, R .J. Jr ., KOHLER, M.A., PAULHUS, J.L .H ., Hydrology for Engineers, McGraw-Hill Book Co. New York (1958), chap.11, p . 2 4 5 .

IAEA-AG-158/9

STUDY OF THE LEAKAGE BETWEEN TWO AQUIFERS IN HERMOSILLO, MEXICO, USING ENVIRONMENTAL ISOTOPES

B.R. PAYNE, L. QUIJANO Isotope Hydrology Section, International Atomic Energy Agency, Vienna

C. LATORRE D. Grupo de Física, Secretaría de Agricultura y Recursos Hidráulicos, Mexico

Abstract

STUDY OF THE LEAKAGE BETWEEN TWO AQUIFERS IN HERMOSILLO, MEXICO, USING ENVIRONMENTAL ISOTOPES. The Coast of Hermosillo is located in the G ulf of California, Mexico. It is a Quaternary alluvial plain of continental origin. Underlying these deposits is a layer of blue clay about 100 m thick which imposes confinement to a deep aquifer in basaltic and pyroclastic rocks. Oxygen-18 and deuterium data support the occurrence of an upwards leakage. The amount of the leakage was evaluated, on the basis of 14C data, to a maximum of 20% of the water pumped by the irrigation wells in the upper aquifer. The stable isotope data also support the occurrence of sea-water intrusion by preferential channels in the south and in the area of K in o Bay.

1. INTRODUCTION

1.1. Description of the area

The irrigation area, known as “Coast of Hermosillo”, is located between the parallels 28° 20' and 29° 55' north and the meridians 11 Io 0' and 112° 30' west (Fig.l). It shares its surface between two basins, Hermosillo and Bacoachi. The irrigation district has an area of about 14 000 km2, but only 1250 km2 are cultivated. The climate is extreme, semi-arid, with deficient precipitation during the whole year. The mean annual temperature is 24°C, the maximum being 50°C and the minimum -5°C. From November to March the mean temperature is about 15°C. The mean annual precipitation is 221 mm and the mean potential

1 1 3 1 1 4 PAYNE et al.

FIG. I. Location o f the study area.

evaporation is 2440 mm. During June and July the monthly potential evaporation exceeds 300 mm [ 1 ]. Two surface streams reach the area. The river Sonora has an annual discharge of about 200 X 106 m3, including the river San Miguel. All this volume is stored in the Abelardo Rodríguez Reservoir by the city of Hermosillo and is used entirely for municipal purposes. Before 1948 the river flowed through the coastal plain and discharged to the sea. The river Bacoachi is rather a wadi, located to the north of the area, and discharges to Laguna Noriega, a small temporal lake known as a playa, where water remains for a few weeks during which time it is lost by evaporation and seepage. The discharge is not measured. During the scarce but heavy rain events, run-off occurs to some extent in the area [2]. IAEA-AG-158/9 1 1 5

PROFILE A-A'

WELL WELL WELL WELL WELL WELL WELL WELL 60. PHO-12 PH0-10 PHB-9 PHB-15 PHO-3 PHO-8 PHB-7 PHO-19 fin 30 GROUND LEV EL .30 0- 0

FIG.2. Lithological profile along a NW-SE direction.

1.2. Geohydrological setting

The Coast of Hermosillo is a Quaternary alluvial plain of continental origin. The deposits comprise gravel and sand with intercalations of clay. The stratigraphie sequences are horizontal with a gentle slope towards the Gulf of California. The deposits become more clayey in the coastal zone where permeability decreases with depth and a horizontal layer of brown clay, a few metres thick, occurs at a depth of about 60 m. This clay layer imposes a semi­ confinement to the deposits lying below. The thickness of the alluvial fill varies between 100 and 190 m. The fill rests on a layer of blue clay of marine origin with a slope similar to the fill. The thickness is variable. It is about 200 m at the coast and in the centre of the irrigation area. Towards the northeast it becomes thinner, disappearing near the city of Hermosillo. Under the blue clay pyroclastic and basaltic rocks occur, resting on the crystalline basement, located at a depth between 500 and 1200 m as a result of the tectonism of the zone [ 1 ]. Both the blue clay and this formation are not known in detail. Figure 2 shows a lithological profile along a NW-SE direction in the centre of the area of study. The water-level contours for the upper aquifer in 1954 show a northeast- southwest direction of flow, that is from the city of Hermosillo to the sea. The 1 1 6 PAYNE et al. water-level contours in 1975 show that the water table has been lowered down to 40 m below sea level in the centre of the area. The groundwater balance for the upper aquifer in 1969 showed a total annual recharge of about 350 X 106 m3, distributed in the following manner: 270 X 106 from infiltration of local precipitation, return flow of excess of irrigation water and a possible recharge through the blue clay, the remaining 80 X 106 came from the lateral underground flow from the periphery of the area [3]. The piezometry of the deep aquifer before groundwater exploitation commenced is not known. Even now, the only evidence at hand is the static level of well PHB-15 located in the centre of the depressed zone. In 1969 it was about 3 m below sea level and 7 m lower in 1974. Nevertheless, the existence of submarine springs a few decades ago, which no longer flow, suggests that the head of the confined aquifer was above sea level. (Sainz Oritz, personal communication.) This aquifer is recharged in that area where the blue clay disappears.

1.3. The problem

Leakage from the deep aquifer

Nothing is known about the relationship between the hydraulic head of both aquifers before water development in the area, but probably the deep aquifer had a higher head as suggested by the existence of submarine springs and the fact that in 1954 the water table of the upper aquifer was less than 10 m above sea level in the area of interest. Therefore, the existence of a steady flow from the deep to the upper aquifer before exploitation was possible. At present the head of the confined aquifer is about 30 m higher than the head of the upper one, in the most depressed area. The fact that, as exploitation of the upper aquifer proceeded, the submarine springs disappeared and the water level of well PHB-15 decreased, would suggest that the flow through the blue clay has increased significantly as a consequence of the drawdown of the water table of the upper aquifer. Nevertheless, the severe exploitation of groundwater has necessarily induced changes up-gradient where recharge to the deep aquifer takes place and probably the amount of recharge to the latter has been reduced, so that the confined'aquifer has lost pressure.

Saline intrusion

Enough evidence is present in the area of the study to affirm that sea-water is intruding into the upper aquifer [4]. Nevertheless, some samples near the coast were taken to show how the.sea-water intrusion modifies the stable isotope content of the upper aquifer which thus causes some difficulty in determining the isotope index of the upper aquifer. IAEA-AG-158/9 1 1 7

2. METHODOLOGY

2.1. Sampling description

In 1973 a reconnaissance sampling was carried out. This included 12 wells from the irrigation area and three wells located along the course of the River Sonora. All these samples were analysed for deuterium,180 and tritium. In 1975 a second sampling was performed — 47 wells from the irrigation area were chosen, covering the cone of depression and the area nearer the coast. These wells were analysed for deuterium and 180 and three of them also for tritium, 13C and 14C. At that time the well PHB-15, which taps water only from the deep aquifer, was sampled and analysed for stable isotopes, tritium and 14C. A third sampling was carried out in 1978, including the following: three wells located outside the occurrence of the blue clay, the same three wells located along the river Sonora, which were sampled in 1973, the river itself before the reservoir, the precipitation of July 1978 at the city of Hermosillo, sea-water and seven holes drilled along the shore which can tap water independently from two different horizons. All these samples were analysed for deuterium and 180 . Isotopic analyses were carried out by the IAEA Isotope Hydrology Section. The results are expressed in the conventional delta units. The overall analytical error (1 a) for 6D is 1.0%o and 0.1%o for 5 180 . Samples from 1975 and 1978 were also analysed for major ions. No field measurements of pH were made. These analyses were carried out by the Commission for Water of the Valley of Mexico. The results are given in Tables I and II.

2.2. Approach to the problem

Leakage through the blue clay

Considering the confined nature of the deep aquifer, it was reasonable to expect a large difference between water from this aquifer and water from the upper aquifer. Hence a difference in 180 and deuterium content could be expected. This difference could then be used to investigate the leakage. The problem of flow through the blue clay can be approached in two ways:

(i) It is possible that under original conditions the flow through the blue clay was downwards and exploitation has caused an inverse flow. On the basis of hydraulic consideration isotope techniques would probably not be applicable for the following reason. Since the permeability of the blue clay should be low and the thickness is about 200 m in the area of interest, it would take hundreds or even thousands of years for the isotopic signal from the confined aquifer to reach the upper one. TABLE I. SAMPLING POINTS AND RESULTS

Total depth Code Name Sampling or screen 5 180 6D Tritium Ca2+ Mg2+ Na+ СГ s o 3 _ HCO3- date location (m) ('%o) (%o) (TU) (ppm) (ppm ) (ppm ) (ppm ) (ppm ) (ppm)

364 Well 15-02 2 1 .6 .7 3 122 - 6 .8 2 - 4 6 .9 2.1 ± 0.3 — — — — — _

365 Well 3 2 -14 2 1 .6 .7 3 - - 6 .4 8 - 4 7 .7 0.1 ± 0.3 ------

366 Well 04-09 2 1 .6 .7 3 - - 6 .3 4 - 4 2 .3 0.1 ± 0.2 ------

367 Well 20-03 2 1 .6 .7 3 - - 6 .3 5 - 4 5 .2 0 .4 + 0.2 ------

368 Well 29-15 2 1 .6 .7 3 - - 6 .7 4 - 4 6 .8 1.1 ± 0.2 - - - - - AN e al. et PAYNE 369 Well 42-06 2 1 .6 .7 3 - - 6 .2 6 - 4 5 .9 0.7 ± 0 .2 ------

370 Well 49-09 2 1 .6 .7 3 50 - 6 .8 5 - 4 8 .9 0.7 ± 0.2 ------

371 Well 44-08 2 1 .6 .7 3 - - 6 .8 7 - 4 8 .2 0.3 ± 0.3 ------.

372 Well 51-12 2 1 .6 .7 3 - - 6 .3 4 - 4 5 .8 1.1 ± 0.2 ------

374 Well 26-03 2 1 .6 .7 3 40 - 5 .4 7 -3 7 .1 6 6 .4 ± 4 ------

375 Well 09-02 2 1 .6 .7 3 - - 6.11 - 4 3 .1 0.8 ± 0.2 ------

376 Well AP-Aconchi 2 1 .6 .7 3 - - 6 .3 2 - 4 3 .0 100 ± 6 - - - - -

377 Well AP-Ures 2 1 .6 .7 3 20 - 6 .0 6 - 4 1 .9 95.2 ± 5.7 ------

378 Well HAP-7 2 1 .6 .7 3 - - 5 .5 2 - 3 7 .8 86 ± 5.5 ------303 Well PHB-15 2 6 .7 .7 5 2 9 8 - 7 3 2 -8 .4 1 - 6 1 .5 0.2 ± 0.2 1.7 0.5 188 108 8 2 .4 224

304 Well 15-02 2 2 .7 .7 5 122 - 6 .4 5 - 4 8 .5 - 60.3 11.7 70.5 68.2 6 8 .7 229

305 Well 30-23 2 2 .7 .7 5 98 - 6 .5 5 - 4 9 .4 — 4 9 .0 11.1 6 1 .3 45.5 65.9 211 TABLE I (cont.)

Total depth Code Name Sampling or screen 6 18o 5D Tritium Ca2+ Mg2+ Na+ СГ s o 3 ' HCO3- date location (m) (%o) (%o) (TU) (ppm ) (PPm) (ppm ) (ppm ) (ppm ) (ppm)

307 Well 30-17 22.7.75 122 -6 .6 1 - 4 6 .6 - 32.2 4 .2 49.3 20.8 3 4 .6 175

308 Well 37-14 22.7.75 152 -6 .6 1 - 4 6 .9 - 3 5 .0 5.8 4 2 .7 20.8 31.1 178

309 Well 37-17 22.7.75 114 - 6 .5 5 - 4 7 .2 - 31.5 4 .2 45.5 20.8 34.1 162

3 1 0 Well 37-21 22.7.75 119 - 6 .6 3 - 4 9 .4 - 39.1 8.5 4 6 .4 - 22.7 3 8 .0 187 ' 311 Well 37-23 2 1 .7 .7 5 131 - 6 .5 9 - 4 7 .2 0.1 ± 0.2 11.4 19.6 4 4 .6 19.0 4 0 .4 175 AAA-5/ 119 IAEA-AG-158/9 312 Well 44-01 22.7.75 114 - 6 .6 5 - 4 9 .0 - 31.5 4.8 4 5 .0 19.0 33.1 169

313 Well 44-07 22 .7 .7 5 ’ 114 - 6 .8 3 - 4 9 .4 - 3 9 .3 5.3 39.3 20.8 20.2 187

3 1 4 Well 4 4 -10 22.7.75 119 -6 .8 1 - 4 8 .2 - 41.1 5.8 4 6 .0 28.1 39.9 181

315 WeM 44-17 25.7.75 122 - 6 .5 6 - 5 3 .1 - 4 2 .0 6 .9 4 4 .2 30.3 39.9 178

316 Well 44-23 25.7.75 122 - 6 .8 7 - 5 0 .2 - 41.1 5.3 4 1 .7 28 .4 32.6 172

317 Well 50-12 25.7.75 65 - 6 .7 3 - 5 0 .0 - 3 7 .6 5.8 42.5 24.6 3 7 .0 172 318 Well 50-04 25.7.75 108 - 6 .7 0 - 5 0 .4 0 .6 ± 0.2 34.1 5.3 44.1 22.7 31.1 175

319 Well 4 3 -0 4 2 2 .7 .7 5 118 - 6 .7 0 - 4 8 .7 - 3 5 .8 6.1 43.1 20.8 34.1 181

3 2 0 Well 3 6 -20 22.7.75 137 - 6 .6 5 - 4 7 .2 - 3 7 .6 5.3 50.3 30.3 4 5 .0 169

321 Well 36-11 22.7.75 128 - 6 .7 6 - 4 9 .5 - 3 5 .0 10.1 51.9 30.3 54.6 175

322 Well 11-02 24 .7 .7 5 61 - 6 .5 0 -4 4 .5 - 3 0 .6 13.3 83.5 70.1 29.2 230

323 Well 19-01 24 .7 .7 5 34 - 6 .4 1 - 4 4 .9 - 56.8 6 2 .6 163 300 118 261 3 2 4 Well 19-03 24.7.75 55 - 6 .4 6 -4 6 .8 — 52.5 14.3 119 92 .9 133 224 120 TABLE I (cont.)

Total depth Code Name Sampling o r screen 6 ls O 5D T ritiu m Ca2+ Mg2+ Na+ cr sol‘ HCO3 date location (m) i%0) (%o) (TU) (PPm) (ppm ) (ppm ) (ppm ) (ppm ) (ppm )

325 W ell 19-05 24.7.75 107 -6 .3 1 -4 7 .1 - 32.3 5.3 59.9 26.5 41.4 187

326 Well 12-06 24.7.75 91 -6 .3 2 -4 4 .8 - 23.6 3.2 67.1 24.6 31.6 190

327 W ell 20-02 24.7.75 91 -6 .3 7 -4 7 .2 - 91.8 25.0 125 2 1 0 114 230

328 Well 19-04 24.7.75 1 0 2 -6 .3 0 -4 4 .5 - 248 43.0 215 502 311 245

329 Well 20-10 24.7.75 76 -6 .5 9 -4 7 .4 - 157 27.1 81.2 258 156 187

330 Well 35-13 24.7.75 113 -6 .2 8 -4 8 .1 - 240 45.1 129 675 31.1 116 PAYNE et

331 Well 35-15 24.7.75 91 -6 .2 5 -4 5 .9 - 47.2 6.9 60.9 1 0 2 23.7 141

332 Well 35-12 24.7.75 130 -6 .5 3 -4 8 .4 - 31.5 17.5 51.9 43.6 55.7 181 al. 333 Well 35-09 24.7.75 . 98 -6 .2 8 -4 7 .6 - 115 35.6 1 2 0 387 39.9 141

334 Well 35-11 24.7.75 6 6 -6 .5 1 -4 8 .6 - 36.7 1 2 . 2 60.3 43.6 60.2 184

335 Well 35-04 24.7.75 85 -6 .4 9 -4 6 .9 - 33.2 19.1 60.1 51.2 85.6 162

336 Well 49-07 23.7.75 92 -6 .8 0 -5 0 .0 - 28.9 19.1 55.4 6 8 . 2 37.0 172

337 Well 49-09 23.7.75 50 -6 .7 4 -5 1 .4 - 35.8 41.4 78.9 173 52.9 162

338 Well 49-17 23.7.75 1 1 2 -6 .7 7 -4 8 .4 - 30.6 27.1 59.9 1 1 2 33.1 159

339 Well 49-14 23.7.75 107 -6.71 -5 0 .1 - 37.6 41.9 138 248 8 6 . 2 159

340 Well 49-03 23.7.75 117 -6 .2 9 -4 5 .7 - 612 104 370 1 820 1 2 2 113 341 Well 49-04 23.7.75 119 -6 .5 8 -4 9 .2 - 30.6 4.8 50.8 34.1 33.1 157

342 Well 55-02 23.7.75 109 -6 .3 6 -4 3 .4 — 490 94.3 406 1 540 187 95.0 TABLE I (cont.)

Total depth Code Name Sampling or screen 5 180 SD Tritium Ca2+ Mg2+ Na+ СГ so|~ HCO3- date location (m) (%o) (%c) (TU) (ppm ) (ppm ) (ppm ) (ppm ) (ppm) (PPm)

343 Well 49-05 23.7.75 96 - 6 .7 5 - 4 9 .0 - 46.3 20.2 60.9 121 38.4 158

344 Well 4 9 -10 23.7.75 96 - 6 .5 2 - 4 7 .5 - 319 85.1 179 861 77.9 110

345 Well 49-11 2 3.7.75 91 - 6 .4 4 - 4 8 .5 - 34.1 7.4 63 .0 73.9 31.4 135

346 Well 50-10 23.7.75 183 - 6 .4 1 - 4 6 .0 - 30.6 6 .4 52.3 56.9 30.2 132

347 Well 50-16 2 3.7.75 93 - 6 .6 5 - 4 7 .9 - 2 4 4 4 7 .8 101 635 55.7 101

348 Well 50-09 2 3.7.75 108 - 6 .4 5 - 4 6 .8 - 647 106 345 1 880 136 76.6

349 Well 50-07 2 3 .7 .7 5 102 - 6 .8 9 -4 8 .1 - 15.7 18.1 4 6 .7 3 2 .2 37 158

350 Well 55-01 2 3.7.75 107 - 6 .7 4 - 4 6 .7 - 20.1 10.6 4 3 .4 30.3 31.1 138

351 Well 50-19 2 3 .7 .7 5 91 - 6 .7 4 - 5 0 .4 - 34.1 30.8 69.7 108 92 141

380 Well HAP-7 6.78 - - 3 .9 7 - 3 0 .3 - 8 1 .0 18.1 108 43.8 123 389

381 Well PSB-10 6.78 1 3 8 -2 9 9 - 5 .9 3 - 3 9 .9 - 2.1 1.9 127 60.2 32 185

382 Well 26-03 6.78 40 - 5 .1 8 - 3 6 .8 - 76.9 12.5 22.3 3 1 .0 38.4 257

383 Well CH -16 6.78 150 - 4 .6 4 - 3 6 .3 - 110 2 2 .4 177 139 242 372

384 Well AP-Ures 6.78 20 - 5 .3 9 - 3 6 .4 - 76 14.3 45.5 18.2 125 235

385 Well AP-Aconchi 6.78 - - 5 .9 0 - 4 2 .2 - 101 17.4 52.9 20.1 150 312 386 River Sonora (Puente el Gavilán) 6.78 _ - 4 .4 9 - 3 6 .2 _ 68 .7 18.7 102 34.7 221 2 40

387 Rain (Hermosillo City) 36.4 mm 7.78 - 0.12 + 0.1 2 PYE etal. PAYNE 122

TABLE I (cont.)

Total depth Code Name Sampling or screen s18o 8D Tritium Ca2t Mg2+ Na+ СГ SO|~ HCC date location (m) ('%0) (%o) (TU) (ppm ) (ppm ) (ppm ) (ppm ) (ppm) (ppn

388 Sea water (Kino) 6.78 - + 0.33 + 2.3 - - - - — - -

389 Well PCH-2 6.7 8 6 4 - 9 6 -5 .5 5 - 4 0 .3 - 815 143 415 2 240 312 96.

390 Well PCH-2 6.78 1 2 7 - 2 0 4 - 5 .5 5 - 4 3 .5 - 18.5 2.5 50.5 32.8 24.8 116

391 Well PCH-4 6.78 6 0 - 1 2 0 - 7 .3 2 - 5 3 .5 - 27.7 3.7 396 503 154 105

392 Well PCH-4 6.78 1 4 4 - 1 9 6 - 7 .5 8 - 5 2 .2 - 20.5 4 .4 394 4 9 4 146 105

394 Well PCH-5 6.78 1 2 0 - 2 0 0 -6 .4 5 - 4 6 .7 - 282 4 6 .7 144 783 55 77

395 Well PCH-6 6 .7 8 3 1 - 9 7 - 1 .3 9 - 1 2 .6 - 1820 828 6 9 3 0 14 3 0 0 2 7 1 0 121

396 Well PCH-6 6.78 1 1 0 - 2 0 0 - 1 .5 0 - 1 0 .7 - 1110 863 7 3 9 0 14 0 0 0 2 6 1 0 135

397 Well PCH-7 6.78 8 2 - 1 0 2 - 4 .2 9 - 3 2 .2 - 99 0 368 2 7 3 0 6 290 9 52 113

398 Well PCH-7 6.78 1 1 9 - 1 9 0 - 3 .2 7 - 2 5 .9 - 1830 6 2 0 4 1 9 0 10 300 1660 110

399 Well PCH-8 6.78 9 3 - 1 1 2 - 1 .9 0 - 1 5 .1 - 1520 791 6 5 5 0 13 100 2 6 1 0 132

400 Well PCH-8 6.7 8 1 2 7 - 1 9 8 - 1 .8 3 - 1 3 .9 - 1500 825 6 5 7 0 13 500 2 3 2 0 132

401 Well PCH-9 6.78 7 6 - 1 2 1 - 6 .4 0 - 4 5 .4 - 35.9 9.3 337 452 67 185 402 Well PCH-9 6.78 1 5 3 -1 9 8 - 6 .4 0 - 4 5 .9 — IAEA-AG-158/9 1 2 3

TABLE II. CARBON CHEMISTRY

Code Name Sampling Water pH 5 13C 14C date temp. (°C ) (lab) (%■>) (pm c)

303 Well PHB-15 26.7.75 50 8.1 - 1 0 .6 9 2.12 ± 0.76

311 Well 37-23 21.7.75 32 7.7 - 1 0 .4 3 7 8 .96 ± 2 .4

315 Well 44-17 2 5 .7 .7 5 32 7.6 - 9 .8 0 8 1 .0 ± 2.4

318 Well 50-04 2 5 .7 .7 5 31 7.7 - 9 .3 9 77.9 ± 2 .5

(ii) On the contrary, before exploitation of groundwater commenced, the piezometric head of the confined aquifer may have been higher than the head of the upper one. If that was the case, an upwards flow has taken place since millenia and the isotopic composition of the upper aquifer should be influenced by this steady but small recharge.

The sampling was oriented to characterize the isotopic content of both aquifers as well as the content of the waters entering the area, that is, underground flow, local precipitation and the river Sonora.

Saline intrusion

To investigate the nature of the saline intrusion a simple linear model of mixing with sea-water is assumed, combining 180 and chloride. This allows the proportion of sea-water present in each sample to be estimated. Wells PCH-2 to 9 (samples 389 to 402) each have two independent tubes with screens at different depths.

3. RESULTS AND DISCUSSION

3.1. The problem of leakage

Isotopic content o f the deep aquifer

Figure 3 shows the 180 and deuterium content of all samples. A meteoric line through the jnean content of the upper aquifer has been drawn. The most depleted sample in heavy isotopes is from the deep aquifer. The delta values are quite distinct from those of the upper aquifer. As éxpected, the tritium content is below the detection limit and the 14C concentration is very low. 1 2 4 PAYNE et al.

óD%o

FIG.3. 180 and deuterium content o f all samples. The meteoric line passes through the mean content o f the upper aquifer. The delta values o f the upper aquifer are situated between those o f the deep aquifer and those o f the groundwater in the periphery.

Considering the confined nature of the aquifer, sample 303 (well PHB-15) can be taken as representative of the central part of the deep aquifer, located under the area where the water level of the upper aquifer has decreased markedly. Anyway, no more wells penetrating the deep aquifer exist in that area.

Age o f waters from the deep aquifer

The WATEQF Model [5] and ISOTOPE Subroutine [6] were applied to interpret the carbon chemistry. The lack of pH measurement in the field, as well as a knowledge of the partial pressure and 13C content of soil carbon dioxide in the recharge area make the model indicative rather than conclusive. Assuming a 5 13C of the soil carbon dioxide equal to - 1 8 %o„ which seems reasonable for arid regions [7], and a 6 13C equal to ~0.6%o for rock carbonates [8], the best fit between the measured 6 13C of sample 303 (-1 0 .6 9 %o) and the calculated (-10.10 %o) correspond to an assumed recharge pH of 7.5 and an assumed partial pressure of soil carbon dioxide equal to 10-215 atm. According to the model, the recharge water has a total dissolved biogenic carbon equal 3.50 mmoles and a 5 13C equal to -1 0 .8 2 %o. On the other hand, the laboratory pH of sample 303 is 8.1 and the total carbon content 3.76 mmoles. The corresponding partial pressure of carbon dioxide is 10-2-57 atm. It seems that 93% of the total carbon content of sample 303 is biogenic. That means that the 14C concentration has decreased mostly by radioactive IAEA-AG-158/9 1 2 5

FIG. 4. The stable isotope content o f the upper aquifer is shown in detail.

decay and not by dissolution of dead carbonate. The chemical composition supports this interpretation. Since the water is almost free of calcium and magnesium, the aquifer should be very poor in carbonate minerals. This agrees also with the volcanic nature of the rock. Taking into account the above considerations it is likely that water represented by sample 303 has an average age of about 30 thousand years. The lighter 5D and 5 180 of this sample as compared with samples from the upper aquifer support the palaeo-origin of waters from the confined aquifer, recharged during a colder and more humid climate than the present.

Isotopic and chemical content o f the upper aquifer

The mean 0 180 and 5D of all samples from the upper aquifer (numbers 304, 305, 307—351, 364—373) are the following:

5 180 = -6.58% o ±0.19 d = 4.9 (1) §D = -47.7%o ± 2.0

The stable isotope content is rather homogeneous (Fig.4). The standard deviation is twice the analytical error. Figure 5 is a graph of chloride vs 5 180 . Samples 3 64 —373 are missing because chemical analyses are not available. Two groups can be distinguished. 1 2 6 PAYNE et al.

Cl", O O o

o o o o o

OO

□ GROUP 1 O GROUP 2

л325 326

6180%

FIG.5. Chloride content vs 5 180 of samples from the upper aquifer. Group one represents waters from the central part and shows an homogeneous composition. Group two represents waters from the coastal areas arid shows an increasing chloride content.

The first group comprises 18 wells from the central and southern parts of the area (samples 307—321, 341, 349, 350). This group has a very homogeneous chemical and isotopic composition:

Ca2+= 32.8 ± 8.7 ppm СП =25.3 ±5.1 ppm § 180 = -6.69% o ±0.11 Mg2+= 7.6 ±4.5 ppm SO4- = 35.9 ± 7.0 ppm (2). 5D = -48.7%o ± 1.7 Na+ = 45.4 ± 3.4 ppm HCO3 = 172 ±12 ppm

Samples 365, 368, 371 and 373, located in the same area, have a similar stable isotope composition to that given in Eq.(2). Sample 372 is slightly enriched. The chloride content of these samples is not available. The tritium measurements obtained on this group (311,318, 365, 368, 371, 372, 373) show a content equal or less than 1 TU. The 13C and 14C concentrations of wells 311, 315 and 318 are similar. The 14C and tritium results are discussed later. IAEA-AG-158/9 1 2 7

Considering the location and the homogeneity in composition of the above samples, it is concluded that the mean values given in Eq.(2) characterize the upper aquifer. Samples 325 and 326, located in the north of the area, have a chemical composition similar to that of group one, but the stable isotope composition is slightly enriched in 180 . The second group (samples 322—340, 342—348, 351) shows an increase in chloride content. This group will be discussed under the topic of saline intrusion.

Iso topic composition o f recharge waters

The stable isotope content of the wells located outside the occurrence of the blue clay (374, 375, 378, 380, 381, 382, 383) show a more positive 6D and S180 in comparison to Eq.(2). The pairs of samples 37 4—382 and 3 7 8 —380 were obtained from the same wells on different dates. Sample 380 is enriched in heavy isotopes as compared with sample 378, along an evaporation line. This enrichment is probably a consequence of seepage from A. Rodriguez Reservoir. By contrast, the other pair shows no significant variation. Samples 376 and 385 were taken from well AP Aconchi and samples 377 and 384 from well AP Ures, on different dates. Both wells are located close to the river Sonora and most probably are recharged by it. The stable isotope content shows variations according to the date of sampling as would be expected in view of the high tritium, content. All these samples also have a higher content of deuterium or 180 than samples from the group defined by Eq.(2). Well 366, located further north, has a similar enrichment. These facts suggest again that waters flowing towards the centre of the area are enriched in deuterium and 180 as compared with waters from the centre.

Assessmen t o f the leakage

Water samples obtained outside the area overlying the blue clay and taken as representative of the inflow to the area show systematically a more positive 5D and 5 180 value in comparison to the mean value for that area (2), suggesting that, in fact, upwards flow through the blue clay occurs, since waters from the deep aquifer are characterized by a more negative delta value. Well PCH-4 (samples 391, 392), located to the south, has a stable isotope content between the figures given in (2) and those for the confined aquifer. Sample 391 was obtained from an horizon between 60 and 120 m and sample 392 between 144 and 196 m. Their isotopic content indicates that leakage from the confined aquifer is more marked in that zone. The flow through the blue clay has occurred before water development under steady-state conditions (cf. 2.2). If flow has taken place, the drawdown of the water level of the upper aquifer has obviously increased its amount. 1 2 8 PAYNE et al.

FIG. 6. Location of the sampling points.

Clppm

FIG. 7. The line represents a simple mixture between sea-water and water from the upper aquifer. The points are wells located in the coastal area and fit well in the mixing line. IAEA-AG-158/9 1 2 9

It is not possible to estim ate the actual am ount of leakage. A m ore careful and detailed sam pling w ould be needed for that purpose. Nevertheless, assum ing that sam ples 311, 315 and 318 are representative o f the water from the central part of the upper aquifer, it is certain that the proportion o f water from the confined aquifer cannot be m ore than 20 % in that central part, since the 14C content of these three sam ples is about 80 % and the 14C content of the deep aquifer is near zero. The tritium content of sam ples 311,315 and 318 (less than 1 T U ) excludes the presence of a significant am ount of water infiltrated after 1952 and, therefore, the contribution of 14C contents higher than 100%.

In the zone of the upper aquifer represented by well PCH -4 (391, 392) the proportion o f water from the deep aquifer seem s to be about 30 to 50%.

This well is located in a zone w here the blue clay is only a few m etres thick

(Figs 2 and 6).

3.2. The problem of saline intrusion

Figure 7 is a plot o f chloride vs <518O %0 of samples 322-340, 342-348,

351, 389, 390, 394-401, located nearer the coast. The hypothetical m ixing line betw een sea-water and fresh w ater is included. It is clear that the chloride increase is due to sea-water intrusion. Nevertheless, som e points should be com m ented upon. The wells PCH -6 (395, 396), PCH -7 (397, 398) and

PCH -8 (399, 400) show the greatest influence o f the saline intrusion; but well

PCH -5 (394) located in the sam e area and closer to the coast show s only a slight influence. This fact supports the opinion that sea-water is intruding into the zone by preferential channels. Both sam ples of well PCH -2 (389, 390) have a very sim ilar stable isotope content, but the chem ical com position is quite different. Probably the bottle 390 for chem ical analysis was m ixed up. Anyw ay, the sam ple 389 fits well in a sea-w ater/fresh-water m ixing line (Fig.7). Finally, well PC H -9 (401, 402) is slightly affected by sea-water intrusion.

4. CONCLUSIONS

4.1. The water of the deep confined aquifer represented by well PH B-15 (303)

m ost probably has an age of approxim ately 30 thousand years.

4.2. The stable isotope data support the occurrence of a flow from the deep

confined aquifer to the upper aquifer before water developm ent in the area.

4.3. It is not possible to estim ate the actual am ount o f leakage. Nevertheless,

according to the available 14C m easurem ents, the proportion of water arising

from leakage from the deeper aquifer, pum ped by the irrigation wells in the 1 3 0 PAYNE et al.

central area o f the upper aquifer, w as less than 20% at the tim e o f the sam pling (1975).

4.4. The available stable isotope data support the existence of sea-w ater intrusion in the south and in the area of K in o Bay.

ACKNOWLEDGEMENTS

The authors acknow ledge the co-operation of the follow ing dependencies of the form er W ater Resources Secretariat: Com m ittee for the Developm ent of the Resources of the Central and N orthern Basins of Sonora, The General

Managem ent of Sonora, and The Residence of G ehydrology in Sonora. The authors appreciate also the com m ents o f M r. Ignacio Sainz Ortiz, and the com m ents on the m anuscript by Gian M aria Zuppi.

REFERENCES

[1] SECRETARIA DE RECURSOS HIDRAULICOS, Estudio Hidrogeológico Preliminar de los Acuíferos de la Costa de Hermosillo, Sonora, México D.F. (1968). [2] SECRETARIA DE RECURSOS HIDRAULICOS, Boletín Hidrológico No.40, Región Hidrológica No.9, Sonora Sur, México D.F. (1970). [3] SECRETARIA DE RECURSOS HIDRAULICOS, Estudio Hidrogeológico Completo de los Acuíferos de la Costa de Hermosillo, Sonora, México D.F. (1970). [4] SECRETARIA DE RECURSOS HIDRAULICOS, Complemento del programa para establecer los medios de detección y control del avance de la intrusión salina en la Costa de Hermosillo, México D.F. (1976). [5] PLUMMER, N.L., JONES, B.F., TRUESDELL, A.H., WATEQF - A Fortran IV version of WATEQ, US Geol. Survey Water Resources Investigation (1976) 76—13. [6] REARDON, E.J., FRITZ, P., Computer modelling of groundwater 13C and 14C isotope compositions, J. Hydrol. 36 (1978) 201—24. [7] DEINES, P., LANGMUIR, D., HARMON, R.S., Stable carbon isotopes to indicate the presence or absence of a gas phase in the evolution of carbonate groundwater, Geochim. Cosmochim. Acta. 38 (1975) 1147—64. [8] HOEFS, J., Stable Isotope Geochemistry, Springer Verlag ( 1973). FIELD INVESTIGATIONS ON GROUNDWATER ORIGIN AND FLOW PATTERNS

IAEA-AG-158/10

UTILIZATION OF NATURAL ISOTOPES IN THE STUDY OF SALINATION OF THE WATERS IN THE PAJEÚ RIVER VALLEY, NORTHEAST BRAZIL

E. SALATI, E. M ATSU I

Centro de Energía Nuclear na

Agricultura (CEN A),

Piracicaba, Brazil

J . M . L E A L

Superintendencia do Desenvolvim ento

do Nordeste,

Recife, Brazil

P . F R I T Z *

University o f W aterloo, W aterloo,

Ontario, Canada

Abstract

UTILIZATION OF NATURAL ISOTOPES IN THE STUDY OF SALINATION OF THE WATERS IN THE PAJEÚ RIVER VALLEY, NORTHEAST BRAZIL. Municipal and agricultural water supplies in the vast regions on the Brazilian Shield come primarily from shallow groundwater systems in weathered bedrock and minor amounts of alluvial fill. The water quality is generally poor. This salination of water supplies is a serious problem because no management schemes have been proposed to solve water shortages and to improve water quality. In this study it is shown that salination is an active process which is not related to the release of fossil sea-water trapped in the crystalline rocks since Cretaceous times, nor is it related to intensive weathering of the bedrock. Evaporation and évapotranspiration must account for it with the salt being brought primarily by precipitations. It is suggested that increased pumping of these shallow aquifers during the dry season will not result in a general improvement of water quality, thus limiting the potential of these aquifers to their present usage.

INTRODUCTION

The silicate m inerals w hich com pose the bulk of crystalline and m etam orphic rocks do not dissolve very fast and, therefore, shallow “aquifers” w ithin such rocks usually yield lim ited am ounts of good quality groundw aters suitable for hum an consum ption and agricultural purposes. How ever, throughout the vast

* UNDP/IAEA/CENA PROJECT/BRA/71/556.

1 3 3 FIG.l. Location map. IAEA-AG-158/10 1 3 5

northeastern region of Brazil shallow groundw aters in crystalline or m etam orphic rocks are often so highly m ineralized that they are acceptable only to anim als.

O nly occasionally is som e better-quality w ater recognized despite the installation o f thousands o f wells during the past decades. This task was prim arily carried out by the Division of H ydrogeology of the Superintendência do Desenvolvim ento do Nordeste (SU D E N E ) w ith the objective not only to find good quality water but also to study the causes for the salination of these groundw aters and to propose

possible solutions leading to a gradual im provem ent in quality of these water

s u p p l i e s .

A report on the water quality and origin o f salt in these w aters w as prepared

by Schoff [ 1 ]. In it, he cam e to the follow ing conclusion:

“It is suggested that the ground w ater contains som e residual sea w ater,

introduced during an U pper C retaceous m arine invasion o f the land, and that this

w ater has been diluted by m eteoric w ater and largely flushed out o f the rocks in

a process that is still incom plete but is continuing. The process has operated

unevenly, in accord w ith diverse rates o f circulation o f w ater in fracture system s

having a w ide variety o f hydraulic characteristics. Thus, the m ineralized and the

dilute w aters are interspersed haphazardly through the region.

A ny action by m an that w ould accelerate the dilution o f the m ineralized

ground w ater with fresh w ater or rem ove it from the rocks w ould tend in the

sam e direction as the natural process, but m an’s efforts are unlikely to accom plish

large observable im provem ents in w ater quality, except perhaps locally. Planned

pum ping o f a w ell over a period o f years m ight w ithdraw enough m ineralized

w ater from a sm all system o f fractures and prom ote the infiltration o f enough

fresh w ater to create w orthw hile changes in chem ical quality. "

Others, as quoted by Schoff [1 ], considered the dissolution o f m icas and

feldspars as the principal source o f m ineralization and all agree that an im prove­

m ent in water quality w ould be achieved if the wells were pum ped.

In this paper, we again analyse the possible reasons for tthe salination o f the

groundw aters, using environm ental isotopes as a basic tool.

The study was carried out w ith U N D P -IA E A support in the Pajeú River Basin

w hich is located in the sem i-arid region o f the State of Pernam buco, betw een

7°30' — 9°0' látitude south and 37°60' — 39°0' longitude west; it covers about

1 7 000 km 2, the m ain channel length is about 250 km and discharges into the

Sao Francisco River (Fig. 1). This research is a continuation of the w ork initiated

by Salati et al. [2] w ho studièd regional hydrogeological problem s using isotope

techniques.

A sem i-arid clim ate (type Bsh in the Koppen classification) is predom inant

in the greatest part o f the Pajeû River valley w ith high tem peratures, little

nebulosity and low relative hum idity. How ever, in the higher head-w ater regions

o f the rivers, the clim ate reaches A w (Koppen). Precipitation decreases along the

course of the Pajeú River from values between 800— 1000 m m in the head-waters 1 3 6 SALATI et al.

to less than 400 m m at the m outh. Rainfall occurs prim arily between Decem ber and June w ith a m axim um during February-M arch of each year.

The Pajeû River Basin is alm ost entirely underlain by Precam brian crystalline rocks w hich consist of a m ixture of gneiss, granite, m igm atites, quartzites, m inor am phibolites, som e m arble and schists. M inor occurrences of possibly Palaeozoic and M esozoic sedim entary form ations are recognized and often associated in tectonic, graben-like structures. Extensive, but shallow alluvial deposits occur also along the river beds and are derived alm ost entirely from the crystalline rocks.

The tributaries of the Pajeú River flow interm ittently, w ith abrupt changes at the beginning of the rainy season and becom ing practically dry tw o or three w eeks after the rains are over. Thus, for exam ple, the discharge from the m ain tributary - the Navio River - varies from 0 to 250 m 3/s over a period of only six days. A s a result, the flow of the Pajeú River is also interm ittent and in a m atter of a few days its discharge can change from near 0 t o a b o u t > 1 0 0 0 m 3/ s .

M ore than 400 w ells have been drilled in the Pajeú Basin, preferentially in crystalline regions w ith large dem ographic density; m ean depth and yield being

30 m and 366 ltr-h_ 1 -m _1, respectively. These w ells have six- or eight-inch diam eters and only their upperm ost part is cased, usually to less than 3 m depth.

Sam ples from precipitations, w ells and rivers have been collected for isotope and chem ical analyses between 1974 and 1977. The results are discussed and an attem pt is m ade to com m ent on present water m anagem ent schemes. The general conclusions obtained from Pajeú River are also valid for m ost other crystalline basins on the shield o f northeast Brazil.

1. EXPERIMENTAL METHODS

1.1. Field m ethods

Precipitation sam ples were collected m ostly at perm anent w eather stations.

M onthly integrated sam ples were obtained by collection under paraffin oil in a

funnel-and-bucket arrangem ent.

W ater from wells was sam pled as close as possible to the pum p w hich was either w ind- or gas-pow ered. How ever, at several stations sam ples could only be obtained from storage reservoirs. These reservoirs (caixa d ’agua) are only partially covered and during the dry season significant evaporation could occur. N ot enough attention had been paid to this during the earliest phase o f this project but w as corrected as soon as it w as recognized.

River sam ples were taken at specified locations during each sam pling trip.

A ll sam ples were shipped im m ediately after collection to the laboratories at

CENA. IAEA-AG-158/10 1 3 7

YEAR FIG.2. Precipitation at Afogados da lagazeira.

N o special sam ples were collected for chem ical analyses but som e constituents were determ ined on portions of the isotope sam ples.

1.2. Laboratory m easurem ents

A ll sam ples were analysed for 180, deuterium and C l~ and som e for tritium concentrations.

Measurem ent of 180 and D were m ade at C EN A , using classical m ethods for sam ple preparation and m ass spectrom etry [3— 5]. The results are expressed in the conventional 6%o notation and refer to the standard SM O W . Repeats on sam ples and standards indicate the overall analytical error is below ± 0.2%o f o r 180 a n d ± 2 %o for deuterium determ inations. Tritium analyses were done by gas counting on C H 4 by the laboratories of the International Atom ic Energy Agency and by the Instituto de Pesquisas Radio­ activas at Belo H orizonte. The results are expressed in tritium units (TU ).

2. RESULTS AND DISCUSSION

2.1. Precipitation

O ne of the m ain characteristics o f the yearly precipitation in the region is its great variability. Variations of up to a factor of 3 have been observed from one year to the other during the short study period. A longer series of precipitation data from the station at A fogados da Ingazeira in the Basin is show n in Fig. 2. 1 3 8 SALATI et al.

8 1е0% 0 (SMOW)

FIG.3. 180 and deuterium in precipitation in the Pajeú River Basin. The regression line (8 D = 6.4 6180 + 5.5j considers only precipitation events exceeding 50 mm/month (A = 0). Months with precipitation below 50 mm/month are shows as small dots (B = . ).

FIG.4. Seasonal variations of 5 180 and rainfall at the Betânia sampling stations. IAEA-AG-158/10 1 3 9

A ll 180 and deuterium data obtained from stations w ithin the Pajeú River

Basin are show n in Fig. 3. W eighted m onthly averages for precipitations exceeding

50 m m during the m onthly sam pling intervals are represented by open circles,

those w ith less are given as sm all dots. A lso show n are w eighted averages for the entire study period ( 19 74 -1 97 7) at the five m ain sam pling stations. The regression

Une was obtained by considering only the m onthly data from periods w ith m ore

than 50 m m rainfall. It thus represents the “local m eteoric w aterline” for w hich

ÔD = 6.4 ô 180 + 5.5

These data also indicate that a decrease in 180 and deuterium w ith altitude

exists, w hich is probably the result o f a com bination o f altitude and continental

effects and leads to about -0 .6 % o change in 180 per 100 m rise. This is only an

approxim ate value, how ever, but its existence is significant because, as show n

below , a sim ilar change is observed in the groundw aters.

Seasonal effects are very pronounced w hereby rains at the beginning and

follow ing the rain season have, on the average, higher heavy isotope contents

than the m ore im portant precipitations o f the rain season. This is dem onstrated in Fig. 4 w hich presents average m onthly precipitation and 5 180 data from Betânia

sam pling stations for the entire sam pling period. It is interesting to note that m inim um and m axim um 6 180 values are delayed w ith respect to m axim um and

m inim um rainfalls, indicating that existing am ount effects alone do not control

the distribution of 180 during the seasons. How ever, because the average annual

tem perature is alw ays above 20°C and the am plitude does not exceed 5°C w ith the hottest m onth being Decem ber and the coldest July, the observed isotope variations are also not sim ply a function of tem perature. A num ber of factors com bine to result in the observed trends and it is thus not surprising to observe very significant differences in both rainfall and isotopic com position from one year to another.

The basin-w ide average o f 5 180 values is close to — 1.5%o during the dry season (June to October) and — 4.0 %o during the rainy season (January to April).

The w eighted annual averages for the entire basin are

S180 = -2.9%o and 5D = -12.0%o.

2.2. Surface waters

2.2.1. Rivers

The isótopic com position of m any river system s is controlled by the inter­

actions between direct surface runoff and groundw ater discharges. The m ore

significant the latter the m ore constant is not only the flow w ithin the river but 1 4 0 SALATI et al.

er (p pm )

1977 FIG.5. The 180 and chloride 'contents and the discharge of the Pajeú River during the rainy season of 1977.

FIG. 6. The comparison of 180 and deuterium data shows that evaporation plays a significant role in the water budget of the rivers in the Pajeú River Basin. Data are for Pajeú River. IAEA-AG-158/10 141

also the isotopic com position of its waters. It is im portant to rem em ber that in this drainage basin m ost rivers go to surficial dryness or com e very close. Because o f the absence o f a significant base-flow com ponent it is not surprising to note very significant seasonal variations of their isotopic com positions, w hich approxi­ m ate the seasonal precipitation values even in these extrem es. This is show n

clearly for an extensive sam pling period during the 1977 rain season and the

m onths follow ing it (Fig. 5). The low est values occur at the beginning of M ay

when the 5180 = ~ l % o approach the low est precipitation values recorded for

the preceding rainy season. Thereafter, both increasing evaporation and increasing heavy isotope contents in the precipitations cause a gradual increase in ô 180 a n d

ôD values and ô 180 values as high as + 1 1 % o have been recorded during the dry

s e a s o n .

Evaporation plays a significant role in this enrichm ent as evidenced by the

180 and D data from all river sam ples plotted in Fig. 6 , w hich show that m ost of

them are to the right of the local m eteoric water line. A separation betw een rainy

season and dry season rains (Fig. 6 ) enhances this picture. How ever, not all creeks

and stream s draining tow ards the R io Pajeû behave in the sam e m anner, and

significant differences are observed. Fo r exam ple, Riacho Triunfo m aintained

an average S 180 value close to — 3.2 % o w ith only about 2 % c seasonal variations,

w hich is a strong indication that in its flow m ore than one of the other stream s

has a significant m ore or less perennial base-flow com ponent. R io Pajeú, being

the m ain drainage channel in the basin, is “average” in its behaviour but the other

extrem e is the sm all Caicarinha Creek w hich has an average 5180 value o f about

— 0 . 8 % o w ith seasonal variations between — 3.3 and +2.8%o, or Riacho do M eio

w hose average 5 180 value is +0.3%o w ith variations between — 2.9 and +8.2%o.

T his very variable behaviour o f stream s w ithin the sam e drainage basin is a strong indication that no m ajor regional groundw ater flow s exist in this region.

The hydrogeology is dom inated by sm all local groundw ater system s w hich in general respond quickly to m ajor precipitation events because o f their lim ited storage capacity.

This also has a profound influence on the salt balance of this basin. Figure 5 show s the dependence o f salt concentrations in the river w aters on precipitation events and a clear parallelism betw een salinity and isotopic com position. The low est salinity w ith only 44 ppm C l- is reached sim ultaneously w ith the low est

S 180 values of — 6.7%o. Im portant to note is that at m axim um river discharge the salinity w as higher (88 ppm ), w hich indicates that surface and, possibly m ore im portant, subsurface discharges aided in the rem oval o f salt from the basin.

2.2.2. Dams

Attem pts have been m ade to retain som e of the rainy season runoff in dam s;

how ever, w ith variable success. A recent study by Stolf et al. [ 6 ] o f a dam w hich 1 4 2 SALATI et al.

er (ppm )

ci- MASS (ton).

2 0 0 180 ■ 160

■ 140 120

FIG. 7. The isotopic and chemical behaviour of the QUEBRA-Unhas dam. Note that the salt load in the dam decreases during the dry season, which indicates that a subsurface discharge does exist. A similar conclusion is found from isotope data (Stolf et al. [6]/

contains, below a surface area o f 1.02 km 2, approxim ately 3.2 X 10 6 m 3 w a t e r

dem onstrates that its survival as an acceptable water supply is only guaranteed if a rem oval o f the salt accum ulating during the dry season is possible. Such is the case in the investigated Q uebra-U nhas dam (Fig. 1 ) and a response pattern is

show n in Fig. 7. A com parison of 180 and deuterium contents show s that increases in salinity and heavy isotope content are alm ost exclusively due to evaporation,

but these data also show that approxim ately 30 % of the water is lost through

subsurface outflow , thus rem oving sufficient salt to m aintain an essentially

stationary situation — the dam w as constructed in 1934 yet its salinity rarely

exceeds a few hundred ppm in total dissolved solids (or 150 ppm C l- ).

M ass balance calculations based on isotopes and chem istry strongly indicate

that virtually all chlorine arriving in the dam has an atm ospheric origin and know ing

the size o f the drainage basin, it can be estim ated that the overall atm ospheric C l-

contribution am ounts to about 3.2 X 1 0 "4 k g -2 a -1 w ith the rain having about

1 ppm C l- . This com pares very well w ith the m easurem ents m ade on rain-water

between 1974 and 1977 where a range between 0.7 and 5 ppm C l- was found. FIG.8. Typical isotopic and chemical response pattern o f wells in the study area. study the in wells f o pattern response chemical and isotopic Typical FIG.8. 8180%o SMOW I.. l80 andFIG.9. deuterium in groundwaters. 1974 4 7 9 1 AEA-AG-158/10 1 / 8 5 1 - G A - A E IA 8 W O M S o % 180 8 975 7 19 5 7 9 1 - 0 0 0 (-2 0 L 0 0 0 1 •

С Г р р т 4 SALATIetal. T A L A S 144

TABLE I. AVERAGE VALUES OF 5180, ÔD, (С Г) AND T OF WATER SAMPLES COLLECTED THROUGHOUT 1974 TO 1977 FROM TH E W ELLS

Location Standard Number of Standard Number o f Standard Num ber o f Tritium range (Cl ) o f wells 5 i 80 % o SMOW deviation analysed 6 D%« SMOW deviation analysed deviation analysed 1 9 7 4 - 1 9 7 7 (ppm ) (F ig -1 ) (%») samples (%») samples (ppm ) samples (TU)

Itapetim - 2 . 6 0.5 2 2 - 1 6 3.1 19 812 2 1 4 15

Sao José do Egito - 3 . 4 0.3 12 - 1 8 1.7 7 5 86 63 5

Riacho do Meio - 3 .1 0.5 18 - 1 6 2.3 16 7 9 0 140 1 0

Tuparetam a - 2 . 7 0 .6 19 - 1 4 2 .6 15 1748 3 0 0 13

Campos Novos 1 - 3 . 7 0.5 16 - 1 8 2 .4 1 2 194 77 1 0

Campos Novos 2 - 3 . 0 0.4 15 - 1 7 2.1 11 1867 196 10

Afogados da Ingazeira 1 - 3 . 0 0 .2 9 - 1 7 2.1 5 6 5 4 361 8

Afogados da Ingazeira 2 - 3 . 7 0.4 9 - 1 9 1.9 5 190 4 2 3

Quixaba - 2 . 8 0.9 14 - 1 3 4 .7 11 176 41 10

Triunfo - 3 . 2 0 .8 19 - 1 4 4 .8 13 4 0 14 13

Calumbi - 3 . 0 0.5 11 - 1 4 2.7 8 4 3 4 2 44 11

Caiçarinha - 2 . 5 1.1 9 - 1 7 3.5 8 983 2 4 4 12

Bom Nome 1 - 3 . 4 0 .4 4 - 2 0 1.5 3 2 2 4 27 6

Bom Nome 2 - 4 . 6 0.3 9 - 2 7 2.4 4 33 8 6

Tupanaci - 3 . 4 0 .2 9 - 1 9 6 .4 3 2 8 2 11 11

Carqueja - 1 . 6 0.3 1 2 - 1 0 3.1 6 1838 40 2 1 0 4.4 to 12.0

Camaubeira - 3 .5 0.9 5 5 2 4 119 7 T A B L E I. co nt.

Location Standard Number o f Standard N um ber o f Standard N umber of Tritium range (СП o f wells 5 ISWoo SMOW deviation analysed 5D%a SMOW deviation analysed deviation analysed 1 9 7 4 -1 9 7 7 -1S8/10 G -A EA IA (ppm ) (Pig-1) (%») samples (%«) samples (ppm ) samples (TU)

Betânia + 0 .3 0.4 23 - 1 3.7 1 0 ' 1039 174 18 2.1 to 9.4

Poço do Pau - 3 .1 0 .4 21 - 1 8 3.0 16 805 6 4 2 18 2.2 to 14.0

Lage das Pombas - 1 .5 0 .8 24 - 1 1 5.5 15 1064 4 9 3 18 5.7 to 11.3

Airi - 2 . 4 0 .6 2 0 - 1 4 4 .4 11 707 154 16 2.1 to 8.7

V arjota - 2 . 4 0 .6 2 0 - 1 3 3.0 13 158 65 15 2.5 to 14.0

L/Ï 1 4 6 SALATI et al.

2.3. Groundwaters

The average 5 180 content of the groundw ater collected at the various sites

is show n in Fig. 1 and show s that a sm all but significant spatial variation is

recognizable: w ells located at higher altitudes at the edges o f the drainage basin

tend to have low er 180 and deuterium contents than those in the low er part.

It is notew orthy that all w ells exhibit seasonal variations in chem istry and

isotopic com positions. Typical variations for the least and the m ost saline w ells

studied in this basin are show n in Fig. 8. This behaviour em phasizes that the

different groundw ater system s exploited here are not interconnected but supplied

locally. Further evidence com es from a com parison of 180 and deuterium data

(Fig. 9), w hich show s that m any wells contain evaporated water. T his is especially

true for those wells w hich are near river banks and are probably tapping river-

connected aquifers.

Tritium analyses have been m ade on a num ber o f wells in the drainage basin and again one notes very significant variations (Table I). N o attem pts have been

m ade to interpret these concentrations in term s o f residence tim es or w ater ages

although it is evident that m ost if not all w ater in these system s is very young.

This agrees w ith the finding of G eyh and Kreysing [7], w ho were able to determ ine

for groundw ater in a sim ilar basin approxim ate residence tim es by com paring 14C

and tritium data, and found in virtually all system s residence tim es to be m uch less

than 100 years. There is thus no evidence to suggest that fossil sea-w ater is still present and responsible for .the salination o f these groundw aters.

W ater-level changes in these aquifer system s were m onitored by S U D E N E at

a few wells inside this drainage basin (Fig. 10). How ever, lithological and physio­

graphic sim ilarities should present a direct com parison from one locality to another.

Thus, a well m onitored at Betânia show s a very rapid and pronounced response to

precipitation events, w hich is taken as an indication that recharge has occurred.

This assum ption is further supported by salinity variations, although the chem ical

response o f these system s is not exactly the sam e as the one observed for water-

level changes. W ith the beginning o f the rainy season, w hen water levels begin to

rise, the salinity o f the groundw aters sam pled in the w ells does not decrease but

show s - som etim es with som e delay — a m arked increase. This is show n in Fig. 10

w hich com pares the changes in salinity from all wells (expressed as ppm C l- )

w ith precipitation events at Betânia. O n ly follow ing the very m ajor rainfalls

from January to A pril does one observe a significant dilution.

O ne m ust rem em ber that these wells have virtually no casing and the water

sam pled w ill not necessarily reflect the average chem ical or isotopic characteristics

o f the aquifers. How ever, the sim ilarity o f response o f m ost w ells perm its at least

an attem pt at interpretation. The significant seasonal variations in isotopic

com position and salinity (represented by C F analyses) indicate that these aquifers

are largely unconfined and that local rather than regional hydrological phenom ena IAEA-AG-158/10 1 4 7

FIG.10 A. Salinity changes in the wells o f the project area. All wells for which data were available are shown. The numbers refer to the following wells: 1 = Itapetim; 2 = Riacho do Meio; 3 = Tuparetama; 4 = Campos Novos 1; 5 = Triunfo; 6 = Calumbi; 7 = Tupanaci; 8 = Carqueja; 9 = Faz. Poço do Pau; 10 = Lage das Pombas; 11 = Airi; 12= Varjota.

FIG.10 B. The regression line indicates that evaporation has affected the groundwaters in the study area. Water-level changes in a shallow groundwater system in response to precipitation events.

control their response. The high tritium contents show that, at least to the depth

sam pled, the residence tim es are rather short. This m ay have been enhanced by

the exploration of these aquifers and — as pointed out above — it w as even argued

that an increased w ithdraw al of water m ay enhance recharge during the rainy

season and thus im prove the water quality.

O ur data show that, although the m agnitude of recharge m ay have increased,

the w ater quality did not im prove. W e suggest that this is the result o f the water

usage patterns — the drilled wells are significantly pum ped during the dry season

and m ay tap salt water w hich was not reached before by earlier hand-dug wells. 1 4 8 SALATI et al.

Thus, an increased am ount of water and salt are brought to the surface. How ever,

because no w ater leaves the local area during the dry season, all salts w ill stay behind. They are highly soluble and w ill be m obilized w ith the onset of the rainy season. A t that tim e infiltration occurs and the salts are sim ply flushed back into

the subsurface, causing a tem porary increase in salinity.

Subsequently, w ith the continuation o f the rainy season freshw ater losses

w ill build up over the deeper salt water and at that tim e surface discharges w ill

also becom e active. This should be the tim e w hen the salts are flushed from the

basin but, because little or no base flow is added to the stream s, the salt w ater

in the deeper aquifers is now essentially im m obile. W ells are now little used or

w ill draw fresh water m ixed w ith som e salt water. The low salinity of the Pajeú

River during m axim um discharge (88 ppm C F vs an average of close to 700 ppm ’

C l- for the well w aters) (Table I) also attests to the absence o f a significant base-

flow com ponent.

3. COMMENTS ON THE SALT BUDGET OF THE PAJEÚ RIVER BASIN

The data accum ulated in the course of these investigations [ 6 , 8 ] and infor­ m ation gathered and sum m arized elsewhere [1, 9— 12] lead to the conclusion that chloride and m ost other dissolved constituents of these groundw aters do not originate from the w eathering of rocks and dissolution of m inerals. Instead they can be explained largely through atm ospheric contributions and m odifications

of this atm ospheric load through subsurface rock-w ater interactions.

The isotopic characteristics of the well w aters strongly indicate that no

fossil sea-w ater is present and that the residence tim es of w ater in these shallow

system s are relatively short. The aquifers are largely unconfined and respond rapidly to atm ospheric inputs.

The absence o f sea-water does not a priori preclude the presence o f residual

sea salt, especially if one evokes local recirculation o f accum ulated salts.

How ever, if one consideres that 98 % o f all water is lost by evaporation and

évapotranspiration then such m echanism s could also account for the high salinities

sim ply through a progressive enrichm ent through water loss. N o significant export

of plant m aterial occurs from these local basins and, therefore, even salt accum u­

lated w ithin plant m atter, and thus tem porarily w ithdraw n from the aqueous

reservoir, w ill also eventually return to the subsurface.

This has serious consequences because, if one accepts a continuous input

through precipitations as béing the dom inant reason for the presence o f salt in

these basins, then any flushing through pum ping and w ater-quality im provem ent

through enhanced recharge becom es m uch m ore difficult than if one had to deal

w ith a finite salt reservoir. Fo r this reason a com parison o f atm ospheric input

and salt discharge through the Pajeú River becom es very interesting. Such a salt IAEA-AG-158/10 1 4 9

budget is unfortunately not an easy task — the great variability of the am ount of

annual precipitations can cause in one year the low areas o f the valley to be

flooded and w ill in another year hardly saturate the soil. T his necessitates long

m easuring periods o f both discharge and salt load both of w hich are available

only for 1976 and 1977.

For the water budget we have to assum e that no subsurface discharge occurs

from the basin and that the surface runoff through the Pajeú River accounts for

the total salt rem oval from the basin. In the light o f the data presented above this

appears to be a reasonable assum ption. U nder these conditions the salt budget

o f the basin is described by

P C p = R C r

were P and R are precipitation and river outflow and C p and C r the corresponding

salt concentrations. C p varies between 0.7 and 3 m g/ltr and averages about

1.9 mg/ltr, whereas during the 1977 runoff season C r varied between 35 and

145 m g/ltr averaging 84 m g/ltr. Thus, the ratio o f salt contents for 1977 is

C p/Cr = 0.02

Under the above conditions this w ould have to be equal to R/P w hich indeed for

1976 was ~0.02 and for 1977 was ~0.05. The sim ilarity of the salt content and

w ater-m ass ratios is an indication that this basin is essentially at a steady state w ith respect to its salt budget.

The tim e required to reach this state w ill depend on the groundw ater reservoirs

and the hydrodynam ics o f the basin. It is difficult to estim ate this although it is

fairly safe to assum e that it w ould not take long, especially since virtually all groundw ater is found in very shallow system s.

CONCLUSIONS

Saline groundw ater in crystalline rocks is not unusual and can be generated

by a num ber of processes. Its presence is, how ever, of m ajor concern if it is found

in shallow aquifers in arid or sem i-arid environm ents. It is often the only source

o f water for the population living in these areas and attem pts have to be m ade

to understand the reasons for the salination of these groundw aters and to propose

rem edies based on hydrogeological, geochem ical, isotopic and geophysical con­

siderations.

In this study the Pajeú River Basin in N E Brazil w as investigated w ith isotopic

and geochem ical m ethods. Com bined with inform ation obtained during preview

investigations it can be show n that virtually all groúndw aters are found in shallow 1 5 0 SALATI et al.

aquifers w ithin w eathered or fractured crystalline bedrock or the shallow alluvial fills derived from it. Their w ater supplies depend on local recharge events w hich m ay occur in the form o f precipitation or river recharge. There is no evidence that regional flow system s are of im portance and even if they exist they cannot yield significant am ounts o f water because it w ould have to be transm itted prim arily through fracture system s in the rock m ass.

The origin of the salt in these groundw aters is m ost probably atm ospheric.

Therefore, the salt reservoir is virtually unlim ited and schem es designed to im prove the groundw ater quality will have to take this into account. M ost previous investigators have assum ed that this reservoir was lim ited and that pum ping w ould rem ove the salt from the groundw ater and lead to the replacem ents of the salty groundw ater by fresh water. W e do not entirely agree w ith this concept.

The seasonal changes o f salinity in the pum ped well water indicate that, w ith the onset o f the rainy season, salts that have accum ulated at the surface ow ing to atm ospheric fallout, irrigation w ith m ore or less salty groundw ater and dom estic practices w ill be flushed into the subsurface rather than flushed from the basin. D uring the latter part o f the rainy season surface runoff does occur and m inor am ounts of salt w ill be rem oved but m ost seem s to rem ain practically im m obile in the subsurface. T o achieve a salt balance w ith a net loss from the basin, one w ould have to pum p salt water during the rainy season into the surface drainage system s and thus ensure its discharge.

This concept has not been verified, and detailed hydrogeological and geo­ chem ical studies w ould be required in order to do so. The rem oteness o f the area has so far prevented the execution of such a program m e. Furtherm ore, it m ay be im practical to execute such pum ping schem es on a large scale and, if so, the ultim ate conclusion w ould inevitably be that any . further m obilization o f deeper ground­ waters through additional w ell installations w ill probably not im prove the situation but lead to an even greater salt loading in the basin, or at least a m ore even salt distribution w hich could m ean a slight am elioration of the water quality of the deeper groundw ater and a gradual deterioration of the m ore shallow resources.

ACKNOWLEDGEMENTS

Parts of this project have been carried out w ith the financial assistance o f the International Atom ic Energy Agency. W e w ish to express our sincere thanks for this help and gratefully acknow ledge the constructive input from

Dr. B.R. Payne and the staff of the Section for Isotope H ydrology. Thanks are also due to Dr. J. G at w ho visited Brazil at the sam e tim e P.F. w as there under an

IA E A technical assistance program m e and w ith w hom m any aspects uf this project were discussed. Drafting was done by Mrs. M . Maziarz, Departm ent of

Earth Science, U niversity of W aterloo. IAEA-AG-158/10 151

REFERENCES

[1] SCHOFF, S.L., Origin of mineralized water in Precambrian rocks of the upper Paraiba Basin, Paraiba, Brazil, Geol. Survey water-supply paper 1663-H, USA (1972) 92 pp. [2] SALATI, E., LEAL, J.M., CAMPOS, M.M., “Environmental isotopes used in a hydro- geological study of northeastern Brazil,” Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp. Vienna, 1974) 1, IAEA Vienna (1974) 259—83. [3] EPSTEIN, S., MAYEDA, T., Variations of 180 content of waters from natural sources, Geochim. Cosmochim. Acta 4 (1953) 213—24. [4] McKINNEY, C.R., McCREA, J.M., EPSTEIN, S., ALLEN, H.A., UREY, H.C., Improvements in mass-spectrometry for the measurement of small differences in isotopic ratios, Rev. Sei. Instrum. 21 (1950) 724—30. [5] FRIEDMAN, I., Deuterium content of natural waters and other substances, Geochim. Cosmochim. Acta 4 (1953) 89—107. [6] STOLF, R., LEAL, J.M., FRITZ, P., SALATI, E., “Water budget of a dam in the semi-arid region of the northeast of Brazil based on oxygen-18 and chlorine contents,” Isotopes in Lake Studies (Proc. Advisory Group Meeting Vienna, 1977), IAEA, Vienna (1979) 57. [7] GEYH, H.A., KREYSING, K., Sobre a idade das aguas subterráneas no polígono das secas do nordeste Brasileiro, Rev. Bras. Geosci. 3 (1973) 53-59. [8] MATSUI, E., Origem e dinamica de salinaz acad da aqua do Nordeste Brasileiro, Ph.D. thesis, Univ. of Sao Paolo, Piracicaba, S.P. May 1978. [9] LEAL, J.M., Estudo Geológico e Hidrogeológico da Bacia Hidrográfica do Rio Pajeú, SUDENE, Recife (1966) 23 pp. [10] CRUZ, W.B., MELO, F.A.F., Estudo Geoquímico preliminar das águas subterráneas do Nordeste do Brasil, Série: Brasil, SUDENE, Hidrogeol. 19 (1968) 147. [11] REBOUCAS, A.C., MARINHO, M.E., Hidrología das Secas, Série: Brasil, SUDENE, Hidrogeol. 40(1972) 126. [12] REBOUCAS, A.C., Le problème de l’eau dans la zone semi-aride du Brésil, DSc. Thèse présentée à l’Université Louis Pasteur de Strasbourg (1973) 291 pp.

IAEA-AG-158/11

ISOTOPE INVESTIGATIONS AS A TOOL FOR REGIONAL HYDROGEOLOGICAL STUDIES IN THE LIBYAN ARAB JAMAHIRIYA

D. SRD O t, Adela SLIEPÍEV IC ,

В. OBELIC, Nada HO RVATINÖ IC

Ruder BoSkovic Institute,

Zagreb, Croatia, Yugoslavia

H. M OSER, W. STICH LER

Institut für Radiohydrom etrie der

Gesellschaft für Strahlen- und

Um weltforschung M bH, Munich,

Federal Republic of Germ any

Abstract

ISOTOPE INVESTIGATIONS AS A TOOL FOR REGIONAL HYDROGEOLOGICAL STUDIES IN THE LIBYAN ARAB JAMAHIRIYA. 14 C-, SH and stable isotope analyses were done on Libyan waters from wells in the Wadi Sawfajjin-Wadi Zamzam-Al Jufrah area during a complex hydrogeological survey. Water from different aquifers can be differentiated by their isotope content. The results generally show I4C ages older than 12 000 years with increasing groundwater age going towards the east and southeast. The hydrogeological concept of a continuous flow from the Palaeozoic aquifer through the Chicla aquifer towards the Mediterranean Sea is confirmed by the I4C measurements. This means that the Socna aquifer is recharged from the Palaeozoic aquifer, and the Eocene aquifer from the Chicla aquifer. The stable isotope content sorts the waters of the investigated aquifers into two groups. The relatively high 2H- and 180-contents of the Upper Cretaceous aquifers and the Chicla aquifer indicate a possible recharge during pluvial times from Jebel Nefusa. Low 2H- and 180-contents of the Palaeozoic, Chicla, Eocene and, in particular, the Socna aquifer, confirm the recharge of the Socna aquifer from deeper aquifers and show that the Palaeozoic and Chicla aquifers belong to an interconnected underground system.

INTRODUCTION

The Consulting Com pany, “Energoprojekt”, Belgrade, carried out an extensive study for the Libyan W ater Authorities in the W adi Sawfajjin-W adi Zam zam -Al

Jufrah area (32°45' to 29°00' north, 12°00' to 16° 15' east) (Fig.l) to determ ine the m ost prom ising areas for groundw ater developm ent w ithin the region, to evaluate existing groundw ater potentials and conditions for their exploitation,

1 5 3 1 5 4 SRDOC et al.

FIG.l. Map o f the area under investigation and o f sampling points with sample Nos (see Table II].

and to initiate som e projects o f groundw ater developm ent. Fo r these purposes,

a com plex hydrogeological survey was carried out including hydrogeological

m apping, a geophysical survey, exploratory drilling, test pum ping, water-level

observations, and chem ical and isotopic analyses o f water. A ll results are to be

found in the Final Report [ 1 ] o f Energoprojekt. This paper sum m arizes the results

obtained from the isotope m easurem ents on w ater sam ples, collected in the areas

o f investigation. IAEA-AG-158/11 1 5 5

TABLE I. LITHOSTRATIGRAPHIC CHARACTERISTICS OF SELECTED

AQ UIFERS IN THE W ADI SAW FAJJIN-W ADI-ZAMZAM-AL JUFRAH AREA

System Series Form ation Aquifer unit Lithology

Holocene Quaternary Pleistocene

Oligocene Limestone

Eocene Limestone Tertiary Paleocene Bechima Floskulina Dolomite

? ? Zman Socna Chalky dol.

Mizdah Mazua Limestone Tigrinna Dol. limestone Upper Nefusa Gharyan Limestone Cretaceous Ain Tobi Limestone dol.

Chicla Formation Sandstone Lower Kabaw Form ation Sandstone

Jurassic Triassic

Palaeozoic Cambrian- Sandstone Ordovician

1. HYDROGEOLOGY

The study area represents the northeastern extension o f the Ham ada al

Ham ra basin, w hose extrem e eastern part includes the Hun-Graben. The geological

structure is a platform w ith a slightly distinguishable syncline, the axis o f w hich

dips northeastwards. The Hun-G raben is caused by a fault system running in a

SE-N W direction. The lithostratigraphic characteristics o f the selected aquifers

are given in Table I.

The follow ing m ain aquifers were identified and characterized in the study

area by Energoprojekt experts [1] (see also Refs [2, 3]). 1 5 6 SRDOC et al.

The Oligocene aquifer, lim ited to the Hun-Graben, is tapped by num erous wells near H un and W addan. How ever, its capacity is lim ited and deeper aquifers yield larger am ounts o f water o f better quality.

The Eocene aquifer also occurs in the Hun-Graben, to the N E of it. Com prising lim estone beds, the Eocene is a confined artesian aquifer. A high artesian head occurs in the SE part o f the Hun-Graben. Som e w ells tapping the aquifer in W adi

Zam zam , W adi Bayy A l Kabir and Abu Nujaym are used for agriculture.

The Palaeocene aquifer contains perched w ater tables throughout the m ost of the study area including the southern and southw estern parts. Inside the H un-

G raben and N E of it, the aquifer is confined and is artesian. O nly in the w adis running in a SW -N E direction between W adi Sawfajjin and Hun-G raben does there occur a water table. The aquifer consists o f m ore hydraulically separate aquifers ow ing to its lithological changes, tectonic structure and topographic factors. It yields only sm all am ounts o f water to the w ells and does not represent water resources for agricultural developm ent.

The Socna aquifer, situated in the Socna area, is one of the m ost im portant aquifers in the study area. Artesian groundw ater occurs in lim estone beds confined by the scale w ith a head up to 60 m . Consequently, drilled wells yield m ore than

200 ltr/s, although transm issivity o f the aquifer is m oderate.

The aquifers w ithin the M izdah Form ation occur in all the lithostratigraphic

units in alternation w ith water-bearing and w ater-confining beds and discontinuities

caused by faulting and w adi erosion. M ost of the existing w ells tap the Tigrinna

aquifer. Still, it seem s that the M azuza aquifer yields larger am ounts o f water.

The aquifers are lim ited to the northern part o f the study area; therefore, tapping

wells are situated on the M izdah and Bani W alid sheets. From west to east, the

M izdah aquifers change the hydraulic characteristics, containing a perched water

table w ithin the M izdah and Bani W alid highlands, a free water table in som e of

the left tributaries o f W adi Saw fajjin, and artesian w ater further to the east.

Their practical im portance is lim ited.

The G haryan aquifer, in its extension, coincides w ith the M izdah form ation

aquifers, i.e. it is lim ited to the northern part o f the study area. Its transm issivity

is low (dow n to 10 -6 m 2/s). Its hydraulic head also increases from w est to east.

Like the M izdah Form ation it is only of lim ited practical im portance.

The A in T obi lim estone and dolom ite beds appear to be a potential aquifer,

but it cannot be considered as one o f great extent. It is m ostly under the piezo­

m etric surface and overlies the Chicla-Kabaw aquifer. Therefore, it is included in IAEA-AG-158/11 1 5 7

the deep groundw ater reservoir constituted by the Palaeozoic-Kabaw -Chicla sand­

stone beds.

The Chicla-Kabaw aquifer underlies alm ost the entire study area, but at great

depths. It reaches lat. 29°N in a southerly direction. O nly in the extrem e N W

do these form ations lie above the regional w ater level. Transm issivity is relatively

h i g h ( 10~3 t o 1 СГ 2 m 2/s), specific capacity o f w ells is from 1— 5 Itr s_ 1- m _1 ;

optim al discharge rates are 30 ltr/s. Fo r m ost of the area, Chicla-Kabaw is the

m ain aquifer.

The Palaeocoic aquifer underlies the entire study area. It is very deep and

therefore drilling o f w ells w ould not be justified except in the south. The Chicla-

Kabaw and Palaeozoic aquifers appear to be in lateral contact and constitute

the sam e groundw ater reservoir. Therefore, it seem s to be reasonable to develop

the shallow aquifer first. Direct groundw ater developm ent o f the aquifer is

provided in the area south of Q asr A sh Shuw ayrif, in the w adis tow ards the H un-

Graben. The Palaeozoic aquifer is one of the m ost im portant groundw ater sources

in the study area.

2. ISOTOPE STUDIES

Forty-one water sam ples, taken from different aquifers were analysed for their 2H and 180 contents in the Institut für Radiohydrom etrie in M unich; and m ost of them for their 14C , 13C, and 3H contents in the Ruder Boskovic Institute in Zagreb (Table II).

The evaluation o f 14C ages, as given below , w as done by m eans o f the 13C

correction Д = % m odern 14C —(2 513C + 50) (1 + % 0 m o d e m — 1000

5 13C values, in general, do not display any larger deviations and range from

— 2.3 to — 7.5 % o . Partially characteristic values o f 13C values in individual aquifers

can be suggested. N o one water sam ple was found to be younger than

« 12 000 years, w hile in the case o f very old w aters the lim iting factor w as the

detection lim it o f 14C activity, i.e. « 3 0 000 to 40 000 years.

The 52H -5180 relation (Fig.2) enables w aters to be classified into tw o groups.

G roup I includes the values 5 180 from — 5.8 to — 7.5 % o and 52H from — 35 to — 50%o.

G roup II includes considerably low er values: 6 180 from —8 t o — 1 1 % o , a n d S D

from — 60 to -80% c. In addition, the groups differ from each other regarding

deuterium excess. G roup I is dispersed around the M eteoric W ater Line

5 2 H = 8 5 180 + 10, w hereas group II is situated on the line 6 2 H = 8 6 180 + 4 .

The sam ples of group II are characterized by a com paratively high apparent

14C age (Fig.3). Thus, the individual aquifers can be characterized by their

isotopic characteristics as follow s: Ul 00

TABLE II. ISOTOPIC COMPOSITION OF SELECTED GROUNDW ATER SAM PLES

FROM THE LIBYAN ARAB JAM AHIRIYA

14c l) ample Typ, Location and/or Name of well Geogr Coord. Water Aquifer áI3c'> 3H1> S 'V > 3) No. N la t E long Depth X mod. age {7.o) T.U. U o) (Zo) (m) (y )

1 DuW, Bir Garania, Uadi Zamzam 3 1°o8 I5°o5' 47.4 Eocene - - - 0 - 55.3 - 7.36 2 DrW, WS-lo, Uadi Zamzam 31°lo 15°o61 172.0 Eocene 3.8 + 0.4 26 З00 •+ 9oo -3 .7 7 3 .8 + 1 4 - 68.4 - 8.99 3 DrW, WS-2o, Wadi Zamzam 31°14 15°o8' 14o,o Eocene - - -2 .3 7 0 - 68.0 - 8.79 4 DrW, WS-21, Wadi Zamzam 31 ° 17 15°11' 75.0 Eocene 4 .8 + 0 . 5 24 З00 + 800 -3 .3 7 0 -6 9 .4 - 9.08 RO e al. et SRDOC 5 DrW, WS-1, Wadi Kabir 3o°52 15°19* 13o.o Eocene 2 .3 + 0 .5 3o 2oo +15oo -2 .1 7 14.6 + 1 7 -7 3 .5 - 9.73 0 6 AW, Hun Old W ell, Hun 29°o5 15°55' 445.0 Eocene 1.8 + o .54) 32 О О CO 0 - 6. 17 0 -6 3 .7 - 8.29 + + 1 7 DrW, Abu Nujaym-I 3o°35 15°27' 45o.o Eocene ? < 0.6 > 4o 000 -3 .8 8 2 .3 + 0 9 -7 4 .2 - 9.59 8 DrW, Abu Nujaym-2 3o°35 15°27 * 45o.o Eocene < 0.6 > 4o 000 -2 .3 9 3 .8 + 0 8 -7 4 .6 - 9.77 9 DuW, B ir al M allah, Wadi Zurzur 31°25 14°37’ 17.5 Paleocene - - - 7.1 + 1 3 -3 6 .7 - 5.81 lo DrW, WS-3/I, Q.A. Sharquiyan 3o°25 13°37’ Зо. о Paleocene - - - 1.6 + 1 6 -5 9 .0 - 7.34 I 1 DrW, Socna-4 29°o4 15°5o' 18o.o Socna 1.4 + 0 .5 31 loo +22oo -4 .7 7 0 -7 7 .3 -I 0 . 2 I 12 DrW, Socna-2 29°o4 15°5o' 2o2 .0 Socna 2.0 + 0.5 31 З00 +17oo -4 .2 9 2.7 + 1 4 -7 7 .9 -1 0 .2 6 13 DrW, Socna-11 29°o4 15°5o' 15o.o Socna < 0.6 ">4o 000 -4 . 18 0 -7 6 .9 -I 0.45 14 DrW, Ferjan J-3T 28°55 I5°38' 332.0 Socna 2 .3 + 0.5 3o 2oo +15oo -4 .7 7 2 .5 + 1 3 -7 9 .8 -I 0.51 4) 15 DrW, Nesma 2 3 l°o2 13 °2 5 1 7o.o Tigrinna 24.7 ♦ 0.6 1 1 15o + 19o -5 .4 7 3 .8 + 1 0 -З6.0 - 6.25 16 DrW, Nesma 2 3l°o2 13°25' 80.0 Tigrinna 6 .5 + 0.У 21 9oo + 56o -1 .4 9 4.0 + 1 2 -3 9 .0 - 6.35 17 DuW, Mizdah 31°26 13i oo v 14.2 Tigrinna - - - 14.7 + 1 4 -3 9 .4 - 6.34 18 DrW, Wadi Mimun 3I°34' 14°23' 95 .0 Tigrinna 6 .9 + o .^ > 21 5oo + 5oo -3 .7 8 - - 6 . 2o Sample Type, location and/or name of well Geogr. coord. Water Aquifer »4C0 5I3C° з„0 i :Hy 6‘*02) No. N lat E long depth % mod. age3) (%o) (TU) (%») (W) (m) (years)

19 DrW, Wadi S a w fa jjin 31°34 I4°23' 5oo.o Mizdah 23.0 + 0 . 6^ 1 1 7oo + 2oo - 6.86 0 -4 1 .2 _ 5.76 2o DrW, Wadi S a w fa jjin 3I°34 I4 °2 3 ' 5oo.o Gharyan - - - - -6 1 .7 - 8.24 £> 21 DrW, Ras Tabal 3 l°o2 13°25* 174.0 Gharyan? .P* О 5oo + 25o - 3 .4 8 0 -5 o .6 - 6.83 + + 1 22 DrW, Bani Walid 3 1°45 14°o 4 ' З оо.о Gharyan 12.8 *_ 0.5 16 5oo + З00 - 3 .7 8 0 -3 8 .9 - 6.18 23 DrW, B ir Sanam, Wadi S aw fajjin 3 J ° 2 1 13°411 45o.o Gharyan 2.9 ♦ 0.54’ 28 З00 + J2oo - 1 .I 9 4) 3 .8 + . 1 -4 7 .2 - 7. 15 24 DrW, Mizdah 1 31 °27 13°oo' 128.Ô Gharyan - - - 3 .7 8 0 -4 1 .9 - 6.51

25 DrW, Khartum 6 31°47 12°1 о ' 18o.o Gharyan 9 .4 + o.54) 19 loo + 4oo -4 .7 7 2 .5 + .6 -4 4 .2 - 6.33 IAEA-AG-158/11 26 DrW, WS-16 31°38 13°oo' 417.0 Ain Tobi - - - - - 4 8 .0 - 6.80 27 AW, ZZ-1, Wadi Zamzam 3l°o9 15°03’ 1000.0 Chicla 1.7 + 0 .5 32 5oo +19oo -5 .4 7 2 .3 + .5 - 68.2 - 9.16 28 AW, ZZ-2, Wadi Zamzam 3 l°lo 15°o5' 1000.0 C hicla 2.2 + 0.3 3o 7oo +l2oo -5 .4 2 1.8 + , 24 >- 68.0 - 9.16 29 DrW, New Well Nura 3I°47 I3°53' 975.0 Chicla < 0.6 > 4o 000 - 6.86 0 -4 5 .3 - 6.69 3o AW, WS-2 3o°58 14°35' lo lo .o Chicla 1.2 + 0 .3 35 З00 + I600 - 4 .6 5 0 -6 8 .7 - 9.43 31 DrW, WS-4 3o°24 13 °3 6 ' 801 .0 Chicla < 0.6 > 4o 000 -5 .5 4 0 -6 7 .4 - 8.94 ^ ° 32 DrW, WS-6 3o oo 14°16 ' 694.0 C hicla - - - 1.9 + ,74) -7 o .l - 9.55 33 DrW, WS-14M, Mizdah 31°27 13°oo' 772.0 Chicla - - -5 .1 7 4. 1 + .7 -47.1 - 7.27 34 DrW, WS-I4M, Mizdah 31°27 13°oo' 772.0 Kabaw 11.6 + 0.4 17 25o + 27o - 7 .4 6 - -4 6 .3 - 7.08 35 DrW, WS-8 29°o2 14°18' 46o.o Paleozoic 5.8 + o .34) 22 7oo + 4oo -3 .4 o 3 .2 ♦ .3 -6 9 .0 - 9.59 36 DrW, No.- 1, Wadi Zimam 29°2o 15°22' 519,0 Paleozoic 4.5 + o.34) 24 9oo + 600 -3 .3 5 0 -7 2 .7 - 9.88

DrW — Drilled Well; DuW = Dug Well; AW — Artesian Well; Well data, water depth and aquifer nomination see Ref.[ 1 ].

^ Measured at the Ruder Bo5kovi6 Institute, Zagreb, Yugoslavia. * Measured at the Institut fur Radiohydrometrie der Gesellschaft fur Strahlen- und Umweitforschung mbH, Neuherberg, Fed. Rep. of Germany. * F o r age calculation see Sect. 2. ^ Not reliable measurement. 9 5 1 1 6 0 SRDOC et al.

62Н(%.)

FIG.2. 5 2H-¿ 180 relation o f water samples taken from different aquifers. E = Eocene; PI = Paleocene; So = Socna; Mz = Mizdah; Ti = Tigrinna; Gr = Gharyan; AT = Ain Tobi; Ch = Chicla; Pz = Palaeozoic.

Eocene aquifer: The concordant quantities of 2H and 180 indicate that the

origin of water sam ples N os 5, 7 and 8 is the same. The waters below to group II, the 14C ages (30 000 to > 4 0 000 years) support this statem ent. M ost o f the rem aining water sam ples from this aquifer (N os 2, 3, 4, 6 ) also belong to group II.

Their 14C ages o f 24 000 to 26 000 years m ay indicate a sm all but definite recharge from W adi Zam zam . Sam ple N o.l is the only one w hich could be con­ sidered as a m ixture o f groups I and II. 14C w as not m easured, unfortunately.

The difference in age of water sam ple N o.5, taken in W adi A l K abir (30 000 years), and that taken from Abu Nujaym N os 7 and 8 (> 4 0 000 years), probably indicates a possible interference o f recent w aters in the W adi A l K abir area.

A certain confirm ation m ay be given by the relatively high 3H value of N o.5, although som e doubts exist in general concerning the 3H values (see Section 3).

Palaeocene aquifer: O nly tw o water sam ples (N os 9 and 10) were m easured.

A s it was considered that recent waters were in question, 14C m easurem ents were

not m ade. This assum ption can be accepted according to the 2H, 3H and 180

contents for sam ple N o.9. Sam ple No. 10, how ever, contains at best considerable parts o f old w ater belonging to group II. IAEA-AG-158/11 161

FIG.3. 62H-14C age relation o f water samples taken from different aquifers (for abbreviations see caption ofFig.2). For calculation o f 14C age from 14C content see Sect. 2.

Socna aquifer: A ll water sam ples (N os 11, 12, 13, 14) taken from this

aquifer belong to group II, characterized by very low 5 2H and 5 l80 values and a

14C age of about 30 000 years. It seem s that no adm ixtures or other hydraulic

effects influenced the w ater o f this closed aquifer (see also Ref.[2]).

M izdah Form ation aquifers: The water sam ples (N os 15, 16, 17, 18 and 19)

belong to group I. The 14C m easurem ents gave no reliable results and therefore

were not considered. The 3H results indicate that m ost sam ples contain portions

of rain-w ater younger than 20 years and thus confirm the hydrogeological

assum ption o f a perched aquifer.

G haryan aquifer: A s far as the S2H and 5 180 values are concerned, water sam ples N os 21, 22, 23, 24, 25, 37 and 38 belong to group I w ith a 14C age of

about 15 000 years. The presence o f a sm all percentage of rain-w ater younger than 20 years in water sam ple N o.23 (and 25) cannot be excluded. W ater sam ple

No. 20, attributed to the G haryan aquifer, belongs m ore to group II and m ay originate from another aquifer.

A in Tobi aquifer: The only sam ple from this aquifer (No. 26) belongs to group I as far as the 52H and S 180 values are concerned and cannot therefore be

connected w ith water from the Chicla-Kabaw -Palaeozoic aquifer (see below ).

3H and 14C m easurem ents were not made. 1 6 2 SRDOC et al.

Chicla-Kabaw -aquifer: The water sam ples taken from this aquifer can be divided into groups I and II w ith regard to their 6 2H and 5 180 values. Sam ple

N os 27, 28, 30, 31 and 32, taken at the eastern and southw estern parts of the study area, belong to group II whereas sam ple N os 29, 33 and 34 taken at the northw estern part of the study area, belong to group I. The 14C ages m ostly occur as follow s — G roup II sam ples are associated w ith 14C ages betw een 30 000 and 40 000 years, whereas group I sam ple N o.34 show s a 14C age of 17 000 years w ith a 3H content of 4 TU . The only unexplained exception, sam ple N o.29 belonging to group I, w ith regard to 5 2H and § 180 values, gives a 14C age of

40 000 years and no 3H content. Therefore, it seem s that in the northw estern part o f the aquifer, lying above the regional w ater level, a certain recharge takes place long after the form er recharge o f the group II waters.

Palaeozoic aquifer: Sam ples 35 and 36 both contain sim ilar quantities of 2H a n d 180, corresponding to group II. 14C ages from 22 000 to 24 000 years are not very reliable.

3. DISCUSSION OF ISOTOPIC DATA ANALYSIS

Tritium analyses: According to the tritium content in groundw aters there are only few cases in the w hole investigated area where the tritium concentration in w ater could indicate contribution from recent precipitation, such as sam ple

N o.5 tapped from the Eocene aquifer, sam ple No. 17 tapped from the Tigrinna aquifer, and sam ple N o.23 tapped from the G haryan aquifer. Nevertheless, it m ust be taken into consideration that contam ination during sam pling or exchange w ith atm ospheric water vapour possibly increased the 3H content of som e of the w ater sam ples. How ever, in m ost cases tritium concentration indicates either an insignificant contribution from precipitation or a total absence o f precipitation w ater in groundw ater in the investigated area, including the relatively elevated regions o f Jebel Nefusa.

T his conclusion, based on tritium analyses, explains the observed fact that the ground-w ater level does not show fluctuations related to the am ount of precipitation water. It could be stated that the contribution o f precipitation w ater to the underground w ater is negligible and if it does occur at all it has a very lim ited range near w adis during the rainy period.

I 4C-analyses: Although the radiocarbon “age” of underground waters could

be altered by m any geochem ical processes, thus giving erroneous and therefore

m isleading results, a useful set o f inform ation can be derived from 14C m easurem ents

of dissolved hydrocarbonates. IAEA-AG-158/11 1 6 3

Generally, the m easured age of Libyan groundw aters is very old, ranging

f r o m 12 000 years to or above the lim its set by the radiocarbon m ethod, w hich is

^ 4 0 000 years. Groundw aters of the northw estern m ountainous region Jebel

Nefusa, from Cretaceous aquifers including the Chicla aquifer (Low erCretaceous),

are relatively younger, w hich indicates contributions from precipitation up to recent tim es, although very lim ited. Therefore, it m ust.be considered that, beneath

the m ain flow in the Palaeozoic from south to north, a second flow com ponent

exists in the Cretaceous aquifers from west to east. Indeed, going eastw ards and

southeastw ards the groundw ater age increases, w hich can be explained by the longer

distance from regions w here precipitation occurs and the very low flow ing rate

of underground water.

It should be em phasized that the underground water in the investigated region

is relatively old. This is true not only for deep artesian w ells w hich belong to the

Palaeozoic and Chicla aquifers, but also for the relatively shallow Socna and Eocene

aquifers. The Socna aquifer is located in the southeast region, w hereas the Eocene

aquifer is near the H un-G raben and is shallow .

The expected very old age o f underground water from the Palaeozoic and

Chicla (Low er Cretaceous) aquifers, located under Upper Cretaceous non-porous

layers, is confirm ed by 14C m easurem ents. But it should be em phasized that water

from the Chicla aquifer is older than water from the Palaeozoic aquifer, w hich is

in agreem ent w ith the concept o f a continuous flow from the Palaeozoic aquifer

through the Chicla aquifer tow ards the M editerranean sea.

A s a result of 14C carbon m easurem ents together w ith other stable isotope

and chem ical analyses, it has been established that the Socna aquifer is recharged

from the Palaeozoic aquifer through H un G raben as opposed to som e previous

speculations w hich considered the precipitation from the Jebel Soda highlands

to be the m ain source o f w ater recharging the Socna aquifer.

The sam e reasoning is valid for the Eocene aquifer, w hich is recharged from

the Chicla aquifer.

Stable Isotopes: Several very interesting and im portant results were obtained

by m easuring the relative concentration of stable isotopes 2H and 180 . I t w a s

possible to sort sam ples into tw o groups according to the diagram show n on Fig.2.

G roup I of the sam ples belongs to the U pper Cretaceous aquifers and the Chicla

aquifer from the Jebel Nefusa highland region. They have the highest 2H and

180 contents, presum ably ow ing to the influence o f the M editerranean clim ate.

T his fact also indicates a possible recharge during pluvial tim es from the Jebel

Nefusa highlands. It cannot be excluded that underground water tapped from

the Chicla aquifer is com posed o f Palaeozoic water flow ing from the south and of

a sm all fraction o f precipitation w ater from the Jebel Nefusa highlands w ith

relatively high 5-values [2].

The first part o f group II, w ith the low est stable isotope content, w as

connected w ith the Socna aquifer. Low 2H and 180 contents indicate a low 1 6 4 SRDO Í et al.

tem perature effect, although it is not clear w hether it w as an effect o f altitude, or a so-called continental effect, or a low ering o f the tem perature during the

Pleistocene. In any case, stable isotope data support the assum ption that the

Socna aquifer is recharged from a deeper aquifer. The recharge to Socna from the

Paleozoic aquifer has also been found by m easurem ents in other wells of the region south o f the area under investigation [ 2 ], and is confirm ed b y sim ilar

13C values o f both aquifers. This conclusion has a very im portant practical con­ sequence since it im plies that an intensive exploitation o f the Palaeozoic aquifer w ould be possible.

The second part of group II belongs to the Palaeozoic, Chicla and Eocene aquifers. The 2H and 180 contents are higher w hen com pared w ith the first part, indicating a higher condensation tem perature. The fact that the stable isotope content o f water sam ples from the Palaeozoic and Chicla aquifers are very close confirm s the assum ption, based on piezom etric observations, that both aquifers are indeed part of a huge interconnected underground system . The assum ption on the recharge of the Eocene aquifer from the Chicla aquifer is based only on data from isotopic analyses.

REFERENCES

[1] ENERGOPROJECT BEOGRAD, Regional Hydrogeological Study: Wadi Sawfajjin- Wadi Zamzam-Al Jufrah, Final Rep. to the Secretariat of Dams and Water Resources, Socialist People’s Libyan Arab Jamahiriya (1977). [2] SALEM, O., VISSER, J.H., DRAY, M., GONFIANTINI, R„ “Flow patterns of groundwater in the western Libyan Arab Jamahiriya — Evaluations from isotopic data”, IAEA-AG-158/13, these Proceedings. [3] GOUDARZI, G.H., Geology and mineral resources of Libya: A reconnaissance, US Geol. Survey Profess. Paper 660, Washington, D.C. (1970) 104 pp. IAEA-AG-158/12

GROUNDWATER FLOW PATTERNS IN THE WESTERN LIBYAN ARAB JAMAHIRIYA EVALUATED FROM ISOTOPIC DATA

O . S A L E M

Secretariat of Dam s and W ater Resources,

T r i p o l i

J.H. V ISSE R

United Nations Food and Agriculture

Organization,

Land and W ater Investigation Project,

T r i p o l i ,

Libyan Arab Jam ahiriya

M. DRAY, R. G O NFIANTINI

International Atom ic Energy Agency,

V i e n n a

A bstract

GROUNDWATER FLOW PATTERNS IN THE WESTERN LIBYAN ARAB JAMAHIRIYA EVALUATED FROM ISOTOPIC DATA. Stable isotope analyses were used to provide evidence of the main features of regional groundwater flow patterns and to show interconnections between aquifers in the western Libyan Arab Jamahiriya. The main flow is from south to north in the Palaeozoic aquifer, but a component deriving from a recharge area in the west is present in the Wadi Ash Shati Valley and increases significantly to the north of Jebel Gargaf. This groundwater, which originated in the west, is also the major recharge source of the important Kicla aquifer which in turn recharges the overlying Upper Cretaceous formations in the area of Misratah. More to the north, the Kicla aquifer is also recharged by precipitation over the Jebel Nefusa. In the north, the Tawurgha spring is fed mainly by the Miocene aquifer and not by the Upper Cretaceous ones.

INTRODUCTION

Generally speaking, the regional flow direction of deep groundw ater in the

Libyan Arab Jam ahiriya is from south to north; that is, from the Sahara Desert to the Mediterranean Sea. How ever, the detailed configuration of groundw ater flow patterns, the areas and periods of recharge, and the interconnections betw een aquifers, are still to a large extent unknow n or not sufficiently proved.

A n attem pt to throw som e light on the subject by using the variations of environm ental isotope concentrations in groundw ater has been m ade as a part of

1 6 5 1 6 6 SALEM et al.

a current investigation, carried out jointly by the Secretariat of D am s and W ater

Resources of the Libyan Arab Jam ahiriya, by the Food and Agricultural

Organization of the United Nations, and by the International Atom ic Energy Agency, w ithin the fram ew ork of the Land and W ater Investigation Project (TF-9184) executed by F A O on behalf of the Libyan Governm ent. O nly part of the results obtained are reported here — those dealing w ith deep groundw ater in western

Libya. The study, initiated in late 1976, is still in progress, but we are convinced that the m ain conclusions already reached w ill rem ain basically unchanged and valid w hen the w ork is com pleted.

The area under investigation is in the west of the country, from the Fezzan to the M editerranean Sea (Fig. 1). The boundaries of the area are approxim ately the

26th parallel on the south, the 13th m eridian on the west, the 16th m eridian on the east, and the coast on the north. T his very large area of about 180 000 km 2 i s essentially flat, w ith only few m ountain ranges of lim ited extent and height, the m ost im portant of w hich are the Jebel N efusah in the north and the Jebel Saw da in the central part. The m ajor part of the area consists o f desert, either of classical sand-dunes (U bari Sand Sea, M urzuk Sand Sea) or of stoney desert

(Ham adah el Ham ra).

The rainfall is about 250 m m /year in the coastal zone, but rapidly drops tow ards the interior to values of less than 50 m m /year. U nder these clim atic conditions, there is, of course, no perm anent surface fresh-w ater body available, and any agricultural activity in the m ajor w adi valleys m ust rely entirely on groundw ater resources for irrigation.

Here we sum m arize the hydrogeological situation of the area. M ost of the inform ation contained in this brief outline is taken from G oudarzi [ 1 ], Pallas [2], and from discussions w ith the hydrogeologists of the F A O team in Libya, especially

P. Pallas, G. Krusem an, and M. Taka w hose contributions are gladly acknow ledged.

Figure 2, show ing a schem atized south-north geological cross-section, can be

used to illustrate the m ajor hydrogeological features of the area. In the south,

in the M urzuk depression and in the W adi Ajal zone, the largest groundw ater

reservoir is probably the Palaeozoic Sandstone (Cam bro-O rdovician).This form ation,

confined and m ostly artesian, is, how ever, generally too deep for exploitation.

Therefore, the w ater-bearing aquifer m ostly exploited in this area is the N ubian-

Posttassilian Sandstone (U pper Prim ary to Low er Cretaceous), also often confined

and artesian, separated from the Cam bro-O rdovician by m arine and continental

deposits, the thickness of w hich is about 1000 m. The m arine layers (Low er

Silurian, M iddle and Late Devonian, Low er Carboniferous) are essentially represented

by shales, som etim es w ith som e fine-grain sandstone intercalations. The continental

deposits (Low er Devonian and Upper Carboniferous) show sandstones and clastic

facies alw ays associated w ith sandy clays.

M ore the north, the Palaeozoic Form ation form s a large anticlinal arch,

oriented west-east, the crest of w hich outcrops in the Jebel G argaf (also called Jebel

Fezzan) at about 600 m a.s.l., im m ediately to the north of the W adi A sh Shati Valley. IAEA-AG-158/12 1 6 7

In the latter, the Palaeozoic aquifer is very near the surface, still being artesian, and is largely exploited for irrigation purposes. According to P. Pallas, the Jebel G argaf zone could be an im portant recharge area for the different groundw ater units flow ing northw ards.

O n the northern flank o f the Jebel Gargaf, the Palaeozoic deepens again and is exploited only in the zone of the A l Jufrah, after w hich the depth becom es prohibitive for current water drillings. Betw een the Jebel G argaf and the A l Jufrah, the w hole sedim entary series is pierced by intrusive basalt w hich has broken

out to form the Jebel Sawda.

In the zone of the A l Jufrah, the Palaeozoic is directly overlain by the Low er

M esozoic sandstones, m ainly belonging to the Kicla Form ation of Low er Cretaceous EXPLANATION

Quaternary : I3sand,sitts,clays Lower Cretaceous: | . | Kicla - sandstones and clays or Nubian sandstones Lower Eocene- Нт^7~Н Waddan Formation^ limestones and gypsum r=— =] , ГТ7ТТТП i- i Jurassic+Trias + Shales and clays P a le o c e n e : F^Jm orls PiV/i/i limestones Upper Primary : or Sands and sandstones Upper Cretaceous: 1Socna facies: Fossiliferous limestones a n d m a rls Lower Primary : Cambro-ordovician sandstones

[ТТТТ limestones and marls Crystalline bedrock! *+l

Extrusive rocks: Basaltic lavas

FIG.2. Geological cross-sections (indicated in Fig.lj IAEA-AG-158/12 1 6 9

age, w hich is probably the m ost im portant aquifer of the w hole northern part of the investigated area. It is believed that the groundw ater com ing from the south is discharging from the Palaeozoic into the Kicla, for w hich it w ould actually constitute the m ajor source of recharge. In the area of A l Jufrah, another form ation currently being exploited is the U pper Cretaceous lim estone exploited in Socna area (artesian) w hich also w ould be recharged by the Palaeozoic.

O ther im portant aquifers exploited in the north are those of the Upper

Cretaceous age - A in Tobi, G harian (also called Nalut), and M izdah, consisting of lim estone and/or dolom itic lim estone. A ll these form ations are supposed to derive m ost of their water from the Kicla aquifer, from w hich they w ould not be separated by any continuous im perm eable barrier. O ne of the m ajor discharge points of all these aquifers is believed to be the im portant spring of Taw urgha near the

M editerranean coast, about 40 km south of Misratah.

In a west-east cross-section, an im portant geological feature is the large H un

Graben, south-north oriented, (approxim ately along the 16th m eridian) starting in the zone of A l Jufrah (Fig. 2). Here, one of the m ajor hydrogeological problem s is to verify w hether or not the Palaeozoic aquifer is discharging across the fault system into the W addan form ation (Eocene) in the Graben.

1. SAMPLING AND AN ALYSES

Selected isotopic data on deep groundw ater sam ples are reported in Table I.

The present w ork is based m ainly on stable isotope variations in groundw ater; how ever, 14C and 13C data, w hen available, have also been reported. Tritium data are not included in the table because the tritium content, m easured in part of the sam ples, w as alw ays negligible as should be expected for w ater from deep, confined aquifers.

The great m ajority o f the isotopic results used in this paper were obtained at the IA E A Isotope H ydrology Laboratory, but a few additional results have been taken from other sources. The data designated by the Letter “K ” in Table I were derived from Ref. [3] and were obtained at the Institut für U m w eltphysik of the University of Heidelberg, Federal Republic of G erm any, on sam ples collected in 1972— 73. Am ong the data reported in Ref. [3] only those obtained on sam ples derived from artesian wells have been used and other data, w hich could have been affected by contam ination w ith water from the shallow aquifer, were discarded.

The data designated by the letter “ E ” were obtained from sam ples collected in 1975 by Energoprojekt, a Yugoslavian consulting firm . The stable isotope m easurem ents were perform ed at the Institut für Radiohydrom etrie in Neuherberg,

Federal Republic of G erm any, w hile the 14C analyses were carried out at the

Ruder Boskovic Institute, Zagreb, Yugoslavia. A ll the isotopic data from this source are discussed in another paper in these proceedings [4]. TABLE I. ISOTOPIC COMPOSITION OF SELECTED GROUNDW ATER SAMPLES

FROM THE LIBYAN ARAB JAMAHIRIYA

Code Location and/or name Depth (m) Aquifer Sl80 6D 14 C% 8,3C MC age (years BP)

MURZUK-WADI AJAL K-62 Murzuk 150-700 Mesozoic -10.4 -74.1 2.5 ± 0.3 -8.5 24 900 ± 3300 T-5 Traghen, Experimental Farm 700 Mesozoic -10.76 -81.4 K-51 Traghen 150-700 Mesozoic -10.4 -80.1 2.7 ± 0.3 -11.8 27 000 ± 2900 T-4 Um El Araneb, Well No.3 273-553 Permo-Trias -10.84 -82.5 K-48 Zawilah 150-700 Mesozoic -11.1 -83.7 2.0 ±0.3 28 100 ±4700“ T-14 Ubari, Agrie. Project, Well No.l 424 Upper Trias -11.28 -81.5 al. et SALEM T-15 Ubari, Agrie. Project, Well No. 5 8 189 Lower Cretaceous -10.90 -80.7 K-26 Ezzighen 200 Mesozoic -76.7 22.2 ± 0.7 8200 ± 4100a

WADI ASH SHATI T-32 Edri, Piezometer No.8 600 Palaeozoic -10,66 -77.2 A-12 Edri, Well 1-A 606 Palaeozoic -10.75 -76.7 0.0 ± 0.4 -13.34 >30 000 T-33 Timisan 250 Palaeozoic -10.78 -78.2 K-19 Wensrick 20-40 Palaeozoic -10.6 -75.8 0.8 ±0.3 -13.8 >30 000 T-27 Gotta 40 Palaeozoic -11.00 -80.6 T-36 Al Gorda 250 Palaeozoic -11.18 -82.4 K-13 Agar 60-100 Palaeozoic -10.9 -78.2 1.8 ± 0.3 -11.5 30 100 ± 3600 T-25 Agar, Artesian Spring Palaeozoic -11.12 -82.2 K-14 Al Maharuga 60-100 Palaeozoic -78.9 0.2 ±0.1 -11.3 > 30 000 TABLE I. (cont.)

Code Location and/or name Depth (m) Aquifer SibO 8D 14C% 5n C 14C age (years BP)

T-37 Ain Noss 200 Palaeozoic -10.93 -79.8 T-22 Brak, Artesian Spring Palaeozoic -11.02 -80.8 K-8 Brak 60-100 Palaeozoic -10.9 -81.7 <0.5 -11.2 >30 000 T-38 Eshkeda, Agrie. Project, Well No.5 340 Palaeozoic -11.36 -82.0 0.4 + 0.4 -11.51 >25 900 A-11 Bir Al Gamajal, Well 1-177 200 Palaeozoic -11.32 -82.6 0.0 ± 0.4 -10.79 > 30 000 K-l 96 Km E of Brak 60-100 Palaeozoic -80.9 0.8 + 0.3 -11.2 >30 000

AL JUFRAH IAEA-AG-158/12 E-33 Well WS-8 460 Palaeozoic -9.58 -69.0 (5.8 ± 0.3)b -3.40 (10 400 ± E-34 Wadi Zimam, Well No. 1 519 Palaeozoic -9.88 -72.7 (4.5± 0.3)b -3.35 (12 300 ± T-39 Al Jufrah, Agrie. Project, Well J-l 8 380-700 Palaeozoic -10.67 -79.4 0.3 + 0.4 -5.55 >16 500 T-l 17 Al Jufrah Upper Cretaceous -10.53 -77.6 0.6 ± 0.4 -4.96 >20*500 A-10 Al Jufrah, Well J-l 8A 193 Upper Cretaceous -10.34 -75.5 0.0 ± 0.3 -4.66 >27 500 K-5 Socna 400 Mesozoic -10.2 -79.6 0.8 ± 0.3 -5.6 >23 700 A-9 Socna, Well S-l 300 Upper Cretaceous -10.51 -76.6 0.0+ 0.5 -5.39 >24 900

ASH SHUWAYREF-WADI ZAMZAM T-43 Ash Shuwayref, Well No.6 650 Kicla -9.68 -70.7 0.2 + 0.4 -5.96 >9600 T-40 Wadi Qirzah, Well WS-2 1000 Kicla -9.50 -69.5 0.0 ± 0.4 -4.80 >25 400 T-48 Well B-l - 39 1000-1400 Kicla -9.21 -68.1 0.2 + 0.4 -6.85 >10 700 T-41 Wadi Zamzam, WeU ZZ-13 1000 Kicla -9.42 -68.9 0.2 + 0.4 -6.23 >9900 T-42 Wadi Zamzam, WeU ZZ-8 1000 Kicla -9.37 -67.8 -J го

TABLE I. (cont.)

Code Location and/or name Depth (m) Aquifer s l8o SD 14 C% S13C 14 С age (years BP)

IEBEL NEFUSAH T-50 Wadi Marmuta 471 Kicla -7.13 -45.9 2.4 ± 1.4 -8.55 >15 300 T-44 Wadi Faysal, Well No.3 505 Kicla -6.82 -43.8 3.2 ± 0.5 -8.31 22 700 ± 3800 T-46 Wadi Faysal, Well 1-A 120 Upper Cretaceous -6.20 -41.0 18.4 ± 1.1 -5.06 4100 + 4100

T-45 Beni Walid, Well Nora-1 975 Kicla -6.94 -45.7 -7.25 al. et SALEM

TAWURGHA T-100 Tawurgha, Well P-22 Gharian -9.49 -68.4 -6.72 T-102 Tawurgha, Well P-18 Mizdah -9.39 -67.8 0.8 ± 0.5 -3.08 >13 800 T-103 TawuTgha 25 Miocene -7.92 -58.2 18.6 ± 1.9 -4.30 2700± 4900 T-101 Tawurgha Spring -8.21 -60.4 2.8 ± 0.8 -3.38 16 400 ± 7400 T-104 Tumminah, Agrie. Project Mizdah -9.55 -68.3 1.0 ± 0.5 -5.29 >19 500 a Values calculated assuming for the total dissolved carbonate specied the value 5 13C = 10 ± 2. b 14C measurements are from Ref. [4], but are believed to be unreliable by the authors. IAEA-AG-158/12 1 7 3

The data designated by the letter “A ” were obtained at the IA E A on sam ples collected in 1978 by Dr. A. Bello of Aquater, an Italian consulting firm operating in the Libyan Arab Jam ahiriya. A ll other data, designated by the letter “T ” , refer to sam ples collected in the period 1976— 78 w ithin the fram ew ork of the Land and W ater Investigation Project executed by F A O and analysed at the IA E A .

The isotopic data are expressed in the usual w ays, i.e. 5 ° / 00 difference from the m ean content of ocean water for 180 and D and from the P D B reference for 13C .

Errors of stable isotope determ inations are about 0.1 °/00 for 5 180, 1 % 0 f ° r ^ D , and 0.5%o f°r 5 13C, the last being m ainly due to the sam pling technique.

The 14C content is expressed in per cent of “m odern”, i.e. of the 14C content of plants in 1890 (before any im portant injection into the atm osphere o f 14C - f r e e

C 0 2 from fossil fuel com bustion and before the 14C release from therm onuclear

explosions in recent years). Errors of m easurem ent (la ) are quoted for each

1 4 C v a l u e .

The 14C content is, in general, very low — often below the detection lim it.

In a few cases, how ever, the 14C is w ell above the detection lim it. The age of groundw ater is evaluated from the 14C and 13C data by using the form ulas:

t(years) = 8267 In (C 0/C)

1 0 0 ( 6 — ôc ) Co = « * Л X О + 2e/1000) SG - 6C +e

where С is the 14C concentration in the sam ple and C 0 is the “ initial” one, corrected

for changes due to interaction w ith the aquifer m atrix; 5 is the 13C content of the

carbonate species dissolved in the sam ple (w hich is assum ed to be m ainly

bicarbonate); öc is that of the aquifer carbonate; 6 G is that o f the soil C 0 2 a t t h e

tim e o f recharge; and e is the fractionation factor between bicarbonate and C 0 2.

The values adopted for the calculation are: S c = 0 ± l ° / 0 0 , 6 G = — 2 5 ± 1 °/0 0 ,

e = 8 ± 0.5°/oo- The age error (2a) given in Table I takes into account the

uncertainty of all these term s and, of course, the error associated w ith the 14C and

13C determ inations. How ever, w hen the 14C content is below 1% of m odern, or is

equal to or less than tw ice its standard deviation, a m inim al age is given, for w hich

a value o f 30 000 years is used w hen the calculation produces a higher figure - a b o v e 30 000 years, in fact, the age resolution of the m ethod becom es too poor.

The reasons for isotope variations in natural w aters and the general principles

o f isotope studies in groundw ater are assum ed to be fam iliar to the reader, w ho

otherwise m ay find this topic discussed in Refs [5,6].

2. INTERPRETATION OF THE ISOTOPIC RESULTS

Im portant variations are show n by the oxygen and hydrogen stable isotopes,

w hich can be used to form ulate hypotheses on the origin of deep groundw ater and 1 7 4 SALEM et al.

FIG.3. Geographical distribution o f 81S0 data in groundwater. Letters designate the aquifers: P = Palaeozoic, PT = Permo-Trias, M=Mesozoic, UT = Upper Trias, LC = Lower Cretaceous, K = Kicla (Lower Cretaceous), UC = Upper Cretaceous (Ain Tobi, Gharian, Mizdah), W= Waddan (Eocene), M = Miocene.

on its m ain regional flow patterns, and to investigate interconnections betw een

aquifers. Fo r these purposes, stable isotopes appear to be alm ost ideal environm ental

tracers because they are conservative, w hile the chem ical com position of groundw ater

is not, but depends on the aquifer lithology and on the duration of interaction

betw een w ater and rock.

The geographical distribution o f stable isotope values is show n in Fig. 3.

In the m ost southern area, that of the M urzuk depression and of the W adi Ajal Valley, IAEA-AG-158/12 1 7 5

DISTANCE FROM EDRI (km)

FIG .4. Variation of the stable isotope composition of groundwater of the Paleozoic aquifer in the Wadi Ash Shati Valley. Distances are calculated from Edri to an easterly direction (samples are listed in the same order in Table I).

the stable isotope com position of the groundw ater derived from the M esozoic aquifer varies w ithin a narrow range. The m ean values are: S 180 = — 10.81 °/00 a n d

5 D = — 80.1 °/00, and the standard deviations are 0.33°/oo and 3 . 2 ° / 0 0 , respectively.

The 14C is low but not negligible - three sam ples from the M urzuk depression analysed by Klitzsch et al. [2] gave sim ilar results w ith a m ean value 2.4% o f m odem .

Surprisingly high (22.2% of m odern) is the 14C content of sam ple K-26, the m ost eastern site sam pled in the W adi Ajal area, probably indicating the vicinity of a recharge area (not necessarily active now ).

In the W adi A sh Shati Valley, on the northern side of the Ubari Sand Sea, the 14C content of groundw ater issued by the Palaeozoic aquifer drops to negligible values. The m ean stable isotope ratios are: 5 180 = — 10.967oo and 5D = — 79.7 °/00 w ith standard deviations o f 0.2 °/00 and 2.3°/00, respectively. These values are practically identical to those observed in the M urzuk-W adi Ajal area, and support the hypothesis that the groundw ater has the sam e origin in the tw o areas. In other words, the water issued by the M esozoic aquifer in the M urzuk-W adi Ajal zone w ould derive from the deeper Palaeozoic aquifer by upw ards leaking through the interm ediate clay deposits (w ith sandy lenses), or the M esozoic and the Palaeozoic aquifers w ould have been recharged under sim ilar conditions, both in term s o f recharge area characteristics and o f clim atic conditions. According to Pallas, the first o f these hypotheses does not seem acceptable, ow ing to the large thickness

(several hundreds o f m etres) o f the im perm eable deposits separating the tw o aquifers.

A closer exam ination of stable isotope data in the W adi A sh Shati Valley reveals that a sm all but regular variation occurs from west to east, as show n in Fig. 4. 1 7 6 SALEM et al.

In this figure, only data obtained at the IA E A (sam ples T and A ) were used so as to elim inate any spread o f values derived from an im perfect intercalibration of laboratories w hich could m ask the sm all variation occurring. A parallel west-east variation is also show n by the 13C values and by the chem ical com position of groundw ater, changing both in salt concentration and in anions per cent distribution. Both the chem ical and the isotopic trends indicate a m ixing of tw o different waters; or, in other w ords, an inflow into the Palaeozoic aquifer of an isotopically heavier water from the west w hich m ixes w ith the water com ing from t h e s o u t h .

In the area north of Jebel G argaf and in A l Jufrah the stable isotope com position

of w ater o f the Palaeozoic aquifer show s a m uch larger variation from west to east, i.e. from the values o f 5 180 = — 9.58°/00 and 5 D = — 69.0°/oo o f sam ple E-33 (w ell

WS- 8) to those of 5180 = -10.67°/oo and 5D = -7 9 .4 °/00 of sam ple T -39 (w ell J-18 o f the A l Fugrah Agricultural Project), passing through the interm ediate values of

E-34 (w ell No. 1 at W adi Zim am ). This again indicates the inflow of an isotopically heavier water from the west, w hich here w ould be m uch m ore im portant than in the

W adi A sh Shati area1. O n the other hand, the m ore negative б-values of sam ple

T-39, sim ilar to those o f the Palaeozoic in the W adi A sh Shati area, w ould confirm the arrival o f w ater from the south. The 14C age, increasing from w est to east, w ould also be in agreem ent w ith this hypothesis, but unfortunately 14 С

m easurem ents of sam ples E-33 and E-34 are unreliable [4]. Concerning 13C , t h e

abrupt change in values w ith respect to those found in m ore southern areas should be noted. This is not in contradiction, how ever, w ith an origin o f water from the

south because the 13C (and the 14C) is strongly affected by dissolution-

précipitation processes o f carbonate — the increase o f carbonate content observed

in the A l Jufrah wells, w ith respect to the m ore southern areas, w ould therefore

fully explain the 13C change.

In the A l Jufrah area, the w ater in the M esozoic aquifer has the sam e stable

isotope com position as that o f the Palaeozoic, confirm ing that the latter is recharging

the aquifers above. This recharge seems, how ever, of lim ited im portance because

m uch less negative ¿¡-values are observed in the M esozoic aquifers in other places.

B y contrast, the Palaeozoic aquifer does not seem to supply w ater laterally to the

W addan form ation in the H un Graben, at least in the A l Jufrah area. In fact, the

isotopic com position o f water from the W addan form ation is quite different, as

observed in tw o wells sam pled by Aquater at H un (IC-21 and IC-22), giving values of

— 8 . 4 °/00 for 180 and o f — 63°/00 for deuterium .

N orth o f A l Jufrah, five wells exploiting the Kicla aquifer (Low er Cretaceous)

were sam pled in the area between A sh Shuw ayref and the W adi Zam zam Valley.

1 P. Pallas suggests that this water could derive from recharge on the outcrops of the Palaeozoic in the Jebel Gargaf. However, the distribution of stable isotope values tends to indicate an origin from the west. It is hoped that future samples might shed light on this question. IAEA-AG-158/12 1 7 7

They do not contain 14C in any appreciable am ount and have a very uniform stable isotope com position w ith m ean values of S 180 = — 9.44 °/00 and 5 D = — 69.0o/oo

(0.18 and 1.2 °/00 are the respective standard deviations). These values represent the isotopic com position of the groundw ater com ponent arriving from the west, perhaps

from the Ham ada al Ham ra, w hich has been already observed in the tw o wells,

E-33 and E-34, west o f A l Jufrah. It m ight be interesting to note that sim ilar values of stable isotopic com position of groundw ater are also found in the area o f Ghadam es on the west side of the Ham ada al Ham ra, near the point where the Algerian,

Tunisian and Libyan borders meet.

Stable isotope §-values, those so far reported, for the Rezzan aquifers and the

Kicla form ation, are definitely m uch m ore negative than those observed in a few shallow groundw ater sam ples of certain recent origin. Sim ilar low heavy isotope contents have been observed in m any other deep wells and springs in the Sahara

[3, 7 -1 2 ] and appear to be characteristic o f old groundw ater w hich w as recharged under m ore hum id clim atic periods that occurred during the Quaternary.

In the Jebel N efusah area, the w ater from the Kicla aquifer exhibits ô-values w hich are definitely m ore positive than those encountered up to now in the deep aquifers. The m ean values o f these sam ples are, in fact, 5 180 = — 6.96°/00 and

SD = — 45.1 % 0 w ith a very lim ited scatter. These values are m ost probably characteristic of groundw ater locally recharged. The 14C content, although low, is not negligible — up to 3.2% of m odem in the Kicla water, corresponding to an age of about 20 000 years, and up to 18.4% in the Upper Cretaceous w hich w ould indicate an age ranging from Recent to 8000 years.

In the area o f Taw urgha, near the coast, tw o different groups o f stable isotope values are found. In the Gharian and M izdah aquifers (sam ples T-100, 102 and 104) the m ean 5 values (5 180 = — 9.48°/00, 5 D = — 68.2°/00) are practically identical to those o f the Kicla aquifer of the area A sh Shuw ayref-W adi Zam zam ; this w ould indicate that the Kicla, the G harian and the M izdah aquifers are all interconnected, and that water flow s from the first into the others. In the M iocene aquifer

(sam ple T-103) the heavy isotope content is definitely higher: S lsO = — 1 . 9 2 ° / 0 0 a n d

S D = — 58.2°/00, w hich m ight result from a m ixing o f water derived from the underlying Upper Cretaceous aquifers w ith w ater recharged b y rainfall west of

Taw urgha. The latter com ponent should have 5-values of about — 5 °/00 for 180 and o f — 30°/oo for deuterium : as a com parison, the average isotopic com position of rainfall at Tunis-Carthage (an IA E A -W M O netw ork station in Tunisia) has been

ô 180 = — 4.6 °/00 a n d 5 D = — 2 6 ° / 00 over a period o f about five years; and that of w ater in the Q uaternary aquifer at M utrad on the M editerranean coast is

5 180 = — 5.03°/oo and S D = — 2 8 . 6 ° / 0 0 . Assum ing these last values, the contribution of precipitation to the recharge o f the M iocene aquifer w ould be about 3 0 % as opposed to 70 % derived from the Kicla form ation.

The isotopicjpm position of water from the Taw urgha springs (5 180 = — 8.2l700,

S D = — 60.4°/oo) is definitely sim ilar to that o f the M iocene aquifer from w hich it 1 7 8 SALEM et al.

m ainly derives. The direct contribution of the U pper Cretaceous aquifers to the spring discharge w ould be m inor, and at the tim e o f this sam pling w as about

20 ± 15% of the total as com puted from 180 and D content. The chem ical com position of the spring also com pares w ell w ith that of the M iocene aquifer.

The 14C content apparently contradicts this conclusion being only 2.8% of m odern as com pared w ith 18.6% of sam ple T-103. How ever, the latter is derived from a w ell only 25 m deep and, therefore, it represents only the water in the upper part of the M iocene aquifer w ith probably a higher 14C content than the bulk of w ater of the w hole aquifer. This w ould explain w hy the 14C content of the spring is m uch less than that observed in sam ple T-103.

3. CONCLUSIONS

W e now try to sum m arize the inform ation gained w ith the environm ental isotopes on the flow patterns of deep groundw ater. The water in the Palaeozoic aquifer in Fezzan flow s northw ards, and it is found until the area o f A l Jufrah where it discharges also in the M esozoic aquifer. The K icla aquifer is not significantly recharged by the Palaeozoic, w ith the exception o f the A l Jufrah area, but it receives its water from the west, possibly from the Ham ada A l Ham ra.

The water in the Kicla then flow s to the north and is found to flow into the

Upper-Cretaceous aquifer in the area of M isratah. The Taw urgha Spring, deriving m ost o f its water from the M iocene, does not appear to be a m ajor direct discharge point of the Upper Cretaceous aquifers. These aquifers, how ever, m ight discharge first in the M iocene, the w ater o f w hich w ould in part derive also from infiltration or local rainfall.

The Kicla aquifer is also recharged in the area o f the Jebel Nefusa, as indicated by the different isotopic groundw ater com position.

ACKNOWLEDGEMENTS

Thanks are due to P. Pallas and G. Krusem an, w ho read the m anuscript and suggested m odifications. Also the useful discussions w ith M. Taka, w ho guided tw o o f us to a field trip in the Fezzan, are gladly acknow ledged. A. E l Dieb and

A. M ’Satar helped in sam pling in the Fezzan. Thanks are also, due to the staff of the Isotope H ydrology Laboratory of IA E A , w ho carried out the isotope analyses. IAEA-AG-158/12 1 7 9

REFERENCES

[1] GOUDARZI, G.H., Geology and Mineral Resources of Libya: A Reconnaissance, US. Geol. Survey Professional Paper 660, Washington, DC (1970) 104 pp. [2] PALLAS, P., “Water Resources of the Socialist People’s Libyan Arab Jamahiriya”, paper preprint presented at Symp. of Tripoli in 1978, Secretariat of Dams and Water Resources, Tripoli (1978). [3] KLITSCH, E., SONNTAG, C., WEISTROFFER, K., EL SHAZLY, E.M., Grundwasser der Zentralsahara: Fossile Vorräte, Geol. Rundsch. 65 (1976) 264-87. [4] SRDOC, D., SLIEPCEVIC, A., OBELIC, B„ HORVATINCIC, N., MOSER, H., STICHLER, W., Isotope investigations as a tool for regional hydrogeological studies in the Libyan Arab Jamahiriya, IAEA-AG-158/11, these Proceedings. [5] INTERNATIONAL ATOMIC ENERGY AGENCY, Guidebook on Nuclear Techniques in Hydrology, Tech. Reps Series No. 91, IAEA, Vienna (1968). [6] BRADLEY, E., BROWN, R.M., GONFIANTINI, R., PAYNE, B.R., PRZEWLOCKI, K„ SAUZAY, G., YEN, C.K., YURTSEVER, Y., “Nuclear techniques in groundwater hydrology, groundwater studies”, An International Guide for Research and Practice, UNESCO, Paris (1 9 7 2 ) Chap. 10. [7] DEGENS, E.T., Geochemische Untersuchungen von Wässern aus der Ägyptischen Sahara, Geol. Rundsch. 52(1962) 625-39. [8] CONRAD, G., FONTES, J.-Ch., “Hydrologie isotopique du Sahara Nord-Occidental, Isotope Hydrology 1970 (Proc. Symp. Vienna, 1970), IAEA, Vienna (1970). [9] GAT, J., Comments on the Stable Isotope Method in Regional Groundwater Investigations, Water Resour. Res. 7 (1971) 980—93. [10] GONFIANTINI, R., CONRAD, G., FONTES, J.-Ch„ SAUZAY, G., PAYNE, B.R., “Etude isotopique de la nappe du continental intercalaire et de ses relations avec les autres nappes du Sahara septentrional,” Isotope Techniques in Groundwater Hydrology 1974 (Proc. Symp. Vienna 1974) 1, IAEA, Vienna (1974) 227-41. [11] SONNTAG, C., KLITSCH, E., LOEHNERT, P., MÜNNICH, K.O., EL SHAZLY, E.M., KALINKE, C., THORWEIHE, U , WEISTROFFER, K., SWAILEM, F.M., “Paleoclimatic information from deuterium and oxygen-18 in carbon-14-dated north Saharian groundwaters: Groundwater formation in the past,” Isotope Hydrology 1978 (Proc. Symp. Neuherberg (1978) 2, IAEA, Vienna (1979) 569-81. [12] EDMUNDS, W.M., WRIGHT, E.P., Groundwater recharge and paleoclimate in the Sirte and Kufra Basins, Libya, J. Ну drol. 40 (1979)215—41.

IAEA-AG-158/13

RECHARGE OF GROUNDWATERS IN ARID AREAS: CASE OF THE DJEFFARA PLAIN IN TRIPOLITANIA, LIBYAN ARAB JAMAHIRIYA

M. ALLEM M OZ

Groupem ent d’Etudes et de

Réalisations des Sociétés

d’Am énagem ent Régional,

N î m e s

P h . O L I V E *

Centre de Recherches Géodynam iques,

Thonon-les-Bains,

F r a n c e

Abstract

RECHARGE OF GROUNDWATERS IN ARID AREAS: CASE OF THE DJEFFARA PLAIN IN TRIPOLITANIA, LIBYAN ARAB JAMAHIRIYA. By means of soil water contents and variations of piezometric levels and tritium concentration, the recharge of groundwaters is determined. If direct infiltration of precipitation is negligible, the major recharge mechanism is the infiltration through the wadi beds during floods.

Tw o quite different m orphological aspects characterize Tripolitania: to the south, Jebel Nefusa and to the north the Djeffara plain (Fig.l). Jebel Nefusa appears as a m ountain area culm inating at 900 m near G haryan and is deeply cleaved by rivers or w adis. The Djeffara plain offers a very level surface, the gradient of w hich does not exceed 1 %. The m inor beds of w adis are distinctly o u t l i n e d .

1. MAIN HYDROCLIMATOLOGICAL OUTLINES

O n Fig.2 are show n the average m onthly values o f precipitation in m illim etres, m oisture in per cent, air tem perature in centrigrade and potential évapotranspiration

(Turc’s form ula) at the A l Aziziyah (125 m ) and G haryan (725 m ) stations.

It should be noted that rain bursts occur in autum n (October, Novem ber,

Decem ber) and in w inter (January, February, M arch). Their distribution in tim e

* Present address: Direction de l’Hydraulique, Divisions des Ressources en Eau, B.P.A 48, Fez, Morocco

181 1 8 2 ALLEMMOZ and OLIVE

FIG.l. The two different morphological aspects o f Tripolitania.

or space varies greatly and the influence of relief is very clear. The daily variations o f air m oisture are such that the dew and the m orning fog are far from negligible.

Finally, let us m ention the suddenness and violence o f floods w hich, since they are in direct relation to precipitation, occur in autum n and in w inter. The m ain part of the flow is over in a few hours. N o w adi is perennial in Tripolitania.

2. EVALUATION OF INFILTRATION

Three kinds o f data have been used to m easure infiltration in this area: the evolution of m oisture m easured w ith a neutron probe in the unsaturated zone, the variations of piezom etric levels, and the therm onuclear tritium content of ground­ w a t e r s .

2.1. Neutron probe

A system atic investigation o f 10 sites for over one year allow ed us to evaluate the real évapotranspiration in the area considered [1]. Site 3, covered w ith IAEA-AG-158/13 1 8 3

80 Precipitations (mm) GHARYAN (1924 -1975) 60 cnd^AL AZIZIYAH - Z /(1920 -1975) ¿0

20 f c

0 Humidity (Vo) . GHARYAN (1926-1975) " ond(AL AZIZIYAH _ '(1926 - 1975) 60

50

40

30 Temperature (”C) GHARYAN (1924 -1975) and AL AZIZIYAH 24 *-"^(1913 - 1975)

20

16

12

8 Potentiol évapotranspiration (mm) GHARYAN (1959 -1975) 160

120

80

40 SONDJFMAMJJA

!. > verage monthly precipitation values (in mm). 184 ALLEMMOZ and OLIVE

H um idity ( % ) 0 5 10 15

vegetation (eucalyptus) and located as show n by Fig.l, allow s the real évapo­ transpiration to be m easured. It goes through m uddy sand, w ith m ore clay at

1.90 m. On 9 M arch 1974, the water stored in the 0— 120 cm section am ounted t o 88 m m (Fig.3). O n 12— 13 M arch, rains of 58 m m resulted in an increase of the stored water w hich was 137 m m high on 17 M arch. This water was subjected to évapotranspiration and on 21 April the stored water again fell to 85 m m . So the average real évapotranspiration calculated during these 35 days (from 17 M arch to 21 April) was 1.5 mm /d.

In fact the real évapotranspiration (E T R ) varies greatly according to the various sites (effect o f vegetation and o f the profile texture) and to the season.

From Novem ber to February (rainy m onths and low tem perature) the E T R varies

from 0.8 to 1.3 m m daily. In M arch and April (beginning of the dry season), it rises from 1.5 to 2.5 m m daily. Then, during sum m er, it is alm ost reduced to

nothing (less than 0.5 m m /d) since w ater reserves available in the soil are exhausted.

The sam e can be said for autum n and w inter m onths if it does not rain. It should be noted that the E T R never has any constant value — it varies greatly in relation to the saturation degree o f the soil’s first decim etres.

Site 7 show s stratified and clay-silt layers near 1 and 2 m . O n

3 Septem ber 1974, the stored water for the 0— 540 cm portion am ounted to

500 m m (Fig.4). O w ing to the flood of 20 October, it was subm erged by 1.140 m m

water, w hich disappeared in six days, and the first m easurem ent recorded after IAEA-AG-158/13 185

Humididy (%)

this flood on 3 Novem ber indicated 900 m m stored water, i.e. an increase of 400 m m in the reserves. It m ust be adm itted that between 3 Septem ber and 20 October the

E T R was at zero (dry soil), and between 20 October and 3 Novem ber it was

negligible w ith respect to the w ater sheet w hich w as resolving. A s a m atter o f fact,

out of the 1140 m m , 400 m m serve to m oisten the ground again; therefore, 700 m m

w ould rem ain that have necessarily infiltrated.

This situation is an outstanding illustration of the dual m echanism of

infiltration. The filling o f the big conduits (m acroporosity) is responsible for a

speedy transm ission o f part o f the w ater (700 m m ) to low er levels, then the filling

of fine conduits (m icroporosity) allow s a slow increase in the m oisture rate (from

500 to 900 m m ). Finally, if the profile o f 16 Novem ber is com pared w ith that

of 3 Novem ber, w hen the m oistening front had reached 420 cm , it is noted that on

16 Novem ber the front had gone beyond the low est altitude (540 cm ) and that the profile began to dry out above 420 cm.

Site 10, com posed of alternating layers of sand (10 % o f m oisture) and clay

silt (20 to 3 0 % of m oisture), brings out lateral inflow s. First let us consider the

0— 250 cm section (Fig.5). O n 9 Novem ber 1974, the water stock was 535 m m

high, it fell to 440 m m on 20 February 1975 after precipitation of 80 m m in

Decem ber and January. During this period, the E T R could be estim ated at about

100 m m . Therefore, 75 m m were, infiltrated. For the 250—490 cm section, the 1 8 6 ALLEMMOZ and OLIVE

Humidity (%)

FIG.5. Site 10: Stored water measurements in relation to precipitation and infiltration from November 1974 to February 1975.

water stock rose from 190 m m on 9 Novem ber to 360 m m on 20 February, i.e.

an increase of 170 m m , w hich could on no account be explained by infiltration into the superficial area, because the E T R should then have been at zero during

these three w inter m onths. The only possible explanation for this increase of the

w ater stock in the 25 0 — 490 cm area is to be sought in a lateral water inflow

issuing from the w adi flow ing close by. T his flow is just below the clay silt level

located at 200 cm. Profiles defined between 9 Novem ber and 20 February

illustrated this phenom enon. It should be noted that this w ater w as preserved

from the E T R by the overlying clay layer and was conveyed to the groundw ater

through sand draining.

Thus, if the precipitations are taken back com pletely by the E T R (site 3),

they nevertheless contribute to m aintaining in the soil a w ater stock w hich is used

by vegetation during the six m onths wet season. The groundw ater recharge occurs

after the floods in the w adi beds, in closed dips constituting the term ination of

w ater distribution areas (site 7), and in flood-slackening areas (site 10). In these

select areas infiltration is m ainly due to m acroporosity.

2.2. Piezom etric variations

First let us consider the seasonal variations. The P ZH 3 piezom eter is situated

in an overflow area of the A l H ira wadi. In this area, pum p flow s are low . It was IAEA-AG-158/13 1 8 7

i ---- 1----- 1---- r"--- 1---- r “I-- - - - 1---- 1---- 1---- 1---- г - 9 6 2 ~ Hi 0 .4m - 9 6 .4 >

- 96-6 Piezometer PZH.3 У -9 6 .8 'S £ С ю - 9 7 S 8 «О s 6 I o 4 Floods | 2 > 0 i i____:_I_u I I______I I______L ______I______I______I------L. J JASON D|JFMAMJJ 19 7 4 1975

FIG. 6. Piezometric variations in the overflow area o f the Al Hira wadi.

noted that the flood o f 20 O ctober 1974 (lag tim e of 20 years) enabled the static level to rise again by 0.4 m (Fig. 6 ). The im pact zone of the recharge of the A l Hari w adi was estim ated at 300 X 10 6 m 2. The groundw ater average upw elling, further to the O ctober flood, am ounted to 0.15 m in form ations where the storage coeffi­ cient, calculated after pum ping tests, varied from 5 to 10%. So the infiltrated volum e was between 2.25 and 4.5 X 10 6 m 3. Considering that the volum e o f the

October flood was 12 X 10 6 m 3 (A l H ira w adi and its affluents), the infiltration rate m ay be evaluated at 30 ± 10%.

O n a pluriannual scale (Fig.7) it appears that pum pings entail a decrease in the piezom etric level w hich increases m ore and m ore rapidly. A sudden disconti­ nuity was noted in 1964. U ndoubtedly this was due to the recharge resulting from the exceptional floods of 1964.

W hatever the tim e-scale m ay be, it appears that the floods alone ensure the recharge of groundw aters. W ith an average annual inflow evaluated at 12 X 10 6 m 3 and an infiltration rate o f 30% , the average efficient infiltration to be expected is about 3 X 10 6 m 3 a year. Pum pings in this area, am ounting at present to 2 5 X 1 0 6 m 3 annually, correspond to seven tim es as m uch as the natural recharge.

This results in the m ost apparent drop in the piezom etric levels, as recorded on

F i g . 7 .

2.3. Tritium contents

W ith regard to the tritium contents m easured in the G ibraltar and Alexandria precipitations [2], and w hat is know n about fallouts in Europe, it is possible to trace back to the average annual values o f the tritium contents in the rains o f this part of N orth Africa (Fig. 8). 8 8 1 concentrotion (TU) FIG.8. Average annual values o f tritium contents in the rains o f the North African region African North the f o rains the in contents tritium f o values annual Average FIG.8. UT under study. under FIG. 7. eraei tepeoerclvloigt tepmigofwater. f o pumping the to owing level piezometric the in Decrease n OLIVE V I L O and Z O M M E L L A

IAEA-AG-158/13 1 8 9

б1во (•/..)

FIG.9. 8D versus 5 180.

During 1974— 75, the 3H content of groundw aters was determ ined at

35 tim es — 14 of them were below 1 T U w ith the rest between 2 and 7 TU.

Further to the system atic study of 3H contents in groundw aters, it is now adm itted that aquifers m ay be considered as w ell-m ixed reservoirs [3— 5]. Thus, know ing the 3H content of the recharge, the 3H content o f groundw aters enables us to calculate the recharge coefficient a (annual recharge/volum e o f the reservoirs) and to infer the m ean residential tim e o f w ater T = 1/a.

The present values o f tritium contents lying between 2 and 7 T U require a recharge coefficient a between 0.001 and 0.004, w hich corresponds to an average confinem ent tim e T between 250 and 1000 years (Fig. 8).

The average m axim um depth of the groundw ater in w hich are found contents between 2 and 7 T U is 200 m. W ith a 10% storage coefficient, this corresponds to a water section of 20 m . Therefore, considering that 0.001 < a < 0.004, the annual water renewal (m ainly through pum ping) is between 0.02 m/a and 0.08 m/a. Here again are the m ere hydrological data previously m entioned, nam ely the average recharge: 0.01 m/a (3 X 10 6 m 3/300 X 10 6 m 2) and the average value o f pum pings:

0.08 m/a (25 X 10 6 m 3/300 X 10 6 m 2). In addition to the fact that hydrological

and isotopic results agree, this com pliance show s that the assum ption o f a good

m ixture of the aquifers (should pum ping be carried out) is again confirm ed, and

for the first tim e in an arid area. 1 9 0 ALLEMMOZ and OLIVE

3. OTHER ISOTOPIC DATA

The great age of the deep groundw aters (200 to 600 m ) has been proved by seven datings w ith 14C — the percentages of m odern carbon are betw een 5 and 10%.

Therefore, it should be adm itted that here [ 6 ], as in other arid areas in Africa, the present recharge o f these deep groundw aters is negligible.

This great age o f deep aquifers is also revealed by an exam ination o f D/H and

180 / 160 (Fig.9). W aters o f superficial and m iddle aquifers are distributed on the straight line o f m eteoric w aters (5D = 8 5 180 + 10%»). Their average value in

180 , 5 = - 6 %o, corresponds to an approxim ate tem perature of 11°C

(5180 = 0.7 t — 13.6 % o), w hich is close to the present tem perature prevailing in w inter (w et season) in the Djebel (Fig.l). B y contrast, w aters o f deep aquifers are distributed along the straight line 6D = 4.2 § 180 — 26%c, w hich indicates that these w aters have been strongly evaporated, as occurs in neighbouring areas [7]. First the water had a 5 180 s - 9.5%o, w hich corresponds to a tem perature of 6° C . B u t the current clim atology o f these areas cannot explain such a param eter (Fig.l).

It is therefore necessary to suppose a very old recharge [ 8 ], m aybe on the upper

Pleistocene and a wetter period than the present one, though rather dry and c o l d [ 9 ].

4. CONCLUSIONS

If the water stock of deep groundw aters (m ore than 200 m ) can be considered fossil, the upper aquifers (up to 200 m ) are now fed w ith natural recharge.

This recharge occurs m ainly in the w adi beds and in the provisional storage areas after the floods. This recharge, corresponding to the third o f the flood waters approxim ately, takes place very quickly (a few days) through the large conduits o f the unsaturated area (m acroporosity). D uring winter the precipitation, w hich is alm ost entirely retrieved b y évapotranspiration, m aintains a w ater reserve located in the soil m icroporosity and used by vegetation.

A t present, pum ping activities are alm ost 10 tim es as large as the natural recharge, w hich results in an obvious drop in piezom etric levels. O nly by the im m ediate im plem entation o f a w ater policy w ill a disaster be avoided in this area.

REFERENCES

[1] ALLEMMOZ, M., Alimentation des nappes en pays aride. Etude de l’infiltration et de l’évapotranspiration en Tripolitaine (Libye), Thesis, Grenoble (1976) 207 pp. [2] INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data: World Survey of Isotope Concentration in Precipitation, Tech. Rep. Series Nos 96, 117, 129, 147, 165, IAEA, Vienna (1953-1971). IAEA-AG-158/13 191

HUBERT, P., MARCE, A., OLIVE, Ph., SIWERTZ, E., Etude par le tritium de la dynamique des eaux souterraines, C.R. Hebd. Séances Acad. Sei. Paris 270 (1970) 908—11. SIEGENTHALER, U., OESCHGER, H., TONGIORGI, E., “Tritium and oxygen-18 in natural water samples from Switzerland”, Isotopes in Hydrology (Proc. Symp. Vienna, 1970), IAEA, Vienna (1970) 373-85. ALLISON, G.B., HOLMES, J.W., The environmental tritium concentration of underground water and its hydrological interpretation, J. Hydrol. 19 (1973) 131—43. SRDOC, D., SLIEPÍEVIC, A., OBELIC, В., HORVATINCIC, N., MOSER, H., STICHLER, W., “Isotope investigations as a tool for regional hydrogeological studies in Libyan Arab Jamahiriya”, IAEA-AG-158/11, these Proceedings. CONRAD, G., FONTES, J.C., “Hydrologie isotopique du Sahara Nord-Occidental”, Isotopes in Hydrology (Proc. Symp. Vienna, 1970), IAEA, Vienna (1970) 405—19. SALEM, О., VISSER, J., DRAY, M., GONFIANTINI, R., “Groundwater flow patterns in the western Libyan Arab Jamahiriya - evaluations from isotopic data” , IAEA-AG-158/12, these Proceedings. ALAYNE STREET, F., GROVE, A.T., Environmental and climatic implications of late Quaternary lake-level fluctuations in Africa, Nature (Lond.) 261 (1976) 385—90.

IAEA-AG-158/14

ASPECTS OF ENVIRONMENTAL ISOTOPE CHEMISTRY IN GROUNDWATERS IN EASTERN JORDAN

J.W. LLO YD

Hydrogeological Section,

Departm ent of Geological Sciences,

University of Birm ingham ,

Birm ingham ,

United Kingdom

A b s t r a c t

ASPECTS OF ENVIRONMENTAL ISOTOPE CHEMISTRY IN GROUNDWATERS IN EASTERN JORDAN. Aspects of the environmental isotope data from a sandstone and a limestone aquifer are given. The data concerning the sandstone are limited but indicate the type of recharge that is occurring in the outcrop area. Although the 14C data indicate some modern recharge in radio­ carbon terms the tritium data show evidence of only localized recharge. Stable isotope data from one well indicate that indirect recharge may be important. In the limestone aquifer it is found that tritiated groundwater occurs chiefly in the vicinity of wadis indicating the importance of indirect recharge. The lack of tritiated groundwater in the ‘recharge mounds’ in the interfluve areas is seen as partly a function of sampling but also as indicating the high permeability zones at the tops of the aquifers. The 5 13C data distribution are examined with respect to possible recharge mechanisms.

INTRODUCTION

The m ain aquifers o f Eastern Jordan consist o f Low er Palaeozoic sandstones in the south w ith an extensive Cretaceous carbonate aquifer extending throughout the rest o f the area.

The rainfall throughout Eastern Jordan is sm all, as show n on F ig .l, and calculations show that the predom inant direct recharge com ponent to the carbonate aquifer occurs only in the west (Lloyd et al. 1966) [1]. The recharge appears to occur to a series of m ounds w ith groundw ater flow m oving tow ards the intervening river valleys. In the southern sandstones no direct recharge is thought to occur.

1 9 3 1 9 4 LLOYD

SOUTHERN SANDSTONE AQUIFER

Tritium data

The isotope data for the sandstone aquifer are lim ited but they do indicate som e interesting inform ation pertinent to the outcrop perim eters o f large regional groundw ater basins in arid areas. The sam ple points are show n on Fig. 2 and the IAEA-AG-158/14 1 9 5

FIG.2. Tritium data distribution for the southern sandstone aquifer in Eastern Jordan.

data are given in Table I. Firstly, it should be noted that the tritium data in the rainfall in the vicinity are of the order of 100 T U and that only som e wells, S I9,

S 6 , S5, PP9 and PP70 have positive tritium ( > 4 T U ) values in their groundw aters.

These w ells are located in the outcrop but other outcrop w ells at sim ilar groundw ater

elevations do not have tritiated water. This is seen as dem onstrating that m odern recharge occurs, but only w ith a variable distribution in tim e and location and is

consistant w ith a hypothesis that sporadic recharge to the system is through infiltration loss from runoff during flash floods.

Stable isotope data

The stable isotope data plotted on Fig.3 clearly show that the groundw aters

in the sandstones do not correlate directly w ith the M editerranean rainfall-type 1 9 6 LLOYD

TABLE I. ISOTOPE DATA FOR SANDSTONE AQUIFER

Well No. Tritium 8D 5 180 s 13c 14C ‘Corrected’ (TU) (.%») (%o) (%o) (% modern) age-years

S3 3.2 - 3 4 - 6 .2 - 4 .6 40.0 Modern

S7 0.5 - 4 1 - 6 .5 - 9 .9 40.0 Modern OO о S19 - 4 0 - 5 .7 - 1 4 .6 41.8 25 0 0

S33 1.0, 0.2 - 4 0 - 6 .1 - 8 .4 12.8 6 0 0 0

S36 1.2, 3.0 - 3 3 - 6 .1 - 3 .2 16.1 Modern

S43 0 .7 , 2.2 - 2 4 - 4 .3 - 4 .0 5.3 8 600 - 3 .4 1.2 19 500

S49 1.4 - 3 5 -6 .1 - - -

S5 5.3, 1.5 - 3 8 - 6 .4 - 7 .4 18.1 3700

PP70 23.0 - - - 4 .1 41.9 Modern

S2 1.7 - 4 4 - 6 .4 - 5 .0 38.1 Modern

S6 74.8 - 2 6 - 4 .2 - 1 3 .0 86.8 Modern

PP9 1.3, 6 .4 - 3 7 - 6 .1 - 7 .0 22.8 1300

PP22 1.4 - 3 5 - 6 .3 - 7 .0 22.1 1500

relationship. This m ay in certain cases be due to different clim atic conditions in the past, such as in the old groundw ater sam ple S43 (see below ), but as m any of the w ells are in the unconfined section of the aquifer it is m ore likely to dem onstrate evaporative effects during the recharge event. This is particularly w ell show n by the highly tritiated sam ple S 6 and indicates that although indirect recharge occurs, the w ater that infiltrates below the run-off areas is subject to heavy evaporation and the recharge m echanism is com plicated. In effect the depletion in the stable isotopes show s that the possible am ount of recharge that can occur in an already severely restricted recharge environm ent is even m ore curtailed.

In exam ining the stable isotope data it is w orth considering the data available from the seven rainfall stations show n on Fig.3. The 5 180 -SD relationships for these stations are plotted as average w inter values w eighted for the am ount of precipitation, against the M editerranean relationship. The num ber o f analyses are lim ited but it is interesting to note that the desert stations o f A zraq and Ram do not relate to the standard M editerranean relationship and Rabba also is som ew hat displaced. The data m ay suggest that m oisture passing over the desert areas is subject to evaporative or possibly virgis effects. IAEA-AG-158/14 1 9 7

s 18 о % °

FIG.3. Stable isotope data for precipitation stations and groundwater samples from southern desert sandstones.

Carbon isotope data

The 14C data for the sandstone aquifer show n in Table I are w ith age corrections carried out using the W igley (1976) [2] technique. It w ill be seen that m ost of the outcrop w aters are ‘m odern’ in 14C term s, w hich is consistent w ith the recharge hypothesis, but that old waters are present under outcrop (PP9, S5, S I9) and that w a t e r s 8 t o 20 000 years old are present in the confined area close to outcrop.

Here the 14C data pose the dilem m a of recharge versus ‘fossil gradient’ (Burdon,

1977) [3]. A s show n on Fig.2 a well-defined hydraulic gradient exists throughout the area, and although it is undoubtedly supported to a certain extent by the m odern recharge it is unlikely that the im plied groundw ater flow m oving under the gradient can be accounted for purely b y such recharge, especially in view o f the very low rainfall am ounts that occur in the area (Lloyd, 1969) [4]. The presence of fairly old groundw ater close to outcrop therefore supports the view that m ixed recharge and fossil gradient decay conditions probably exist in the area w ith m uch of the old water possibly having been recharged som e 10 —20 000 years ago. 1 9 8 LLOYD

FIG.4. Sample area over the limestone aquifer showing tritium distributions.

The 14C data from well S43 provide interesting inform ation. The first

sam ple obtained at a pum ping rate of 24 m 3/ h gave a m arkedly low er age

(8600 years) than the second sam ple (19 500 years) w hich was obtained w hen the

well was pum ped at 60 m 3/h. The other chem ical param eters, how ever, show ed no

change. The higher pum ping rate obviously drew water from a greater depth and

distance, the depth probably being m ore significant. The im plication is then that

stratification o f w ater occurs, w hich is revealed by 14C analysis but not from other

inform ation. The origin, nature or distribution o f this postulated stratification is IAEA-AG-158/14 1 9 9

I I + I + 500 400 300 200 100mm Precipitation

be a fraction of stratagraphical unit

FIG.5. Conceptual model o f groundwater flow in limestone aquifers.

not know n, but these 14C differences indicate som e o f the problem s o f obtaining definitive groundw ater age data from norm al pum ped sam ples. In this particular

case the ‘age’ obtained is related to the yield and depth influence o f pum ping.

LIMESTONE AQ UIFER

Tritium data

If we now turn to the carbonate aquifer environm ental isotope data, som e

interesting inform ation is im m ediately apparent from the tritium values. O n Fig.4,

in a representative sam ple area o f the aquifer, the positive tritium levels are seen to occur predom inantly outside the m ain rainfall zone w here tritium levels in precipitation are around 100 TU , and w here large volum es of direct recharge are

considered to occur annually (Parker, 1970) [5]. Further, in the classical style the

recharge m ounds occur under the high precipitation areas and groundw ater gradients

slope radially from these areas despite the fact that m uch o f the m ound water is

non-tritiated. W hat then do the tritium data infer and what possible flow

m echanism s occur in the carbonate aquifer? 2 0 0 LLOYD

FIG.6. Areas o f groundwaters with differing 8 13C ranges.

Firstly, the tritiated w aters aw ay from the m ound areas are located in valleys

and indicate the im portance o f indirect recharge as transm ission losses from floods

into the fissured sections — a point not generally appreciated from hydraulic data.

Secondly, the non-tritiated water in the m ound areas im plies that the large volum es

of direct recharge can only m ove through a thin high perm eability upper section of

the aquifer over the top of underlying older waters w hich are know n to occur

(Parker [5]). The basis for this statem ent is that the tritium sam ples are pum ped

sam ples taken from wells penetrating into the deeper zones o f the aquifer so that

integrated sam ples are obtained resulting in reduced tritium levels. The hypothesis

is supported by other evidence. Frequently in drilling, total circulation losses

occur at the water-table indicating high perm eability, w hich is a com m on feature IAEA-AG-158/14 2 0 1

-16- Г-\ \ \ ♦

/ ; ■ \ ' « / ♦ ' i ♦ \ и 4 (- i ♦ E \ c • j \ • • ♦ \ ------12 Чл ♦ N. .

I - 1 . ^ - • I i Y

К -10 - \ ч «

\. A 'j ) - ' л •/

-8 - ! V ' ' I j .^ v.• N o n - t r it ia t e d / D 7 ( * 44 Triti ated and j / ^ ♦ undoubtedly mixed waters

A etc. relates to areas on Figure 6

ь

10 20 30 U 0

14 С % Modern

FIG. 7. Ь1ЪС and 14C%o modern data for limestone aquifers.

o f carbonate aquifers w ith m arked solution o f lim estones resulting from water- table fluctuation. Further, lim ited tem perature-conductivity profile data indicate greater Assuring in the shallow er part o f the aquifer than at depth. W here serial sam pling has been carried out as at Shaubak, it has been dem onstrated that tritium levels decline w ith increasing w ell yield, for exam pling tritium reducing from 7 to

0 T U w ith the yield increasing from 80 to 120 m 3/h, w hich is consistent w ith a

stratified groundw ater condition. U nfortunately, depth profiles o f tritium are not

available but in any case m ay not be o f value in fully open w ells penetrating

variable perm eability m aterials. 2 0 2 LLOYD

TABLE II. RECHARGE M ECHANISM S INFERRED ON THE BASIS OF

S13C DISTRIBUTION AND LO CAL SUPERFICIAL GEOLOGY

Area General 8 13C %o Recharge feature characteristic

A - 9 to - 1 4 Average direct recharge rates, important vegetation cover. В - 6 to - 7 Local mixing of resident groundwater with incompatible indirect recharge, very poor vegetation cover. С - 1 3 Average recharge rates, important vegetation cover. D - 4 t o - 1 0 Large outcrop area with variable recharge rates of both direct and indirect type resulting in mixing of waters. Poor recharge volumes per unit area. Limited vegetation cover. E -1 1 to -1 6 Rapid direct recharge from high rainfall areas. Limited direct recharge elsewhere due to thick low-permeability soil cover. Local indirect recharge in valley beds. Important tree and vegetation cover.

Although undoubtedly a high transm ission of recharge occurs in the upper

layers o f the aquifer the dearth o f positively tritiated w ater is disturbing and m ust

bring into question the assessm ents o f direct recharge. The fact that tritium data

indicate low recharge cannot be overlooked.

A conceptual m odel of the flow m echanism thought to be operating in the

carbonate aquifers based on the isotope data available is illustrated in Fig.5.

Carbon isotope data

The 5 13C data from the lim estone aquifer are surprisingly negative w ith very

few sam ples having values m ore positive than - 7 %o, signifying that a ‘tem perate’

type of photosynthesis has been operative historically unless norm al carbonate

reactions have not taken place. The rock m atrix 5 13C value is about 0 % o b u t

considerably different values have been obtained for surface sam ples w here caliche

developm ent is im portant. Such sam ples have recorded values ranging from

-8.9 to-10.3%^. IAEA-AG-158/14 2 0 3

The distribution o f 6 13C in the aquifer is interesting in that it appears to be possible to delim it five areas as show n on F ig .6 w ith the particular characteristics given on Fig.7. The distributions do not appear to be due to lithological differences in the aquifer and are not easily reconcilable w ith any hydrochem ical zonation relating to carbonate under-saturation or ion exchange etc. If differential m odi­ fication to 5 13C has not occurred below the water-table then the differences in

5 13C should reflect predom inantly the effects o f differing recharge m echanism s

and possibly under-saturation and m atric dissolution due to the m ixing of incom patible resident groundw ater w ith recharge water. The postulated conditions

are sum m arized in Table II and it m ust be pointed out that they are postulated as

having existed over very long periods of tim e. O bviously such postulations are

speculative but the evidence o f 5 13C distributions indicates that this isotope m ay be o f value in its ow n right and not just in conjunction w ith 14C.

CONCLUSIONS

In conclusion it m ay be said that the regional environm ental data for Eastern

Jordan have provided inform ation essentially relating to recharge m echanism s. The

im portance o f indirect recharge in the low rainfall areas has been dem onstrated

by tritium and the presence o f old water, identified by 14C dating, has indicated

that ‘fossil’ groundw ater conditions exist even in the recharge m ounds. These

tritium data support.other inform ation concerning the presence o f high perm eability

zones at the top o f the lim estone aquifer overlying less perm eable zones at depth.

The isotopes clearly show that m ixing of groundw aters is occurring ow ing to

different recharge inputs so that som e o f the ‘dating’ m ay be open to question.

Such m ixing precludes the use o f the isotopes to determ ine groundw ater velocities

and transit tim es. The stable isotopes of lsO and deuterium in the sandstone

groundw aters have largely been depleted, show ing that the infiltration has been

subjected to evaporation as w ould be expected. The <513C data can be differentiated

on a regional basis and m ay indicate variable recharge conditions over the area.

The isotope data have not provided definitive or quantitative inform ation in the area. They have, how ever, provided qualitative inform ation that w ould not

have otherw ise been available and have m ost im portantly questioned the reliability

o f the direct recharge estim ations.

ACKNOWLEDGEMENTS

The author w ould like to acknow ledge the staff o f the Food and Agriculture

Organization w ho collected the sam ples for isotope analysis. The analyses were

carried out by the International Atom ic Energy Agency, Vienna. 2 0 4 LLOYD

REFERENCES

[1 ] LLOYD, J.W., DRENNEN, D.S.H., BENNELL, B.M.U., A groundwater recharge study in north-eastern Jordan, Proc. Inst. Civ. Eng. (1966) 36. [2] WIGLEY, T.M.L., Effect of mineral precipitation on isotopic composition and carbon-14 dating of groundwater, Nature (London) 263 (1976) 219-21. [3] BURDON, D.J., Flow of fossil groundwater, Q.J. Eng. Geol. 10 (1977) 97—124. [4] LLOYD, J.W., The Hydrogeology of the Southern Desert of Jordan, FAO Rep. AGL:SF/Jor 9, Tech. Rep. 1 (1969) 207 pp. [5] PARKER, D.H., The Hydrogeology of the Mesozoic-Cainozoic Aquifers of the Western Highlands and Plateau of East Jordan, FAO Rep. AGL:SF/Jor 9. Tech. Rep. 2 (1970) 285 pp. IAEA-AG-158/15

A CONCEPTUAL HYDROCHEMICAL MODEL FOR ALLUVIAL AQUIFERS ON THE SAUDI ARABIAN BASEMENT SHIELD

J.W. LLO YD

Hydrogeological Section,

Departm ent of Geological Sciences,

University of Birm ingham , Birm ingham ,

United Kingdom

P . F R I T Z

Departm ent of Earth Sciences,

University of W aterloo, W aterloo,

O n t a r i o

D. CHARLESW ORTH

Hydrogeological Section,

Jam es F. M acLaren Ltd.,

W illow dale, Ontario,

C a n a d a

A b s t r a c t

A CONCEPTUAL HYDROCHEMICAL MODEL FOR ALLUVIAL AQUIFERS ON THE SAUDI ARABIAN BASEMENT SHIELD. Situated across the Precambrian Basement shield of Saudi Arabia is a sequence of alluvial- filled wadis which contain the main groundwater resources of the area. The groundwater in some of the wadis indicates a central wadi fresh groundwater zone with highly saline groundwater occurring at the edges of the major alluvial thicknesses. The distribution of the chemistry in the groundwaters is examined and a conceptual model is given for one main wadi. Both the major ion chemistry and the environmental isotopes are used to demonstrate that the edge effect high salinity is the result of evaporative action and the concentration of salts in groundwater entering the main wadi from side wadis containing thin alluvial fill deposits.

INTRODUCTION

The Precam brian Basem ent in Saudi Arabia form s a shield area of som e

7 7 0 0 0 0 k m 2 w hich dom inates the western part of the country and extends from the Jordanian border into the Yem en. Developed upon the shield are a sequence of alluvial-filled w adis w hich contain the m ain groundw ater resources of the area.

The m ain precipitation occurs in the higher part of the w adi catchm ents w ith the

2 0 5 2 0 6 LLOYD et al.

FIG.l. Rainfall distribution over the Wadi Bisha catchment.

m ajor recharge to the groundw ater in the alluvium occurring through bed trans­

m ission losses during flood runoff.

The groundw ater in som e of the w adis indicates the presence of central wadi,

low salinity groundw ater zones, w ith higher salinity groundw ater occurring at the

edges of the m ajor alluvial thicknesses. The distribution o f the higher salinity

waters is som ew hat unusual as is their com position, w hich is predom inantly of

‘sodium sulphate’ type.

T o exam ine the origin of these hydrochem ical distributions m ajor ion and

isotope data from the groundw aters in the W adi Bisha, one of the largest w adis in the

area (see Fig.l), have been used. In this w adi the m ain transm ission losses from

floods occurs in the w adi bed between H ifa and Junaynah. The precipitation giving

rise to the floods occurs predom inantly in the upper catchm ent as indicated in

Fig. 1. Elsew here in the catchm ent, how ever, over an extrem ely large area, where

the m ean rainfall ranges between 125 and 150 m m , only very sporadic flooding is

know n to occur. IAEA-AG-158/15 2 0 7

FIG.2. Groundwater conductivity o f central Wadi Bisha.

HYDROCHEMICAL DATA

O n Fig.2 the groundw ater conductivity for the m ost extensive aquifer area of the W adi Bisha is show n and the presence o f the higher salinity groundw aters at the aquifer edge is clearly dem onstrated. It is im portant to note that the aquifer boundary show n on Fig.2 represents the boundary of significant alluvial thickness 8 0 2 LY e al. et LLOYD

FIG.3. Schematic section across Wadi Bisha showing some major ion and conductivity data at A-A on Fig.2. IAEA-AG-158/15 2 0 9

FIG.4. Durov diagram indicating general major ion characteristics o f Wadi Bisha groundwaters.

(> 5 m ) and that aw ay from the boundary vast areas of very thin alluvium w ith good perm eability are present. Clearly the low -salinity waters align w ith the present w adi course from w hich the dom inant recharge elem ents occur.

O w ing to extensive groundw ater abstraction for irrigation purposes m uch of the groundw ater chem istry has been m odified and m ixing has occurred; however, the hydrochem ical types present can be dem onstrated as is show n in Figs 3 and 4.

O n Fig.3 the total salinity, sulphate and chloride data are show n for the section across the w adi m arked A -A on Fig.2. Figure 3 show s the low -salinity indirect w adi recharge w ater occurring at tw o localities, w hile a sulphate dom inance over chloride is clearly show n at the edges o f the aquifer. W here high-salinity w ater occurs centrally it is attributed to salt concentration by irrigation return infiltration. T w o types of return seem to occur, one in w hich the sulphate water is concentrated, and the other in w hich chloride dom inant water develops, probably as a result of return, through the irrigation use of the m ain w adi recharge water. NJ

О Conductivity Conductivity S jj

FIG.5. Schematic section across Wadi Bisha showing isotopic data in the vicinity o f A-A (Fig.2.). IAEA-AG-158/15 211

Figure 4 illustrates the type of m ajor ion chem istry present in the W adi Bisha and indicates som ething of the origin and m odifications of these waters. A s can be seen the m ain w adi recharge is bicarbonate in character w hile the edge w aters are sulphate w ith chloride waters developing from irrigation return.

O n Fig.5 tritium and 6 180 data are plotted against conductivity and ground­ water tem perature for a wadi section adjacent to A -A . The isotopes show , as w ould be expected, that the m ain w adi recharge water is m odern w ithout m arkedly depleted 5 180 levels. O ne central sam ple in the section, how ever, is depleted

(5 180 = — 1.31) and is probably attributable to evaporated irrigation return water.

It is interesting to note that the edge w aters have m uch low er tritium values than the central waters and that the 5 180 levels are relatively depleted. Lim ited 14C data that are available for the w adi also show that activity decreases aw ay from the w adi centre.

INTERPRETATION

The interpretation of the unusual groundw ater chem istry in the W adi Bisha is show n diagram m atically in Fig. 6 . The chem ical types indicated relate to a Durov-

style hydrochem ical classification. The significant feature is the incorporation into

the hydrogeology of the effects o f the very thin alluvial spreads adjacent to the m ain aquifer. Conceptionally the developm ent of the hydrochem istry is seen

as follow s:

(i) Precipitation produces m inor recharge (probably indirect) into the thin highly receptive alluvial spread. This water is probably C a (H C 0 3)2 in character w ith a

negative gypsum saturation index (-SIG ).

(ii) The water flow s through the thin alluvium dissolving gypsum that is know n to

occur, and is subjected to partial evaporation. It also w ould appear to dissolve Nacl

to acquire an overall ‘N a 2S 0 4’ classification. This classification is som ew hat

m isleading in that the groundw aters are sim ply rich in the salts o f N aC l and C a S 0 4.

W ith the evaporation 6 180 is enriched. O w ing to its undoubted slow m ovem ent

such water w ould have low tritium values and 14C activity w ould decrease tow ards

the m ain aquifer.

(iii) The ‘N a 2S 0 4’ water recharges the m ain alluvium producing an edge effect of

relatively high salinity groundw ater. The edge effect is m aintained by the flow of

the groundw ater in the m ain alluvium . This water is C a (H C 0 3)2 in character,

relatively non-enriched in 6 180 and enters the aquifer as indirect recharge from

f l o o d s . 1 LOD t al. et LLOYD 212

FIG. 6. Wadi Bisha conceptual flow model. IAEA-AG-158/15 2 1 3

(iv) Salinity anom alies aw ay from the alluvial edge are attributed to irrigation return effects. ‘N a 2S 0 4’ waters can be concentrated by this m eans and N aC l waters are produced from the m ain wadi recharge. Such waters show enrichm ent of 5 180.

CONCLUSIONS

The conceptual m odel devised provides a reasonable explanation of the hydrogeological m echanism s believed to be operating and dem onstrates the use of isotopes together w ith conventional m ajor ion chem istry in hydrochem ical

interpretation.

It is concluded that over a vast area of thin, good perm eability alluvium ,

the long-term recharge becom es concentrated as edge water in the m ain w adi and is

seen to be highly saline w ith low tritium values and enriched 5 180. A lthough this recharge is significant over long distances along the edges o f the m ain alluvial

aquifer, the fact that it is retained in its position by the m ain recharge pulses along

the central w adi bed indicates that the flow volum e of the saline groundw ater is relatively sm all. Therefore, the unit recharge under present-day conditions in the

thin alluvial areas, where the m ean rainfall ranges between 125 and 150 m m , m ust

be very sm all.

ACKNOWLEDGEMENTS

The authors w ould like to thank Jam es F. M acLaren Ltd., Canada, for

perm ission to publish the paper w hich includes data collected for the M inistry of

Agriculture and W ater in Saudi Arabia. The isotope analyses were m ade in the

Departm ent of Earth Sciences, University of W aterloo, Canada.

IAEA-AG-158/16

ISOTOPE METHODS AS A TOOL FOR QUATERNARY STUDIES IN SAUDI ARABIA

H. H Ô TZL*, C. JO B**, H. M O SER+, W. R A U ER T +,

W. STICH LER* J.G. ZÖTL*,

* Institut für Geologie,

Universität Karlsruhe,

Federal Republic of Germ any

** Institut für Balneologie,

Universität Innsbruck, Austria

+ Institut für Radiohydrom etrie der

Gesellschaft für Strahlen- und Um w eltforschung,

Neuherberg, Federal Republic of G erm any

H+ Abteilung für Hydrogeologie,

Technische Universität Graz, Austria

A b s t r a c t

ISOTOPE METHODS AS A TOOL FOR QUATERNARY STUDIES IN SAUDI ARABIA. Within the framework of a combined sedimentological, hydrogeological, hydrochemical, geomorphical and climatological investigation in central and eastern Saudi Arabia various environmental isotope studies were performed. 14 С measurements on shells from different shell banks in the Gulf coastal region confirmed the existence of sea-level fluctuations during the Würm time and during the Holocene. Measurements of 2H, 180 , and 3H contents of water samples from the Al Qatif and Al Hasa areas gave valuable information on the origin and the fate of these waters as well as on hydraulic connections between aquifer strata. 14C measurements on calcite crusts in the region As Sulb Plateau confirm that the precipitation of the “Neolithic Pluvial” reactivated the karst development in this region. In its southwestern part, different I4C values of stalactites and of duricrust cementing the stalactites show the development of duricrust during late Pleistocene and/or Holocene. In the upper part of Wadi Ar Rimah, a close insight into the causes of salination of groundwaters was obtained by studying their contents of 2H, 180 , and 3H in combination with hydrochemical data. It is shown that secondary and tertiary evaporation during irrigation and infiltration must be taken into consideration. In Wadi Ad Dawasir and its hinterland the 2H and 180 enrichment and that of the mineralization in hand-dug wells were explained by an admixture of recent shallow groundwater with old sandstone water.

2 1 5 216 HÖ TZL et al.

INTRODUCTION

The results reported here were elaborated within the framework of a combined sedimentological, hydrogeological, hydrochemical, geomorphical and climatological investigation in central and eastern Saudi Arabia. The project was carried out by an interdisciplinary team of 20 scientists from Saudi Arabia, Austria, the United States of America and the Federal Republic of Germany under the guidance of Professor Zötl. The whole investigation was recently published.1 In regional investigations environmental isotope studies contributed to solving problems concerned with sea-level fluctuations during the Quaternary period, with climatic variations and with hydrochemistry. The measurements of 2 H and 180 contents in water samples were performed by W. Stichler. W. Rauert measured the 3H and 14C contents in water samples; while 14C contents in sedimentary rocks were determined partly by W. Rauert and H. Felber2 , and the 34S contents by E. Pak2. The interpretation of the isotope contents resulted from discussions with C. Job (hydrochemist), H. Hötzl and V. Maurin (hydrogeologists)3.

1. QUATERNARY SEA-LEVEL FLUCTUATIONS INVESTIGATED IN THE GULF COASTAL REGION

To obtain a good knowledge of the Quaternary sea-level fluctuation along the east coastal zone, three areas were investigated (Fig. 1): the coastal area south of Al Jubayl and the shorelines north of Ras Tannurah and near the border with Qatar. 14C measurements on shells from different shell banks (Table I) gave some information on sea-level fluctuations during the Würm time and during the Holocene. A marine transgression during the interstadial period of the Middle Würm (40 000—26 000 years BP), where the maximum water level reached a few metres higher than the present sea level, proved to be possible by 14C measurements. The post-glacial transgression began about 15 0Ö0 years ago BP. The sea-level rise occurred in stages interrupted by brief standstill phases of and partly by regression. The existence of the Holocene transgression and a higher sea level during the Middle Holocene in the east coastal zone of Saudi Arabia provides a well-developed abrasion terrace in the area north of Ras Tannurah where well-cemented shells were collected. 14C measurements revealed a 14C age of about 4000 years BP (Table I, Nos 1,2, 3).

1 AL-SAYARI, S.S., ZÖTL, J.G . (Eds), Quaternary Period in Saudi Arabia, Vol. 1, Springer Verlag Vienna-New York (1978). 2 Institut für Radiumforschung und Kernphysik der Österreichischen Akademie der Wissenschaften, Vienna, Austria. 3 Institut für Geologie, Universität Karlsruhe, Federal Republic of Germany. IAEA-AG-158/16 2 1 7

Qaternary ( in general ) ИР;«! Sabkhahs sandstone, mafP1 E S 3 and limestone Hofuf Formation Dam Formation Hadrukh Formations 47* Dammam Formation Dammam and Rus Formation Rus Formation \ Umm er Radhuma Formation / Al Alah Jabal Midra At Janubi JabaI Umm Ar Run Ю0 km Ar Rub' Al Khali 4ST '¿ ¡¡y s o 1 5Г long

FIG.l. Generalized geological map o f the Gulf coastal region and its hinterland.

2. ISOTOPES AS A TOOL FOR INVESTIGATING AL Q ATIF AND AL HASA

W ATER SOURCES

The A l Q atif oases cover an area of about 75 km 2 between Ad Dam m am and

Ras Tannurah (Fig. 1). Figure 2 show s ’A yn A l Labaniya, one of the largest springs of the Q atif area. F o r the entire cultivated area one m ay assum e a total discharge o f 8 0 0 000 m 3/d from springs and boreholes.

The oasis of A l Hasa is about 70 km tow ards the interior of the country from the G ulf coast. A l Hasa is one of the largest oases in the w orld and covers a cultivated area of about 200 km 2. W ithin its m ain tow n, A l Hofuf, and about

60 villages are about 250 000 inhabitants. In 1977 the total discharge of the springs and som e boreholes was about 1 0 8 0 000 m 3/ d .

Measurem ents of 2H , 180, and 3H contents of waters from the A l Q atif and

A l Hasa areas gave valuable inform ation on the origin and the fate of these waters.

In the A l Hasa area, w ater sam ples were only collected in spring 1973 (Table II). 1 HTL t al. et HÖTZL 218 TABLE I. RESULTS OF 14C M EASUREM ENTS OF SAM PLES CO LLECTED IN 1973 AT THE SH O RELINE OF THE

ARABIAN GULF AND AL HASA

Geographic 14 С age Locality Laboratory Reg. No. Material position (years BP)

1. Shoreline, wave-cut bench Lat. 26° 53’ N Shells of Long. 49° 56' E IRMa Lab. 36 4 9 Cardies and Pectes 4 6 7 0 ± 190

2. Shell bank Lat. 26° 50' N Shells of Long. 50° 00' E IRM Lab. 3650 Cardies and Pectes 3 3 8 0 ± 180

3. Shell bank Lat. 26° 30' N Cemented shells of Long. 50° 00' E IRKWb VRI-406 Cardies and Pectes 3990 ± 90

4. Shells 1.5 km west of the shoreline Lat. 24° 45’ N Long. 50° 45' E IRM Lab. 3652 Oyster shells > 3 8 800

5. Small hill in Al Hasa Lat. 25° 30' N Long. 49° 37' E IRM Lab. 3653 Snail shells 14 280 + 430

6. Al Hasa, 2 m below surface Lat. 25° 30' N Long. 49° 40' E IRM Lab. 3654 Snail shells 2180 + 210

7. Small hill in Al Hasa Lat. 25° 30' N Long. 49° 37' E IRKW VRI-405 Peat and charcoal 8290 ± 120

a Institut für Radiohydrometrie, Munich. b Institut für Radiumforschung und Kernphysik, Vienna. IAEA-AG-158/16 2 1 9

FIG.2. ’Ayn Al Labaniyah, Al Qatif oases (photo taken by J.G. Zötl in 1975).

In the A l Q atif area three series o f sam ples were collected in the spring of 1973,

1974 and 1975. The results (Table III) show that the data obtained are independent o f sam pling tim e and it w as assum ed that this is also the case for the w ater sam ples taken from A l Hasa.

From the ¿>2H -5180 relation of A l Q atif and A l Hasa waters (Fig.3) it can be seen that the values lie distinctly below the line 5 2H = 8 ; 6 180 + 10, valid for precipitation in tem perate clim ates, e.g. central Europe. There can be no doubt that the equation line o f the A l Hasa and A l Q atif w ater sam ples is based upon a clim atic effect. The waters recently discharging from springs in the A l H asa and

A1 Q atif oases originate from precipitations evaporated from the sea under clim atic conditions other than those today. The low 2H excess values, t (defined by

ô2H = m 6 180 + t), lead to the conclusion that the water vapour of this condensate w as in a better equilibrium w ith the sea-water than is the case now . The evaporation rate w as evidently low er than today, and this could be due to a cooler as w ell as a m ore hum id clim ate than that today.

B y contrast to the A l Q atif and A l Hasa waters, the water sam ples from

W adi H anifah show , as expected for recent waters, the ô 2H -ô180 relation of recent waters in the M editerranean region. The stable isotope results are confirm ed by 220 HÖTZL et al.

TABLE II. STABLE ISOTOPE DATA FROM AL HASA SPRINGS 1973

Location No. of sample 52 H(°/oo) 5180 (%„)

24 -39.8 -5.36 25 -40.1 -5.24 26 -39.9 -5.27 35 -39.9 -5.10 36 -39.2 -4.87 Agricultural farm 37 -39.8 -5.36 ’Ayn Sumbor 38 -40.0 -5.05 ’Ayn Haql 39 ' -39.4 -5.12 ’Ayn Shabah 40 -39.6 -5.20 ’Ayn Khudud 41 -38.8 -4.85 ’Ayn Najm 42 -40.0 -5.32 Al ’Uyun 43 -36.7 -4.86

3H and 14C m easurem ents — the A l Q atif and A l Hasa waters are practically

tritium -free (Tables IV and V), whereas som e o f the W adi H anifah w ells or springs

(46, 48, 49, 50) have higher tritium concentrations, thus show ing a m ore or less pronounced contribution of present rainfalls to the feeding of shallow groundw ater

wells. These water sam ples have the highest 2H and 180 contents. Later the feeding m echanism of the w adi waters is discussed in m ore detail (Section 5).

Four 14C m easurem ents of A l Q atif and A l Hasa waters (Table V I) show old waters, w hich infiltrated according to the 180 and 2H contents under clim atic conditions

different from those prevailing now . IAEA-AG-158/16 221

TABLE III. STABLE ISOTOPE DATA OF A L Q ATIF AREA IN 1973-1975

Location No. of sample 62H(%o) 6 ,sO (°/oo)

’Ayn Al Labaniyah I 1973 2 -26.3 -3.63 1974 57 -26.4 -3.77 1975 126 -26.3 -3.82 ’Ayn Al Labaniyah II 1973 3 -26.5 -3.59 1974 58 -26.4 -3.83 1975 127 -26.3 -3.84 ’Ayn Hussain 1973 4 -26.2 -3.72 1974 59 -26.3 -3.70 1975 128 -26.5 -3.76 ’Ayn Subia I 1973 5 -25.7 -3.41 1974 64 -26.2 -3.68 1975 129 -26.3 -3.76 ’Ayn Subia II 1973 13 -26.9 -3.59 1974 65 -27.2 -3.75 1975 130 -26.6 -3.78 ’Ayn Al Jaiamiyah 1973 15 -27.3 -3.88 1974 61 -27.1 —3.75 1975 131 -27.2 -3.89 Drainage near ’Ayn Al Jaiamiyah 1973 n.s. n.s. n.s. 1974 62 -25.8 -3.60 1975 132 -25.6 -3.69 ’Ayn Muhairig 1973 14 -27.7 -3.75 1974 63 -28.2 -3.83 1975 133 -27.2 -3.95 ’Ayn Obede (Al Ajam) 1973 11 -31.7 -4.11 1974 70 -31.7 -4.40 1975 141 -31.4 -4.41 Hammam Balousa 1973 17 -26.8 -3.34 1974 78 -27.3 -3.44 1975 143 -24.6 -3.44 Sea-water Dawhat Zaloom 1973 n.s. n.s. n.s. 1974 67 +24.2 +5.04 1975 145 +27.6 +4.84 As Sabacha 11 (Al Ajam) 1973 n.s. n.s. n.s. 1974 69 -30.0 -4.16 1975 123 -29.4 -4.18 ’Ayn Hussain (Al Ajam) 1973 n.s. n.s.' n.s. 1974 71 -32.2 -4.29 1975 124 -31.5 -4.36 ’Ayn Towairit (Al Ajam) 1973 9 -27.8 -4.68 1974 73 -28.1 -3.92 1975 125 -28.5 -3.89 n.s. — no sample. 2 2 2 HÖ TZL et al.

СГ2Н % .

FIG.3. Relationship between S2Hand 81S0 contents in water samples taken from Wadi Hanifah and Al Qatif oases and the Al Hasa area, in 1973.

In the A l Q atif region a very strong positive correlation exists betw een 5 2H and salt content (correlation coefficient r = 0.9) as w ell as sulphate content (r = 0.95).

B y contrast, there is a negative correlation betw een nitrate and 52H (r = — 0.85).

T his m akes it possible to estim ate the com position of the m ixing com ponents.

A n intensive study by C. Job on these m ixing effects can be sum m arized as follow s:

A ll aquifers of the lim estone layers of the Tertiary strata are hydraulically connected.

The strata series contain w aters of different salt content ow ing to local differences in the geochem ical com position of the rock m aterial. Sea-water intrusion and

seepage o f drainage w aters have no significant influence on the salt content of the

karst waters. Salty underground waters (high S 2H values) m ix w ith waters of a

low er salt and heavy isotope content. In general the salty waters com e from N,

whereas the fresh water com es from S (Fig.4). Sim ilar com bined hydrochem ical- isotope investigations have also been m ade by C. Job in other oases. IAEA-AG-158/16 2 2 3

TABLE IV. 3 H-CONTENT IN A L QATIF W ATERS IN 1973-1975

Sample number 3H content (TU) Location 1973 1974 1975 1973 1974 1975

’Ayn Al Labaniyah I 2 57 126 < 0 .8 < 3 .5 0.1 ± 1.3

’Ayn Al Labaniyah II 3 58 127 < 0 .8 < 4 .5 3.3 ± 1.3

’Ayn Hussain 4 59 128 < 2 .3 < 2 .6 1.0 ± 1.4

’Ayn Subía I 5 64 129 < 0 .9 < 2 .2 0.6 + 1.3

’Ayn Subia II 13 65 130 < 2 .3 < 4 .4 1 .0 ± 1.3

’Ayn A1 Jalamiyah 15 61 131 < 2 .7 < 4 .5 1.3 ± 1.2

Drainage near ’Ayn Al Jalamiyah n.s. 62 132 n.s 3.4 ± 2.2 1.5 ± 1.3

’Ayn Muhairig 14 63 133 < 2 .7 < 4 .0 0 ± 1 .2

’Ayn Obede 11 70 141 < 0 .9 < 2 .2 0.6 ± 1.0

Hammam Balousa 17 78 143 < 2 .2 2.5 ± 1.8 1.2 ± 1.0

Sea-water Dawhat Zaloom n.s. 67 145 n.s. 24.4 ± 2.7 1 3 .2 ± 1.3

As Sabacha II (Al Ajam) n.s. 69 123 n.s. < 2 .2 0.4 ± 1.4

’Ayn Hussain (Al Ajam) n.s. 71 124 n.s. < 3 .4 0 .5 ± 1.4

’Ayn Towairit (Al Ajam) 9 73 125 < 2 .7 < 4 .6 0.3 ± 1.4

3. KARSTIFICATION INVESTIGATED IN THE REGION OF THE AS

SULB PLATEAU

The thick com plex of lim estones in A s Sulb Plateau (Fig.l) are strongly

karstified, but karst phenom ena lie below the surface. In the caves o f the northern

and eastern part neither stalactites nor stalagm ites were found, only sm all w arty

calcite deposits. 14C m easurem ents revealed a 14C age of 5000 years B P for these

calcite crusts. From these data it m ay be concluded that the precipitation of the

Holocene “ N eolithic Pluvial” infiltrated into the underground, thus proving the

activity o f karstification until the Holocene.

In the southw estern part of the Plateau stalactites cem ented into duricrust

have been found. For these the follow ing genetic developm ent m ay be deduced:

Developm ent o f the stalactites on the roof o f a cave;

Filling of the cave by aeolian sand;

Cem entation o f this sand by carbonate solutions, (re)-crystallization of calcite; and

Erosion o f the lim estone overlying the caves and reactivation o f karstification. 2 2 4 HÖTZL et al.

T A B L E V . 3H-CONTENT IN AL HASA W ATERS AND OF W ELLS IN

W ADI H AN IFAH (1973)

Al Hasa Wadi Hanifah Sample Sample Sample (TU) (TU) No. No. No.

24 < 0.7 35 < 2 .5 44 < 1 2 25 < 2.6 35a < 2 .6 45 < 4 26 < 2.3 36 < 2 .3 46 7.0 ± 2 27 < 2.5 37 < 2 .6 47 < 3 .8 28 < 0.5 38 < 0 .3 48 74 ± 10 29 < 2.5 39 < 2 .6 49 67+8 30 < 1.2 40 < 1 .2 50 16 ± 2 31 < 2.8 41 < 2 .4 51 < 4 32 < 0 .9 42 < 1 .9 52 < 0 .9 33 < 0 .9 43 < 2 .9 53 < 3 .8 34 < 2 .6 54 < 3 .4

a Twofold standard deviation

TABLE VI. RESULTS OF 14C, 13C AN D 3H M EASU REM EN TS OF W Af ER

SAMPLES COLLECTED FROM THE AL QATIF AND AL HASA AREAS ( 1 9 7 3 )

14 С age 14C content“ 13 Cb 3H content Place or No. of sample (years BP, (°/o modern) (°/oo) (TU) un corrected)

’Ayn Al Labaniyah I (Al Qatif) < 5 .9 > 2 2 000 - 9 .0 < 0 .8 ’Ayn Al Labaniyah II (Al Qatif) < 1 .2 > 3 4 500 -8 .8 < 2 .7 No. 32 (Al Hasa) < 1 .4 > 3 3 000 -1 0 .3 < 0 .9 ’Ayn Mansur (Al Hasa) < 1 .4 > 3 3 000 -1 0 .0 < 0 .9

a An initial 14C-content of 85% modem was assumed. b The I3C content is given as the relative per mille deviation from the limestone standard PDB. IAEA-AG-158/16 2 2 5

О 50 100 150 — — — ...... PIPELINE KILOMETERS

FIG.4. Mineralization and flow directions o f underground waters between the outcrop region o f the Umm Er Radhuma Formation and the Gulf coastal area.

A s was expected, no 14C could be m easured in the stalactites (> 3 7 000 years).

Duricrust showed, how ever, a sm all 14C content (uncorrected 35 000 years BP).

Despite som e uncertainties — content of fossil carbonate, volum e and type of recrystallization — these 14C m easurem ents show a partly progressive developm ent of duricrust during the late Pleistocene and/or Holocene and a repeated reactivation of the cave system .

4. MINERALIZATION OF GROUNDW ATER DEMONSTRATED BY THE

EXAM PLE OF W ADI AR RIM AH

The Arabian Peninsula show s three large old river system s crossing the entire

Arabian SheJf from west to east — W adi A r Rim ah, W adi A s Sah’ba (w ith its form er upper part, the present W adi Birk), and W adi A d Daw asir.

The Quaternary studies in the upper part o f W adi A r Rim ah were carried out over a distance of about 250 km west of Buraydah and ’Unayzah, along about

Lat. 26°N, between Long. 42°30’ and Long. 44°E (Fig.5). 2 2 6 HÖTZL et al.

FIG.5. Generalized geological map o f the area o f Wadi Ar Rimah, with numbers o f wells or water samples, Qes, Qs, Qg = Quaternary aeolian sand, silt and gravel; S, Saq = sandstone; Gn = Gneissic gray granite; Gp = red and pink granite; Mu = Murdama schist; Di = diorite.

Riyadh A l Khabra includes a few sm all oases in W adi A r Rim ah. W ater sam ples

were collected by C. Job from drilled wells (6 0 -70m ), reaching dow n the Saq

sandstone, where the water level lies 16 to 20 m below ground surface. W ater

tem perature is 27.5°C and the water is saturated w ith oxygen. The degree of

m ineralization is highly variable (1.6— 8.7 g/ltr) and increases w ith the age of the,

w ells (Fig. 6). The salt content (N a-Ca-Cl-S0 4-solution, nitrate up to 370 m g/ltr and boric acid up to 10 m g/ltr) decreases, how ever, during daily pum ping. (Fig. 6).

The enrichm ent of various ions does not increase regularly w ith the total

concentration. Thus, it is m ost striking that brom ide and nitrate are less concentrated

than chloride, although both brom ide and nitrate are very soluble. A close insight

into the causes of m ineralization of these groundw aters is obtained by studying

t h e i r 2H, 180 and 3H contents. In this case, besides prim ary evaporation of sea­

water, the subsequent secondary and tertiary evaporation m ust be taken into

consideration — if a part of the rain-w ater or surface water evaporates again, an

enrichm ent of heavy isotopes occurs in the liquid phase. In this case the slope,

and therefore also the 2H excess t in the ô 2H -ô180 relation, are no longer constant

but depend on several clim atic factors (quantity and intensity of rainfall, hum idity,

air tem perature, w ind velocity etc.) w hich determ ine the extent of this secondary

evaporation. Varying isotope contents of precipitation infiltrating into

groundwater produce an average content by m ixing and exchange. If gound-

w ater again reaches the land surface through w ells and springs, kinetically IAEA-AG-158/16 2 2 7

RIYADH AL KHABRA

Watertable 16-20 m b.s. Borehole 60 - 75 m

N

FIG. 6. Parts o f the well area o f Riyadh Al Khabra with numbers o f wells or water samples. The tabulated data give sample number, age o f the well (i.e. time since extraction by pumping), time interval between the start o f the pumping and sample collecting (= sampled hours after start), water temperature, oxygen content (mg/ltrj, total mineralization in g/ltr (TDS) and content o f stable isotopes (8 2H, 8isO). b.s. = below surface.

influenced evaporation w ill becom e active again (tertiary evaporation). Infiltrating once m ore, these w aters have low er t values in the relation 6 2 H = 8 5 1 8 0 + t 8 ( w h e n m = 8 , t w ill be w ritten as t 8 in the follow ing text). In this infiltration w ater the salt concentration w ill be higher, too; this is not so m uch a result o f evaporation as that due to the high salt content of arid soils. W hen these w aters are recycled into wells, the enrichm ent of the-stable isotope content by tertiary evaporation w ill

therefore be com bined w ith an increasing salt concentration.

Regarding the content of deuterium and 180, the waters of Riyadh A l Khabra differ strikingly from the isotopically lighter old karst waters. The 5 2H values are between -8 .5 and — 10.9°/oo; and the S 180 values between -1.25 and -3 .3 7 °/00

( F i g . 7 ) .

The deuterium content show s no significant relation to the total m ineralization.

How ever, the 5 180 values becom e significantly higher, corresponding to the increasing degree of total m ineralization. The difference betw een the § 2H and

6 180 values depends on the fact that the 5 2H values, in relation to m easurem ent accuracy, are m uch less influenced by the kinetics of evaporation than 5 180 values.

In the relation S 2H = 8 5 1 8 0 + t 8 t h e t 8 value is significantly correlated w ith the

salt content. W e found that this is typical for w aters of tertiary evaporation. 228 HÖTZL et al.

FIG.7. 82H-SlsO relation of recent well waters in the area o f Riyadh Al Khabra (...), in Wadi Ar Rimah (A), and in Wadi Maraghan (xxx). Samples with a 3H content o f ~>4 TU are marked by circles. Note the different slopes o f the equation lines o f Riyadh Al Khabra waters (influenced by evaporation) and the mixed waters o f Wadi Ar Rimah.

The very low tritium content show s that recent rain-w ater cannot be the m ain agent

w ashing soil salts into the wells.

Sum m arizing all observations m ade in the area of Riyadh A l Khabra the

follow ing argum ents speak in favour o f a reflux of drainage waters into the wells

(recycling): The decreasing degree of m ineralization of the water during pum ping

(i.e. the low ering w ater level) is evidence that the upper groundw ater zone has a

higher salt concentration than the low er one. Consequently, the salt infiltration

m ust com e from above. Furtherm ore, m ineralization proved to be correlated w ith

isotopic enrichm ent. Finally, the fact that the increasing degree of m ineralization

corresponds to the age of the w ell supports the theory of recycling; the longer this

recycling process lasts, the m ore surface-evaporites accum ulated in the Q uaternary

sedim ents are dissolved and carried into the aquifer.

In the W adi A r Rim ah and W adi M araghan area (a tributary to W adi A r Rim ah

west of R iyadh A l Khabra) the m ineralization decreases w ith increasing 3H content.

T h e t 8 values decrease significantly w ith the m ineralization. In com parison w ith

the waters of R iyadh A l Khabra it seem s that the waters of higher m ineralization m ay

have undergone a stronger evaporation process.

How ever, as can be seen from sam ple N os 93— 98 in Table V II and Fig.7,

a decreasing t 8 value is not linked to an increasing 180 content — as it is in the

Riyadh A l Khabra waters w hen evaporation is involved — but m ainly to a

decreasing deuterium content. In this case it seem s that the change of t 8 value - and IAEA-AG-158/16 2 2 9

TABLE VII. ISOTOPE CONTENTS OF WATER SAMPLES COLLECTED IN THE AREAS OF RIYADH AL KHABRA, WADI AR RIMAH, WADI MARAGHAN AND RAIN-WATER SAMPLE FROM A THUNDERSTORM AT ’UNAYZAH, 18 MARCH 1974 (AIR TEMPERATURE 18.5°C). WELL- WATER SAMPLES COLLECTED BETWEEN 18 AND 25 MARCH 1974 fs calculated fro m 62H = 8 X 8 1S0 + ts.

No. SJH i0/«,) 5lsO (°/oo) Í8 TU

87 -10.9 —2.37 8.06 0.8 ± 1.7 88 -9.7 —2,20 7.90 0.2 ± 1.7 89 -8.5 -1.73 5.34 2.5 ± 1.9 90 -9.3 -2.24 8.62 0 ± 1.7 91 -9.2 -2.18 8.24 1.3 ± 1.8 Riyadh Al Khabra 100 -9.6 -2.35 9.20 0.4+ 2.3 101 -8.5 -1.25 1.50 2.2 ± 1.9 102 -9.6 - 2.11 7.28 1.9 ± 2.1 103 -9.9 -2.06 6.58 0.6 ± 2.3

92 + 0.6 -0.18 2.04 2.9 ±1.8 93 -1.9 - 1.66 11.38 5.4 ± 1.8 94 ' -1.5 -1.99 14.42 59.1 ±3.9 Wadi Ar Rimah 95 -4.0 -2.05 12.40 9.1 ± 1.9 96 - 2.1 -1.98 13.74 40.2 ±3.1

97 -5.6 - 2.21 12.08 23.0 ±3.1 98 -5.7 -1.81 8.78 22.7 ±3.1 Wadi Maraghan 99 -7.7 -2.05 8.70 1.7 ± 2.3

8 6 +9.2 - 0.0 2 9.36 33.5 ± 2.8 Rain-water

therefore also the differences in mineralization — depends on the fact that older waters, free from tritium, poor in deuterium and more strongly mineralized, become diluted by younger waters of a lower mineralization, carrying tritium and an enriched deuterium content. Therefore, a decreasing t8 value combined with an increasing mineralization is not always a proof of tertiary evaporation, but can also occur in mixed waters. The gradient of the 52H-S180 relations shows which of these two possibilities is effective. On the other hand, recycling is not always the only reason for a high degree of total groundwater mineralization. Thus, the high salinity of the wells in Wadi Maraghan (samples 97 and 98) is probably attributable to a contamination by an 2 3 0 HÖTZL et al.

100 km

FIG. 8. Geological map o f the recharge area o f Wadi Ad Dawasir (generalized after US Geol. Survey and Arabian American Oil Company, 1963). 1. Precambrian metamorphic rocks, mainly schists; 2. Basic igneous rocks, partly metamorphic; 3. Granite and granite gneiss; 4. Permian (Wajid sandstone and K hu ff limestone) and Triassic (mainly silt and sandstone); 5. Jurassic (mainly limestone); 6. Cretaceous (mainly sandstone); 7. Alluvium and related surficial deposits; 8. Aeolian sand; 9. Tertiary and Quaternary basalts; 10. Fault.

infiltration o f river water, w hich infiltrates into the w ell through salinated

Q uaternary deposits. The inflow of waters of high salt content from Q uaternary deposits explains the occurrence of highly m ineralized w ater in less soluble rocks, e.g. granite, schists and sandstone.

T his general conclusion from hydrochem ical investigations m akes it clear that, for the agricultural developm ent of arid regions, not only the water supply is im portant for irrigation but also a w ell-controlled drainage system . From this point of view cultivation should generally not use w adi floors and basins but higher IAEA-AG-158/16 2 3 1

located areas and flat slopes w here there is a free runoff of irrigation water, and where no infiltration of salt water from Quaternary accum ulations m ay occur.

T o calculate the depth o f boreholes and their casing, the possibilities o f groundw ater m ineralization through reflux or inflow of highly m ineralized waters from Quaternary w adi sedim ents should be taken into consideration.

5. ENRICHM ENT OF STABLE ISOTOPES IN HAND-DUG W ELLS AS AGAINST

ARTESIAN W ELLS AND CORRELATION BETW EEN M INERALIZATION

AND ISOTOPE CONTENT, INVESTIG ATED IN W ADI AD DAW ASIR AND

ITS HINTERLAND

W adi A d Daw asir is the old river system crossing the cuesta landscape of

Saudi Arabia in its southern part. The m ain tributaries are the present W adi Tathlith,

W adi Bishah, W adi Ranyah and W adi Subay’ (Fig. 8). The name W adi A d Daw asir is generally used dow nstream of the junction of W adi Tathlith and W adi Bishah.

W adi Ranyah has cut its w ay through a basalt flow. This basalt flow show s intensive weathering. Large areas of the accum ulation plain, especially south of the w adi channel, consist of autochthonous basalt detritus and aeolian sand of sm all thickness. The underlying clayey weathered soil reaches a thickness of about 4 m.

In som e pits used for rem oving m aterial for buildings .several lim ey soil horizons, signs of form er groundw ater levels are detectable.

These old soil В horizons, together w ith the clayey layers, em phasize the intensive chem ical w eathering process that follow ed the eruption of the basalts.

Sam ples for 14C m easurem ents were collected from the lim ey crusts at depths of

0.7 and 1.0 m below the land surface. The m easurem ents revealed 14C ages of

26 400 and 29 840 years B P for the origin of these calcite horizons (see Table V III).

In contrast to our expectations, these results lead to the conclusion that there was enough precipitation during the W ürm interstadial to produce such soil caliche for this period.

W hile the plain between the villages of M uqabil and Raw dhah show s a certain relief caused by basalt flow s and pedim ents w ith outcropping rocks and different detritus covers, the flat basin east of Raw dhah oasis takes on m ore and m ore the character of an accum ulation plain tow ards the east. The alluvial fans from the south still show a northw ards flow direction, i.e. tow ards the old east-west trending wadi. N orth of the w adi the accum ulation plain displays m ore and m ore a hom ogeneous eastw ards dip. A t present the old w adi channel is accom panied by dunes on both sides. These aeolian accum ulations dem onstrate the w ind transport o f fine m aterials blow n out from the upper areas of the accum ulation plain.

In m ost cases at W adi A d Daw asir, hand-dug wells supply water. W ater occurs

8 -1 2 m below ground surface w ith a tem perature of 2 7 -3 1°C and a strong

m ineralization, 2 8 -1 0 4 meq/ltr. Chem ically, these are m ostly N a-Ca-Cl-S0 4 w a t e r s , 2 3 2

TABLE VIII. RESULTS OF 14C M EASUREM ENTS IN THE AREA OF W ADI AD DAW ASIR AND W ADI RAN YAH

( 1 9 7 5 )

Locality Geograph. Laboratory Reg.-No. Material 14 С age position (years BP)

1 Bir Juqjuq Lat. 21° 18' N IRMa Lab.-No. 4 231 Weathered limey crust, > 3 5 000 Long. 43° 42' E soil horizon in terrace

2 Bir Juqjuq Lat. 21° 18'- N IRM Lab.-No. 4 232 Limey crust in terrace 10 820 ± 320b

Long. 43° 42' E HÖTZL

3 Loam pit Muqabil Lat. 21° 14' N IRM Lab.-No. 4 233 Limey weathered material 26 400 ± 1970 Wadi Ranyah Long. 42° 46' E overlying basalt; sample t l. a et 0.7 m below land surface

4 Loam pit Muqabil Lat. 21° 14' N IRM Lab.-No. 4 234 Limey weathered material 29 840 ± 2600 Wadi Ranyah Long. 42° 46' E overlying basalt; sample 1.0 m below land surface

5 Sha’ib Hathag Lat. 21° 15' N IRM Lab.-No. 4 235 Calcite sinter 6 7 0 0 ± 280 NW of Al Jirthamiyah Long. 42° 44' E

6 Sha’ib Hathag Lat. 21° 15' N IRM Lab.-No. 4 236 Calcite sinter 6110+ 250 NW of Al Jirthamiyah Long. 42° 44' E

a Institut für Radiohydrometrie, Munich. b Twofold standard deviation. IAEA-AG-158/16 2 3 3

TABLE IX. ISOTOPE CONTENTS OF THE W ATERS OF W ADI AD DAW ASIR

(SAM PLES TAKEN BETW EEN 21 AND 27 FEBRU ARY 1975) i8 is computed from 82H= 8 X 8lsO + t8

No. 3H content S2H(°/oo) 5180(°/oo) *8 of sample (TU)

164 -46.5 -6.52 5.66 2.8 ± 2.0 166 -42.9 -6.06 5.58 1.9 ±2.5 167 -56.5 - 8.02 7.66 1.6 ± 2.0 170 -49.0 -6.98 6.84 1.5 ± 2.0 171 -47.9 -6.73 5.94 0.7 ± 1.9

157 -38.5 -5.60 6.30 0.3 ± 1 - 158 -44.8 -6.08 3.84 0.7 ± 1.9 159 -44.3 -5.68 1.14 0.9 ± 2.0 160 -41.2 -5.64 3.92 0 ± 2.0 161 -35.7 -4.93 3.74 2.3 ± 2.0 162 -26.3 -4.39 8.82 0.5 ± 2.0

163 -33.7 -4.89 5.42 11.0 ± 2.2 165 -26.3 -3.61 2.58 1.5 ± 1.9 168 -5.7 - 2.00 10.30 45 ±3 169 -31.4 -4.71 6.28 2.8 ± 2.0

151 - 6.8 -2.48 13.04 28 ± 2 152 -5.7 -1.97 10.06 35 ± 2 153 -7.7 -2.03 8.54 44 ± 2.3 154 -4.8 -1.87 10.16 30± 2 155 - 6 .2 -1.76 7.88 30 ± 2

partly w ith a high M g content. Sam ple No. 168 (Table IX ) contains strong portions

of recent rain-w ater according to the 3H and stable isotope contents. A ll other

waters belong to a considerably older storage area w ith w idely ranging values.

The artesian wells can be differentiated by their gypsum content. They have

a low degree of m ineralization (8— 12 m eq/ltr). A s can be expected, all artesian

w aters are free o f 3H. The isotope data show no relation to the tem peratures.

M ost hand-dug w ells show less negative 5-values than artesian w ells (Fig.9).

A t first sight one m ight, therefore, be tem pted to explain the phenom enon of

isotope enrichm ent as resulting from an evaporation process, beginning in artesian 2 3 4 HÖ TZL et al.

<Г*0'А'

FIG.9. 5 2Я -5180 relation o f the waters o f Wadi Ad Dawasir and Wadi Ranyah, together with the equation lines for artesian waters (lj and waters from hand-dug wells (2). The broken line stands for mixtures o f artesian waters with the groundwater originating in the Shield.

wells and continuing in hand-dug wells to an ever-increasing extent. A nother

argum ent seem s to be the positive relationship betw een the degree of m ineralization

and the enrichm ent of deuterium as well as 180 .

T w o questions are o f decisive im portance concerning these w aters — first,

w hat is the reason for the enrichm ent of stable isotopes in hand-dug w ells as against

artesian w ells? Second, how can the correlation between m ineralization and isotope

content be explained if there is no evaporation? It has been observed that drilled

w ell No. 162 (the water o f w hich is not connected w ith the upper layers o f the

wadi floor ow ing to a casing reaching dow n 30 m ) has a relatively low degree of

m ineralization yet it is strongly enriched isotopically. This enrichm ent cannot have

occurred in the wake of evaporation because of its high ts value. It w as indicated

in Section 4 that isotope shifting, together w ith changes in the extent of

m ineralization m ay be explained as due not only to evaporation but also to the

m ixing of waters.

In W adi A d Daw asir such a potentiality arises through a shallow groundw ater

flow com ing from the vast recharge area in the crystalline rock of the shield,

follow ing a channel filled w ith Q uaternary sedim ents. W aters taken from valley

fillings of the shield in W adi Ranyah situated som e 200 km from A l Kham asin were

poorly m ineralized (4— 13 m eq/ltr) and show ed a m edium deuterium content of

S 2 H = — 6 % 0 and a m edium 180 content of 5 180 = — 2 % 0. Assum ing that these

isotope values were likew ise valid for the groundw ater of W adi A d Daw asir originating IAEA-AG-158/16 2 3 5

in the shield, and assum ing that artesian w ell N o. 164 (Table IX ) is representative

of the old sandstone water in the area of A l Kham asin, the m ixtures of these tw o

com ponents should then lie on the broken line draw n in Fig.9. This is indeed true

for the isotope values o f w ell No. 162, w hich is provided w ith a casing dow n to

low er parts o f the w adi filling. B y contrast, except N o. 157, all hand-dug wells

(probably because of stronger evaporation) are grouped around a slightly flatter

equation line draw n bn the right in Fig.9(02H = 7.95 X 5 180 + 3.89). This equation

line show s that the shifting of isotope content m ay occur parallel to 5 2H = 8 X 5 1 8 0 ,

ow ing to a com bination o f m ixing and evaporation, although it has generally been

assum ed that a slope of m = 8 is a sure sign that no kinetically influenced evaporation

has taken place.

These deliberations lead to the conclusion that the enrichm ent o f isotopes as

well as the m ineralization in hand-dug wells m ay be explained by an adm ixture of

recent shallow groundw ater to old sandstone water, provided that the recent water

in the low er parts of the w adi filling displays a poorer degree o f m ineralization than

that in the upper parts.

IAEA-AG-158/17

ENVIRONMENTAL ISOTOPE STUDY OF GROUNDWATER SYSTEMS IN THE REPUBLIC OF DJIBOUTI

J. Ch. FO N T ES*

Laboratoire de Géologie Dynam ique,

Université Pierre et M arie Curie, Paris

P . P O U C H O N

Laboratoire de Géodynam ique,

Université de Bordeaux III, Talence

J.F. SALIEG E, G.M. ZU PPI*

Laboratoire de Géologie Dynam ique,

Université Pierre et M arie Curie, Paris,

F r a n c e

A b s t r a c t

ENVIRONMENTAL ISOTOPE STUDY OF GROUNDWATER SYSTEMS IN THE REPUBLIC OF DJIBOUTI. Environmental isotopes and hydrogeochemistry are being used to shed new light on the occurrence of present-day recharge and on the origin of groundwater systems in the Republic of Djibouti. Furthermore, an attempt is also being made to evaluate palaeohydrological conditions during the past 6000 years. From stable isotope data which he along a correlation line at a slope of 8 in the diagram 6 2H-6180 , it can be concluded that recharge occurs by rapid seepage in fractured rocks without evaporation. Some waters from hot springs show an oxygen shift, indicating the occurrence of an exchange process with rocks at high temperatures. The following conclusions can be reached from tritium and 14C content of waters. Groundwaters can be divided into two groups: one deriving from recent recharge (last five or six years) corresponding to water with rather fast circulations in fractured media; and a second group, pre-bomb recharged corresponding to water with low flow rates in porous media. Only one sample (Yoboki) seems to derive from about 10-year-old recharge. In the case of Abhè hot spring, a 14C age of about 1200 years may be evaluated. The calcite concretions of the Abhè Lake Basin are believed to have formed as a result of the mixing of lake water (sodium-carbonate type) with groundwater (sodium-chloride, calcium-sulphate type). From the 13C and 14C content it appears that the dis­ solved carbon of present-day lake water is in, or close to, equilibrium with the atmosphere. Consequently, it is assumed that such was also the case during the whole Holocene. The 180 content of palaeolake water, evaluated from the calcite isotopic composition with the palaeo- temperature equation, was originally more negative than the present one. This is interpreted as due to the fact that the Holocene lake was fed by large floods and that significant seepage occurred through the lake bottom with a consequent reduction of the evaporation effects.

* The present address of Mr. Fontes and Mr. Zuppi is given in the List of Participants.

2 3 7 238 FONTES et al.

TABLE I. CLIM ATIC CONDITIONS IN THE REPUBLIC OF DJIBOUTI

Location Altitude Annual Rainy days Annual Annual (m ) rainfall evaporation temperature (average) (average) (average) (mm ) (mm) - (°C )

Djibouti + 10 177 17 2810 29.9

Arta 700 246 28 2500 26.1

FIG.l. Location map o f Djibouti area and distribution o f sampling points.

1. HYDROGEOLOGICAL FRAMEW ORK AND PROBLEMS

The Republic of D jibouti (Fig.l) is located in the arid zone of eastern Africa

(11 to 13° lat. N). Precipitation is random ly distributed on the coast and becom es

m onsoon in type in the inland (sum m er rains). Tem peratures are high and show

little m onthly variations. Clim atic data (average annual values) are sum m arized

in Table I. The ratio precipitation/evaporation w hich can be used to define the

aridity varies betw een 0.05 (D jibouti) and 0.1 (Arta) according to the altitude

o f the station.

The w hole country is practically covered by lava and volcanodetrital deposits

from Tertiary and Q uaternary ages, som e of them very recent. This volcanic

activity is due to the location o f the area on the southern part o f the A rar rift. IAEA-AG-158/17 2 3 9

Vertical m ovem ents along the faults o f the rift structure give rise to isolated basins w ith internal drainage (Grand Bara, Abhè, Henlé, Gobaad, Asal). During the U pper Pleistocene and until about 4000 years ago, lacustrine episodes took place in these depressions [1,2]. A t present tw o basins are still active, both occupied by highly saline lakes. Lake Abhè is the term inal lake of the Aw ash

River system w hich com es from the Ethiopian Plateau. It is a sodium -carbonate type water (T D S~ 15 0 g). Lake Asal is m ainly supplied by sea-water underflow

and reaches saturation in sodium chloride [3].

The surface netw ork at present consists of interm ittent wadis. G roundw ater ,

system s are governed by geological and sedim entological features. Precipitation and/or floods can infiltrate through highly perm eable fractured lavas and accum ulate w ithin sedim ents in the depressions w here porosity and perm eability are extrem ely

low ow ing to the occurrence of a high load in clay m inerals.

Piezom etric surfaces are very deep and range from 200 m below ground level

in the north o f the country to 30 m in the coastal region o f Djibouti. Because of the high geotherm al flux in the rift area, groundw aters are w arm (30 to 40°C for

shallow system s to boiling point for deep system s). The purposes of the environ­

m ental isotope investigation were to determ ine:

The occurrence and im portance o f the present-day recharge;

The origin o f groundw ater (local infiltration, supply from higher altitude,

old w aters, i.e. palaeow aters);

The influence of evaporation before or during the recharge; and

The age of palaeogroundw aters, if any, w hich could be involved in ground­

w ater system s.

The irregular regim e o f a 25 to 224 m m precipitation for annual rainfall

in the city of Djibouti (over the period 1901— 1945); the poor know ledge of

évapotranspiration values; and the difficulty of groundw ater flow study by

conventional m eans in fractured rocks and in basins of internal drainage, m ake

the hydrogeological study difficult and thus its isotopic com plem ent attractive.

Furtherm ore, som e lacustrine concretions o f Holocene provide suitable m aterial

for an attem pt in the field o f palaeohydrology.

Previous isotopic investigations in this area w ere carried out on surface w aters and lake system s [3, 4], and a general reconnaissance survey on the stable isotope

content of groundw ater w as reported by Schoell and Faber [5].

2. HYDROGEOLOGY

2.1. Sam pling

Sam ples were collected from available boreholes for the w hole country.

Special attention w as paid to the borehole field (see Table II), w hich supplies the 4 FNE e al. et FONTES 240 TABLE II. AQUIFER OF W ADI AMBOULI (W ATER SUPPLY OF DJIBOUTI)

A l k a l . ^ No. D ate ô 13o / show 62h/ smow 6 3H TU 6 13C PDB 14C $ mod. pH meq. kg

E 1 0 2 -7 3 - 1-54 - 1 1.83 5 2 .З + 4 .5 8 .1 4 .1

E 2 0 2 -7 3 - 1 .6 6 - 1 .2 24 + З

E 3 0 2 -7 3 - 1 .4 5 + 0 .7 13+3

E 4 0 2 -7 3 - 1 1 .1 5 7 9 .З + 2.7 3 .4 ( 4 . 2 )

E 5 0 2 -7 3 — i . 66 - 3 .3 28 + 3 7 3 .4 + 1 .8

E 6 0 2 -7 3 - 1 .7 0 - 1 .4 11 + 3

0 2-71 - 1 .4 5 - 8. ó

E 7 0 2 -7 3 - 1.49 + 2 .0

0 2 -7 1 - 1 .4 5 . + 2 .9

E 8 0 2 -7 3 - 1 .5 4 + 0 .6 43 + 3 - 1 2.03 7 3 . 4 + 2 .2 7 .5 ( 3 . 9 )

E 10 0 2 -7 3 - 1 .7 5 + 0 .6 23 + 3 - 1 З.ОЗ 7 5 .6 + 2 .0 8 . 4 - 4 .O

02-71' - 1 .6 2 - 3 .3

E 11 0 2 -7 3 - 1 .6 2 - 4 .0 26+3

E 12 0 2 -7 3 - 1 .7 3 - 0 .3 13 + 3

0 2 -7 1 - 1 .87 - 0 .4

E 13 0 6 -7 6 - 1 .6 2 13+3 - 1 2 .4 5 86.6+7.0 7 .2 4 3 .3 5

E 1 5 0 6 -7 6 - I . 4 8 i7 + 3 7 З . 5 + 3 . 0 7 .1 2 3.3 0

E 17 0 6 -7 6 - I .6 9 6 + 1 - 1 1.45 7 9 .6 + 4 . 0 6 . 9 2 3 .4 5

RG 2 0 2 -7 3 - 1 .7 5 1.1 27 + 3

RG 3 0 2 -7 3 - 1 .7 5 1 .1 27 + 3

RG 4 0 2 -7 1 - 1 .37 2.7

RG 6 0 2 -7 3 - 1 .8 0 - 1 2 .3 2 7 7 .1 + 7 .0 8.1 ( 4 . 0 )

Values in brackets were obtained during a separate sampling. Temperatures are uniformly close to 40°C. IAEA-AG-158/17 2 4 1

city and the harbour of Djibouti w ith fresh water (consum ption 24 X 10 3 m 3/ d )

from a slightly confined aquifer. Several springs generally correlated to faults

and hot circulations were also sam pled.

W hen possible, the pH, tem perature, conductivity and alkalinity were m easured on location. Chem ical determ inations 1 were done on ultrafiltrated sam ples w ith

both acidified and non-acidified aliquots. Results w ill be published in detail

e l s e w h e r e .

2.2. The problem of present-day recharge

Tritium m easurem ents (see Table II and follow ing) show values from back­

ground to 43 ± 3 TU. M ost of the sam ples thus contain a part of recent recharge.

Because m easurem ents o f 3H activity in rain-w ater are not available, the

tritium content o f groundw ater is com pared w ith precipitation data o f the IA E A

N etw ork [6-11]. The selected stations (Table III) of Khartoum , M inicoy,

Bahrain and Jeddah m ay be not fully representative of the Djibouti area. For

instance, Khartoum rains show a rather high 3H content due to the continental effect.

How ever, the record w as chosen there because its com pleteness allow s relative

estim ations. From the com parison of these results, it appears that the follow ing

average ranges o f values can be proposed as rough guidelines to estim ate the tritium

content of recharge in the Djibouti area:

1963-1965 peak 100 to 200 TU

1 9 6 5 - 1 9 7 0 40 to 50 TU

1 9 7 0 - 1 9 7 6 10 t o 2 0 T U

In M ay 1976 the waters o f the Asal lake had a hom ogeneous tritium content

o f about 9 T U [3], w hich reflects the average o f 3H activity o f the environm ental

atm ospheric m oisture since this activity is only gained by m olecular exchange.

D uring the sam e period, the tail o f the flood o f the interm ittent W adi Am bouli,

near Djibouti, exhibited a 3H content o f 7 ± 1 T U w hich is also attributed to the

exchange w ith atm ospheric vapour rather than to rain or to soil water.

Taking into account the age effect, one w ould estim ate that all groundw ater

show ing a tritium content ranging from 10 to 43 T U (Tables II, IV , V ) was

recharged during the last 5 to 10 years. Since such tritium contents are observed

in different unconfined aquifers — 75/3, 76/6 D ikkil, 75/39 Dorra, 76/2 Tadjourah

Magale, 76/31 G ourabous, 76/33 Yoboki, and m ost of the sam pling points of the

aquifer of W adi A m bouli (Table II) ■- in the w hole region one m ust conclude that

these aquifers are rapidly recharged through fractures and cracks o f the basaltic

outcrops. Furtherm ore, the occurrence of recent waters in the discharge indicates

1 Chemical studies involve collaboration with the US Geol. Survey (B.F. Jones) and the University of Aachen (H.R. Langguth). 2 4 2

TABLE III. ANNUAL AVERAG E TRITIUM CONTENT OF RAINS OF SOME SELECTED STATIONS FOR

CO M PARISON W ITH THE DJIBOUTI AREA. From Ref. [6 t o 1 1 ].

Station Co-ordinates Alt. 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 al. et FONTES (m)

Khartoum 15°60'N;32°55'E 382 89 71 379 1432 771 387 269 171 104 219 130 149.5 82 45.1 53.7 50.1 35.3 Minicoy 8°30'N; 73°00'E 1 67 53 56 26 Bahrain 26°27'N;50°62,E 2 370 222 242 172 59.8 37.6 97.7 96.5 34.4 32.1 24.3 33.7 17.8

Jeddah 21°50'N;39o20'E 11 19 15 TABLE IV. GROUNDW ATERS FROM THE REPUBLIC OF DJIBOUTI

(Sam pling 1975)

0 A lk a l. Drainage . -1 13 14„ No. Name Area Type t°C pH /iScm "1 meq kg 6 0 SMOW 62H SMOW 3H TU 613C PDB С prac

75/1 Doubdoubbol el e Grand Bara В 90m 36.6 7.62 3500 2.33 - 4 .0 6 -3 2 .0 < 4 - З.4 0 27.6 + 2.7

75/2 Mouloud Grand Bara B BOm 36.3 7.40 2400 4 .З 4 - 3 .9 3 - 3 2 .О . <: 4 -1 0 .7 9 5 2 .2 + 2 .5

75/3 D ik k il Henlé В 110m 4 2 .0 7.30 2150 6 .1 5 - 1 .3 6 23+2 -11 .3 1 75/4 Abhè Alaylou Abhè Lake 30.1 9 .3 0 + 3 .7 3 - З.5 0 37 + 7 + 0.63 110.0 + 1.1 Abhè Spring Abhè S prin g 70.0 3.26 0.72 -2 2 .0 -1 7 .0 5 30.0 + 5 .0 75/17 -2 .7 7 < 4 243 IAEA-AG-158/17 75/19 Abhè Spring Abhè S prin g 99.0 -З. 5З -3 2 .5

75/19 Abhè Spring Abhè S prin g 36.0 -3 .4 9 -З 1 .9 75/20 Abhè Spring Abhè S p rin g 33.0 -З .7 7 -3 1 .6

75/21 Àbhè Spring Abhè S p rin g 100.0 -З .4 0 -3 4 .3

75/22 Abhè Spring Abhè S p rin g 95.0 -3 .7 7 -3 1 .0 .

75/33 K o r il i Asal S p rin g 55.0 7 .0 5 O.9O -1 .2 3 + 9-7 < 5 - З.З9

75/34 Agna Henlé S p rin g 60.0 3.34 3.20 -3 .3 5 Í 4 - 2.61 49-7 + 4 .0

75/35 H a ll o i A sal S prin g 69.0 7 . 4 I 0.66 - 2 .1 4 - 1 4 .З £ 5 -1 7 .4 4 75/36 A sa l SE2 A sal S p rin g 59.0 7 . 4 I O.4 9 -1 .2 8 -4 2 .5 ¿ 5 - 6.99

75/37 Obock Soublali Piedmont В 120m 39.5 7.39 5. 4 I - 2 .О4 -I 7 . 5 8 + 2 -1 2 .3 4 9O. 9 + 2 . 5 P la in

75/33 PK 50 Coastal В 39*0 ( 6 . 50 ) 1.30 -I .2 9 7 t 1 -1 1 .6 0 <60 A q u ife r

75/39 Богга Piedmont B 250m 43.0 3.60 4 .I 3 -0 .4 3 - 3.6 24 + 5 -IO .4 6 63.3 + 2.0 P la in

75/40 Andaba Piedmont В 43.0 8 .12 2.13 -3 .6 8 < 5 - 3.33 30.3 + 2 .5 P la in

В = Borehole and depth 4 FNE e al. et FONTES 244

TABLE V. GROUNDW ATERS FROM THE REPUBLIC OF DJIBOUTI

(Sam pling 1976)

Conduct. Alkal. , -1 No. Name Drainage Area Type t°C pH /iS-cm-1 raeq. kg 6180 SMOW6 ^ SMOW 3H TU 6 13C PDB pmc

76/1 Ambouli Coastal Aquifer Wadi flood - 3.01 7 ± 1 7 6 / 2 Tadjourah Coastal Aquifer В 100m 37 7 . 5О 4.7 1 8 + 2 -12.00 9О.7 + 1.1 Magale

76/i Tadjourah PK.9 Coastal Aquifer В 100m 35 7 . 2 2 4.1 - 1 .5 8 5 + 1 - 13.20 9 З.9 t 1л 76/4 Pit 50 Piedmont Plain В 40 7.65 II50 5.З - 1 .5 7 -II .69 80.4 + 1.0 76/5 Abhè Spring Abhè Spring 69 0.7 - 2.61 <3 -I 7 .O5 9 2 .0 + 8.0 76/6 Dikkil Henlé В 110m 7.22 3100 5-5 - 1.66 15+2 -11.81 37.7 + 1.7 76/7 Korili Asal Spring 80 7.42 О.52 - 1.44 - 11.8 0.3 + 0.3 - 8.53 <37 76/8 Manda Aeal Spring 32 7.51 2 . 2 2 + 0.68 + 6.3 3.1 + 0 .2 - 9 .2 6 76/14 Hallol Asal Spring 70 7.32 0.35 - 2 .4 0 - 17.3 0 . 8 + 0.2 -12.18 1 0 1 + 18 76/19 Gubbet Coast Coastal Aquifer Spring 45 + 0.33 4 + 1 76/30 Daddato Piedmont Plain Spring 37 (7 . 50) (10.0) - 1.42 19 + 2 - 15.00 9 8 .2 + 2.1 76/31 Gourabous Piedmont Plain В 27 7.65 1506 6 . 4О 2 0 + 2 -11.99 76.1 + 3.0 76/32 Oulma Obock Coastal Aquifer B 120m (40) 7.07 367 2.39 - 2.37 ' - 7.33 7 З.4 + 1 .6 76/33 Yoboki Henlé B 70m (40) 7.55 6 0 2 (6.39) 43 + 3 - 7.46 43.1 + 0.7 76/34 Minkile HenlÔ Spring 64 300 1.44 - 2.06 -17.35 76/35 Koutabouya Gobaad В 90m 40 8 . 1 0 2188 (2.44) 5 ± 1 - 5-62 3 0 .6 + 1.7

Type: Wadi B: Borehole and depth Values in brackets were obtained during a separate sampling IAEA-AG-158/17 2 4 5

that the storage is low and that the porous m edium , w hich m ay exist in these locations, does not contribute to the recharge as expected from sedim entological criteria (large content o f clay m inerals provided b y w eathering o f basaltic rocks).

Nevertheless, som e sam ples — 75/37 O bock Soublali, 75/38 P K 50,

76/3 Tadjourah P K 9, 76/19 Coastal Spring in the bottom of the G ulf of Tadjourah,

76/35 Koutabouya, and the borehole E 17 o f Djibouti water supply (in 1976) — show tritium am ounts w hich are at the low er lim its of anticipated activity for recent groundw aters. These low tritium contents could be explained by a m echanism o f recharge involving single episodes o f tritium -poor m onsoon rains.

This kind of recharge is com patible w ith local clim atic and hydrogeological conditions, i.e. sporadic episodes o f heavy rains and rapid infiltration in fractured lava. Such a m echanism w ould also im ply isolated circulations and low storage capacities. Because o f the location o f these boreholes in rather thin detrital deposits and the fact that E 17 w as sam pled during drilling operations, the occurrence o f m ixing between “old” (tritium -free) and recent w aters could also be invoked. In that case, “old” m eans aged at least m ore than 20 years and w ould correspond to poorly circulating water bodies.

Som e w aters are clearly old as indicated by the background level in tritium activity. This is the case for hot and thus deep circulations in the therm al spring of Lakes Abhè and Asal (sam ples 76/5, 76/7, 76/14, 75/17, 75/33, 75/35, and

75/36), but also for norm al groundw aters in large basins filled w ith argillaceous sedim ents (sam ples 75/1 Doubdoubbolele, 75/2 M ouloud, 75/34 Agna, 75/40

Andaba). This reinforces the hypothesis of the occurrence of waters older than

20 years in the m ain depressions. Furtherm ore, because the aquifers are uncon­ fined, the lack o f 3H in these basins o f internal drainage suggests that thin sedi­ m ents allow an active evaporation and évapotranspiration o f rain and interm ittent floods in the low est points o f these system s w hich w ould not benefit any significant vertical supply.

2.3. O rigin and m echanism s of groundw ater recharge

The stable isotope content of groundw aters is reported in Tables II and V I and Figs 2 and 3.

From the distribution of the points on the diagram 52H -5180 of 1972 sam pling, w hich is the m ost detailed (Fig.2), it can be concluded that:

M ost o f the values fall on a range w ith a slope o f 8 indicating that no

significant evaporation occurred before or during infiltration; this fact is

in agreem ent w ith the concept o f a fast infiltration through fractured lavas;

Along w ith this tendency, the values range between — 4.6 and about 0 % o

i n 180 concentrations; interpreted in term s o f altitude effect, this range FONTES et al.

ÎLE VI. GROUNDW ATERS FROM REPUBLIC OF DJIBOUTI

npling 1972)

No, Name Type t ° c Conductivity 5 lsO SMOW 5 2H SMOW /uS-сп Г 1

1 Seik Sabir W 3 .0 m 280 - 0 .2 + 3 .0

2 Teoao W 2.5 m 480 0.0 - 2 . 4

3 Moutrous W 2 .0 m 4.8 X 103 + 2 .6 + 4 .2

4 Goma hele W 2 .0 m 3.5 X 103 - 1 .0 - 1 0 .8

5 Guidoli W 2 .0 m 2.8 X 103 + 1.7 + 5.3

6 Halouli S - 1 .6 - 2 .4

7 Mouri Hani S 3.3 X 103 - 1 .7 - 1 3 .7

8 Lahibouya S 670 + 1.5 + 5.1

9 Boule yare W 2.5 m 670 - 1 .1 - 1 . 4

10 Nihile S 1,5 X 103 - 1 .9 - 1 .3

11 Gourabous В 2 0 m 540 - 0 .2 - 0 .9

12 Djibouti R 2 7 /0 4 /7 2 - 1 .5 - 6 .5

13 Djibouti R 2 1 /0 9 /7 2 + 2 .4 - 8 .2

14 Yoboki R 0 8 /7 2 + 4.5 - 6 .5

15 Oueah В 25 m - 0 .7 - 7 .0

16 Hindi W 2 .0 m - 0 .4 + 5.1 17 PK 50 В - 0 .5 - 3 . 0

18 Ambouli В 30 m - 1 .1 + 4 .0 19 Douda В 30 m - 1 .5 - 7 .5

2 0 Koussourale W 0.0 - 7 . 0

21 Galerie Ambouli В - 0 .7 + 1.8

22 Hallol HE 7 S 3.1 X 103 - 3 .5 - 2 7 .1

23 Hallol HE 11 S 3.85 X 103 - 3 .5 - 2 8 .0

24 Hallol HE 19 S 7.1 X 103 - 3 . 4 - 2 3 .0

25 Hallol HE 2 -1 S 2 .9 X 103 - 3 .1 - 2 4 .0

26 Hallol HE 2 -2 S 2 .9 X 103 - 3 .6 - 2 9 .8

27 HaUol S III S 80.0 15 X 103 - 2 .7 - 1 4 .9 1 28 Canal Sakallol S + 0 .2 OO 00

29 Harallol В s 45.0 7 .4 X 103 - 2 .4 - 2 0 .0

30 Harallol A s 50.0 8.7 X 103 - 2 .6 - 1 9 .8

31 Harallol IIHS s 6.7 X 103 - 2 .6 - 2 0 .9

32 Oued Toha s - 1 .3 - 5 .1 IAEA-AG-158/17 2 4 7

TABLE VI. (cont.)

No. Name Type TeC Conductivity S 1!0 SMOW 6 2H SMOW /LiS-cm”1

33 Bondarra W 6 .0 m + 1.3 + 5 .4

34 Ali Sabieh R 0 8 /7 2 + 5 .2 + 6 .3

35 Korili S - 0 .3 + 14.2 36 Canyon SE 1 S 4 7 .0 76 X 103 - 0 .9 - 1 .3 5

37 Crevasse Manda S 30.0 36 X 103 + 1.9

38 Manda 3 s 37.0 51 X 103 + 2 .0 + 15.6

39 Asal N 1 s 105 + 2 .7 - 3 8 .3

40 Asal SE 2 s 58.0 8 2 .103 - 0 .6 - 3 9 .3

41 Hallol A s 40.0 1 5 .103 - 2 . 4 - 1 7 .5

42 Hallol B s 29.0 + 6.5 + 2 3 .2

43 Hallol C s 46.5 - 2 .5 - 2 0 .7

44 Hallol D s 79.5 - 2 .4 - 1 5 .9

45 Hallol E s 4 8 .0 - 4 .6 - 3 4 .6

46 Hallol F s 5 2 .0 - 4 . 0 - 3 1 .3

Symbols: В = Borehole and depth; R = Rain and data; S = Spring; W = Dug well and depth.

suggests that infiltrations occurred on m ore than 1000 m o f vertical develop­

m ent if one takes into account an isotopic gradient o f 0.3 to 0.5 % o p e r

100 m ; it m ay be noted that the stable isotope content o f groundw aters

from the Ethiopian highlands at altitudes higher than 2000 m , reported by

Schoell and Faber [5], do not show values low er than in the low lands

of Djibouti (m axim um elevation of the escarpm ents ~1 30 0 m ); this suggests

that the altitude effect is not responsible for the observed range o f variations

in groundw aters w hich could rather be due to sporadic recharge by isolated

episodes o f m ajor rains;

The intercept o f the correlation (deuterium excess, d), low er than the

present-day typical value of + 10 for oceanic rains, is sim ilar to w hat w as

found for hot springs o f the area [5] but quite different from the intercept

encountered by these authors in groundw aters from Ethiopian highlands

( d = + 1 5 ) ;

Isolated sam ples o f rain-w ater fall below the local m eteoric w ater line and

are thus clearly evaporated during rainfall; this deviation show s that only

heavy episodes öf non-evaporated rains can contribute to the recharge; FIG.2. D-ibO correlation for 1972 — 73 sampling. The large range o f values is attributed to sporadic recharge by single heavy episodes o f rain and floods in fractured rocks and, to a minor extent, to altitude effect. Evaporation does not affect the waters except in some shallow wells and in available rain samples: only major rains, non-evaporated, contribute to the recharge. Values obtained by Schoelland Faber [5] fall within the observed range and the distinction between “Wadi-type”and “Rift-type” water is not obvious, even if coastal samples (e.g. Wadi Ambouli aquifer j show a high heavy isotope content associated with a deuterium excess close to +10, which could characterize the first stages o f condensation o f monsoon rains. Geothermal exchange with rock materials affects samples 39 and 40 from the Asal Rift system, which show an “oxygen shift” towards positive values. IAEA-AG-158/17 2 4 9

Sam ples from the aquifer o f W adi A m bouli (Table II) fall w ithin a very narrow

range (— 1.9 < 5 180 < — 1.5; —8 < 52H < +3), close to the m eteoric water line w ith an intercept o f + 1 0 suggesting that recharge occurred from heavy

non-evaporated m onsoon rains w hich generated heavy floods and recharged

the aquifer;

Som e sam ples clearly show the effect of evaporation, their heavy isotope

content is shifted below the correlation line; these sam ples are generally

shallow wells - sam ples 3,5, and 33 (dug wells); or springs — sam ples 8 a n d

28 — w hich m ay have undergone evaporation near the soil surface;

Except for som e sam ples of the Lake Asal basin (sam ples 39 and 40), therm al

w aters do not show a significant 180 shift tow ards positive values, w hich

reveals an exchange w ith the m inerals of the aquifer. The sam ples w hich

show such an oxygen shift have the low est deuterium content of the w hole

sam pling and thus w ould correspond to the supply com ing from the highest

altitude correlated to the deepest circulation; for sam ples 41, 43, 44, 45, and

46 w hich correspond to waters com ing from the highest parts o f the Lake

A sal basin, an altitude effect could account for the total difference o f 2 % o

i n t h e 180 content o f these waters.

The stable isotope analyses repeated in 1975 (Fig.3) and 1976 on som e of the sam e locations basically confirm the previous statem ents. O ne can define a local m eteoric water line w ith an intercept close to zero. The geotherm al exchange is still m arked in the Asal basin (sam ple 75/36; Asal SE 2) but a slight isotopic exchange betw een water and rocks is noticeable in the boiling sam ples 75/18 to

75/21 from the Abhè Lake system .

It is interesting to note that the w ater from the K orili Spring still exhibits a deuterium excess greater than 10. This isolated feature m ay be due to an exchange w ith fluids bearing H S - or H 2S. The circulation of this hot spring (70°C) is located near the gypsum deposits of the Lake Asal basin, w hich could be reduced at depth in the peri-volcanic environm ent o f the Asal rift giving rise to com pounds depleted i n 2H, w hereas the w ater is enriched in this isotope and the representative point of the sam ple shifted above the m eteoric water line in a 62H -5180 diagram ..

In several springs or boreholes, the stable isotope content show s variations between the sam pling, i.e. P K 50 (5180 = —0.5 in 1972; — 1.29 in 1975, and

-1.57 in 1976), Dikkil (0180 = -1.36 in 1975, and -1.66 in 1976). These variations favour the hypothesis of a sporadic recharge. O n the other hand, the

aquifer of W adi Am bouli appears m ore hom ogeneous in each borehole: E 6

(0180 = — 1.45 in 1971,-1.70 in 1973); E 7 (S180 = -1 .4 5 in 1971,-1.49 in

1973); E 10 (6180 = -1.62 in 1971, -1.75 in 1973); E 12 (S180 = -1.87 in 1971,

— 1.73 in 1973). This is attributed to the texture of the aquifer (Fig.4) w hich can be considered of m ixed (porous and fractured) type. The floods can percolate to о

FIG.3. D-nO correlation for 1975 sampling. The range o f values o f Fig.2 (1972 - 73) is confirmed for hypothermal and hydro- thermal systems. The occurrence o f tritium in most samples indicates that the present-day meteoric water line with a deuterium excess (d = 52tf-88180 = 0) is representative o f present-day recharge in the inland. IAEA-AG-158/17 2 5 1

N S ELEVATION mo.s.l.

FIG.4. Geological setting of the Wadi Ambouli confined aquifer.

through the ancient alluvial deposits o f the w adi or through the fractured old

(Pliocene) basalt o f the southern river bank.

In conclusion, regarding the stable isotope study it appears that infiltration occurs on lim ited and localized catchm ent areas and that groundw aters undergo reduced m ixing w ithin the aquifer w here the isotopic variations o f the input are preserved. These variations are m ainly due to the individual labelling o f single episodes of recharge. How ever, a contribution of waters from the escarpm ent of the Ethiopian plateau is felt to explain the m ost depleted heavy isotope contents.

The variability o f the stable isotope content of groundw ater confirm s the interpretation draw n from 3H m easurem ents on a lim ited storage.

2.4. “Age” of groundw ater

The initial 14C activities o f total dissolved carbon (T D C ) were determ ined using 13C content of T D C , p H and field alkalinity values according to the closed system treatm ent proposed by Fontes and G am ier [12, 13], and the respective approaches of Tam ers [14], Pearson [15], and M o ok [16]. For calculations, one has to assum e param etric values for the 13C and l4C content o f soil C 0 2 and solid carbonate. To leave the m inim um of freedom to the m odels, the sam e values were adopted for the stable isotope content of soil C 0 2 (— 21 %o) and o f solid carbonate 252

TABLE VII. CARBON ISOTOPES GEOCHEMISTRY SAMPLES FROM VARIOUS AQUIFERS FROM THE INLAND

No. a X 103 a/4. c X 103 mC A A o (M) /1 (F and G) A o (T ) Rem arks M meas. 0 * 0( r )

Î5 / 1 O.II 52 ■ 0 .0461 0 .0 0 6 2.50 I.I 9 27.6 4 .9 ЗО.9 4 0 .0 5 2.4 A 0( g ) = 100# 75/2 0 .3 4 8 3 О.О742 0.006 4 .6 9 2.17 52.2 30.0 4 9 .7 51.4 53.7 A0(g ) = 100$ <

75/17 О.ОО85 O.OII5 0.010 0 .7 3 8 0.36 80.0 9 3 .6 91.9 81.2 50.5 О = 100$ O.OO O 0.0 4 2 З I 38.4 4 7 .2 = 100, A (1)=50 75/34 0.0290 9 .27 .60 4 9 .7 56.2 7 5 .5 Ao ( s ) 75/37 0 .4 3 7 9 0 .0 7 4 8 0.008 5.86 2.705 9O.9 88.1 86.0 7 9 .5 7O.O A 0(g ) = 130$ 75/39 0 .0 2 0 3 0 .0 0 4 8 0 .1 0 3 4 .2 5 2.O65 6 8 .3 57.7 63.4 ' 6 4 .8 6 6 .8 A0 (g ) = 130$ 75/40 0.0317 0 .0 1 4 5 0 .0 1 8 2 .1 8 I.O 65 30.3 15.2 32.3 39.7 51.1 A 0 ( g ) = 100$ FONTESetaJ. 76/2 0.2992 О.О598 O.OO9 5.OI 2.35 90.7 9 6 .4 98.1 95.7 9 2 .5 AQ(g ) = 130, A o ( l ) - 5 0 76/3 0.5040 O.IO 94 0 .0 0 4 4 .6 1 2.05 9 3 .9 10 6 .4 104.6 100.3 9 4 .4 A0 ( g ) = 130$,Ao(l)=50$ 76/4 О.2352 О.О424 0 .0143 5 .55 2.65 8 0 .4 71.6 75.4 72.4 6 7 .9 A 0 (g ) = 130$ 76/5 0 .0081 0 .0114 0 .0 1 0 0 .7 2 0 0.35 92.O 9 4 .3 91.9 81.2 51.4 A0 (g ) = 100$ 76/6 O.65I 8 O.IO 59 0 .0 0 6 6.16 2.75 87.7 90.5 95.5 9 5 .0 9 4 .3 A ( g ) = 130,Ao(l)=50$ i <

76/7 0 .0 4 7 1 0 .0 8 2 9 0.0 0 1 О.568 0.26 Í 37 33.8 37.0 4 0 .6 54.2 0 = 100$ 76/14 0 .0 3 5 8 О.О928 0 .0 0 1 0.386 0 .1 7 5 101 7 8 .0 7 9 .5 7 9 .0 7 7 .3 A0 ( g ) = 100,Ao(l)=50$ 76/ЗО 0 .6 3 6 7 О.О 598 0 .1 8 2 IO .65 5.000 98.2 1 0 3 .0 84.8 7 1 .4 6 9 .0 A 0 ( g ) = 100$ 76/31 O.1986 0 .0 3 0 0 0 .0 2 2 6 .6 2 3.20 76.I 8 5 .0 8 0 .4 74.2 6 7 .2 A0 (g ) = 130$ 76/32 0 .4 0 3 2 0 .1443 0 .0 0 2 2 .7 9 I.I 95 7 8 .4 4 2 .7 6 8 .8 7 9 .8 9 5 .7 A0 (g ) = 130$,Ao(l)-50$ 76/ЗЗ 0 .3 5 7 0 О.О528 0 .0 1 4 6 .76 3.195 4 3 .1 — 38.3 5 6 .8 8 4 .4 A0 (g ) = 160$ 76/35 0 .0 3 8 4 O.OI54 0 ,0 1 8 2.50 1.22 30.6 2 8 .8 57.9 7 1 .4 9I .0 A 0 ( g ) = 130$,Ao(l)=50$

Symbols: a, c, 2: Calculated activities of aqueous CO^ + Hj COj , carbonate ion and total dissolved carbon, respectively, according to Mook [16]. mCT, mCM: Calculated molalities (millimoles) of total dissolved carbon and of carbon of inorganic origin according to Fontes and Gamier [12, 13]. A meas.- Measured 14C content o f total dissolved carbon. IAEA-AG-158/17 253

(0%o). It was checked (Fontes, unpublished data) that the 13C content of lacustrine carbonate of this area does not differ significantly from that of marine . carbonates because of a nearby complete equilibrium with atmospheric C 02. According to the respective locations, two hypotheses were adopted, taking into account the fact that solid carbonate can be either “dead” with respect to 14C decay (case of old marine carbonate) or “active” (case of carbonates of Holocene lacustrine origin). In the latter case, a 14C content of 50 pmc was adopted according to numerous data obtained on these deposits [1,3]. The 14C content of soil C 02 was fixed at 100% when the tritium content of the water was lower than 4 TU and was thus regarded as “pre-bomb” carbon. When tritium is present, even in amounts as low as 7 TU, a 14C content of 130% was adopted for soil C 02, i.e. all the dissolved carbon of organic origin was attributed to what could be expected for air C 02 activity at these latitudes during the ’seventies. An exception was made for sample 76/33 (Yoboki in the Henlé basin) whose 3H content (43 TU) is clearly higher than in any other location and corresponds to the period 1965—1970 on the basis of the available tritium record in rains (see Section 2.2). A value of 160 for the 14C content of soil C 0 2 was thus adopted by comparison with the values published by Nydal et al. [17] for the temperate zone, assuming a slight correction because of the latitude. From this review of the parametric values, it appears obvious that the application of the various models could be easily refined in each single case, but our purpose was mainly to discuss the general value of the 14C age estimations in a given area with expected and measured values rather than to observe the parameters “reasonably”. Taking into consideration the statistical uncertainty on the 14C measurements, it appears that the measured 14C content of total dissolved carbon of inland groundwater fits generally within about ±6 pmc with calculated initial activities calculated according to the treatment by Fontes et Garnier (see Table VII). The first conclusion of this global treatment is that no contradiction appears between 3H and model 14C contents. This means that one deals with two basic groups of recent groundwater: a group which accepts a 14C content close to 130% in the atmosphere, i.e. which is aged less than 5 or 6 years; and a group which corresponds to a 14C content of 100% within the atmosphere, i.e. which infiltrated before nuclear tests (1952). The first group corresponds to samples 75/37 (Obock Soublali, coastal aquifer); 75/39 (Dorra, piedmont plain); 76/2 (Tadjourah Magale, coastal aquifer); 76/3 (Tadjourah PK 9, coastal aquifer); 76/4 (PK 50, piedmont plain), 76/6 (Dikkil, closed basin of Henlé); 76/31 (Gourabous, piedmont plain); 76/32 (Oulma Obock, coastal aquifer). These groundwaters correspond to systems in which fracture porosity plays a greater role than matrix porosity and where the storage capacity is low. Groundwaters are thus rapidly renewed by drainage, withdrawal from wells, and boreholes and évapotranspiration. TABLE VIII. CARBON ISOTOPE GEOCHEMISTRY Aquifer o f Wadi Ambouli (Confined aquifer)

Parameters

613C (soil COj) -2\%o (thom-tree steppe) 613C (carbonate) 0 %0 (marine carbonate) A14C (soil C 02) 130 %o (post 1970 recharge) A14C (carbonate) 0 %0 (preholocene marine carbonate)

mGp No. a X 103 А/ mCM A (m eas.) A0 (M) Ao (F and G) A0 (P) A o (T )

E l 0 .0 5 2 0 .0 1 5 4 4 .20 2.05 52.3 7 8 .6 78.3 73.5 66.5

E4 0 .0 2 7 0 .0 0 7 7 4 .3 0 2 . 1 0 79.3 6 6 . 6 70.7 6 9 .0 66.5

E8 0 .1 9 6 0 .0 5 8 9 4.15 1.95 7 3 .4 75.5 79.3 74.5 6 7.3

ЕЮ 0 .0 0 8 0 .0 0 7 7 4.07 2 . 0 0 7 5 .6 1 0 0 . 0 90.5 8 0 .7 ' 6 6 .1

E 13 0.3 5 1 0 .1 0 2 3 4 .29 1.93 8 6 . 6 7 7.3 80.7 77.1 7 1.7

E 17 0 .6 5 7 0 .1 9 2 4 4 .27 1.73 7 9 .6 4 7 .8 6 6.4 7 0 .9 77.5

RG 6 0 .0 5 0 0 .0 1 5 4 4 .09 2 . 0 0 77.1 8 6 .4 81.1 7 6 .3 69.1

Symbols: a: calculated activity of aqueous C02 plus HaC 0 3 A/: calculated activity ratio of aqueous C02 plusH2C03 versus total dissolved carbon: Mook [16] mCT: calculated molality (millimoles) of total dissolved carbon Fontes and Garnier [12, 13] mCM: calculated molality (millimoles) of dissolved carbon from inorganic origin Fontes and Gamier [12, 13] A (meas.): measured 14C activity (pmc) of total dissolved carbon Aq : calculated 14C initial activity (pmc) according to the models of Mook [16]: A0 (M); Fontes and Garnier [12, 13]: Ao (F and G); Pearson [15]: Ao (P); Tamers [14]: A0 (T) Ao : calculated initial 14C content (pmc) of the total dissolved carbon according to Mook [16]: Aq (M); Fontes and Garnier [12, 13]: Ao (F and G); Pearson [15]: Ao (P); and Tamers [14]: Aq (T ). Parameters:5 !3C (soil COj) = — 21%o A14C (soil CO j) = 100% when 3H activity < 4 TU, 130% for tritium-bearing waters except for sample No. 76/33 (Yoboki) where a value of 160% was adopted because the high ^ content (43 TU) implies a recharge during the 1965—1970 period. 0 13C (soil carbonate) = 0 %o for both basins containing marine carbonates and lacustrine Holocene carbonates ( l3C content controlled by a close to complete exchange with the atmosphere in both cases). A14C (soil carbonate) = 0 %o for basins containing marine carbonates and 50% for basins containing Holocene carbonates. IAEA-AG-158/17 255

The second group corresponds to samples 75/1 (Doubdoubbolele, Grand Bara basin); 75/2 (Mouloud, Grand Bara basin); 75/17 and 76/5 (Abhé thermal spring, Abhé Lake basin); 75/34 (Agna, Henlé basin); 75/40 (Andaba, piedmont plain); 76/7 (Korili thermal spring, Asal Lake basin); 76/14 (Hallol, Asal Lake basin); 76/30 (Daddato, piedmont plain); 76/35 (Koutabouya, Gobaad basin). These systems are those where groundwater circulations take place through porous media. Most of these waters are modern, i.e. no age affect can be identified and the recharge is no older than some tens of years, or at most one or two centuries. However, the Abhè spring (75/17 and 76/15), which is probably supplied on the edges of the catchment area, shows in 1975 and 1976 a similar difference in 14C content with the initial modelled activity (Ameas = 80% A0 = 92%). Interpreted in terms of age, this difference is equivalent to about 1200 years. Between these two groups the samples from the Yoboki borehole (76/33) would be the only ones aged around 10 years. The sample from Koutabouya is difficult to interpret because the alkalinity was not measured on an aliquot of the sample used for the isotope study. Despite its homogeneous stable isotope content, its restricted extension and its confinement under an argillaceous level (Fig.4), the Wadi Ambouli aquifer does not, in any of the models, correspond to a single set of parameters (see Table VIII) for the evaluation of the initial 14C activity of the total dissolved carbon. This observation, which is in agreement with the variations observed in pH and tritium contents, is interpreted in terms of heterogeneity of the recharge and of differences in the processes of carbon mineralization according to the various recharge areas in the Wadi bank. It is even possible that in some places magnetic or volcanic C 02, coming through the fractured basalt, contributes to the total dissolved carbon. It must be pointed out that such a supply of “dead” deep C02 (at 40°C, a 5 13C of about —7%o) could only be identified on the basis of a decrease in 14C activity since its contribution to the 13C content would be similar to that of the marine carbonate. The supply of deep C 02 would lead to a 5 13C value close to —1 for the dissolved carbon, if the pH tends to be increased towards values greater than 8 under the influence of alkaline and earth-alkaline, as it should in the case of lava; when isolated from the C 02 source by ground­ water flow, this dissolved carbon would give a contribution to the total dissolved carbon which would be indistinguishable from that of leaching of solid carbonate from marine origin. Such an effect can be significant in groundwater systems from rift structures and volcanic areas like the southern Afar rift. In conclusion, regarding 14C age estimation, it appears that most of the hypothermal waters are very recent and can be attributed either to post-1970 or to pre-1950 recharge. Although the dissolved carbon content is very low in hot waters, and thus the measurement is rather difficult (with large uncertainties — see Tables II and V), it seems that these waters are no more than some centuries old (Abhè spring, for which one calculates an age of 1150 years with Ameas = 80% and A0 s 92%), or could even be recent (Sakallol). 2 5 6 FONTES et al.

As inferred from 3H measurements, it is likely that hot water circulations through fractured lava are rather rapid as compared with circulations in porous media.

3. PALAEOHYDROLOGY

3.1. The concretions of Lake Abhè

On the southern shore of Lake Abhè numerous accumulations of porous carbonate (stoichiometric calcite) deposits can be observed. These accumulations, clearly linked to major fracture directions, can reach some tens of metres in elevation. As indicated by the occurrence of boiling springs (99°C at 450 m) in their vicinity and sometimes by vapour emanations at their surface, these masses are correlated to thermal fluid circulations drained by faults. However, no construction of that kind can be observed from the area of maximal extension of the palaeolake indicated by its upper shore line. Thus, it appears that these hydro- thermal concretions were also strictly correlated to the aquatic environment of the lake. Nowadays, brines from Lake Abhè correspond to the concentration by evaporation of waters from the River Awash. These sodium-rich waters give rise to high pH values and to an active reaction with atmospheric carbon dioxide. The result is a sodium-carbonate type water. On the other hand, the boiling springs which are observed at the basis of some major hydrothermal concretions are sodium-chloride, calcium-sulphate in type. Using the WATEQ programme [18], it can be shown that when river and spring waters are mixed they are supersaturated with respect to calcite even for small amounts (5%) of any of the two components [19]. Thus, the hypothesis is made that the formation of the hydrothermal concretions was due to the precipitation of calcite when sublacustrine hot springs mixed with lake water.

3.2. Principles of the use of carbonate concretions for palaeohydrological reconstruction

If the carbonate ion which gives a solid carbonate is in open system equilibrium with the atmosphere it means that: (i) the temperature of crystallization can be calculated, assuming a value for the gaseous C 02 and knowing the thermo­ dependence of the enrichment factor e = [(RcaC0 3/^C0 2^ 4 * 1 0 0 0 , where R = 13C/12C; and (ii) the 14C ages measured for the solid carbonate approximate sideral ages. If, furthermore, the carbonate précipitants are at equilibrium with the environ­ mental water, it means that the knowledge (or any assumption) of the temperature of crystallization will allow the isotopic composition of the mother water of the IAEA-AG-158/17 2 5 7

FIG.5. Schematic representation o f the formation o f hydrothermal concretions from the Abhè Lake area. The mixing o f calcium-sulphate and sodium-carbonate type waters is saturated with respect to calcite. If the total dissolved carbon o f lake water is in equilibrium with atmospheric C02, one can calculate the temperature o f crystallization using the fractionation between COi gas and calcite. The isotopic composition o f the mother water will then be determined using the paleotemperature equation. Furthermore, the calcite o f the concretion will represent a suitable material fo r 14C dating.

precipitation to be calculated. The origin of palaeowaters at a given time indicated by 14C measurement can thus be discussed. Basic requirements for this isotopic treatment can be investigated on the basis of present-day isotopic composition of Lake Abhè total dissolved carbon (sample 75/4 on Table IV). The 5 13C of the total dissolved carbon (mainly CO32 ions at pH 9.8) is +0.63 % o vs PDB at 30°C and is thus very close to equilibrium with atmospheric C 02. One concludes that calcite precipitation within the lake should at present occur in equilibrium with the atmospheric reservoir. It is assumed that such was also the case during the Holocene. From an aerial reconnaissance survey of concretions at present forming in lake waters (but which could not be sampled), it was checked that they reach the lake surface. Thus, precipitation occurs near the surface where thermal water comes up through previously precipitated carbonate and mixes with lake water. The precipitation mechanism of these carbonate concretions is shown on Fig.5.

3.3. Data and interpretation

Carbonate samples were taken on the highest concretion and measured for 13C, lsO, and 14C contents (Table IX). For calculation of temperature and the stable isotope content of mother water, the following equations were used: t°C = 147.7-14.8 (513Ccarb - 0 13CCo2gas) + 0.266 (S13Ccarb.^S13Cbo2gas)2 according to Fontes et al. [20] from fractionation factors calculated by Bottinga [21 ]. 5 FONTES al. et 258

TABLE IX. PALAEOHYDROLOGY: ISOTOPIC COMPOSITION,14C AGE OF THE CALCITE FROM HYDROTHERMAL CONCRETION OF ASBAHALTO Calculated temperature and isotopic composition of the water of palaeolake Abhè

Calculated Water Sample ô13c »ieo Calculated Water 1Я 14C Ages BP Location vs FDB vs PDB Temperature Range S O Range vs SMOW Uncorrected Remarks

314 m (top) +2.30 - 1 0 .5 8 19 to 39°C - 9 .9 to - 5 . 8°/oo — 1

314 m (top) +1.85 - 1 1 .6 0 23 to 44°C - 1 0 .0 to - 6 . 0°/oo 1570 + 60 2

295 m +1 . 2 8 - 7.40 28 to 50°C - 4.7 to -0.7% ° — 1

295 m +1.15 - 7.80 30 to 51°C - 4.8 to -0.9°/oo 2360 + 80 2

285 m +1.79 - 7 .8 6 24 to 44°C - 6 .2 to - 2 . 1 °/oo — 1

285 m +1.19 - 9.30 30 to 51°C - 6.4 to -2.4°/oo 2720 + 12 0 2

255 и (tase) +1.63 - 6 .1 0 25.1 to 48°C - 4 . 1 to - 0 . 1 °/oo — 1

255 и (Ъаэе) - 1 .8 8 -13.8 9 61 .4 "to 86°C - 5.3 to -1.7% ° 6290 + 12 0 2

The range of temperature values is calculated using the range —6.4 to — 8.5%o for ,3C content of atmospheric CO; . These values correspond respectively to the general steady-state 13C content of atmospheric CO: estimated by Craig and Keeling [23] and to a value corrected for the contribution of C 02 that originates from fossil fuel use during the last century. The right value is thus certainly close to -6.4%> for the time interval, 6300 to 1600 BP. 1 = Average of 2 measurements from the same sample 2 = Measurements from Fontes and Pouchan [19] IAEA-AG-158/17 259 t°C= 16.9-4.2 (Sl8Ocarb - 5 18Owater) + 0.3 (5 18Ocarb - 5 l8Owater)2 according to Craig [22]. Further available data since previous measurements [19] are reported. All calculated temperatures are obtained from the 13C palaeotemperature equation. The values used for the 13C content of equilibrating atmospheric C 02 are —6.4%c and — 8.5%o, respectively. The former corresponds to the 13C content of the atmospheric carbon-dioxide free of any contamination with C 0 2 produced by fossil fuel combustion. Hence, this value is likely to apply to the time interval of crystallization (6300 to 1600 BP). The value —8.5% o is taken as a reference for an average 13C content of present-day carbon dioxide from the atmosphere; the corresponding calculated temperatures are thus minimum values. Results suggests that waters were rather warm and even hot in the vicinity of the thermal springs. On top of the concretion, where the lake was more than 60 m higher than its present-day level, a temperature effect is still noticeable in the precipitating carbonate. This would indicate that thermal springs discharge was very important at lake maximum. At the base of the concretion, two facies of carbonate can be distinguished. One is very hot and suggests that thermal water mixed with lake water at, or near, boiling point. The calculated 180 content of mother water of carbonates show rather large variations. From base to an elevation of 40 m on the body of the concretion, the 18 О content of the solution is high and corresponds to an active evaporation (—2 < S180 < O). However, the enrichment is far from that of the present-day lake (5180 = +3.73, sample 75/4 in Table IV). This suggests that the residence time of water within the lake was lower or that the relative humidity was higher than at present. On top of the concretion the calculated stable isotope content is low (518 О = - 6%o). The interpretation is that, during high-level stage, the water was actively renewed and leakage occurred from the lake towards the Henlé basin. This isotopic composition corresponds to a supply of water precipitated under rather cool conditions. Thus, the recent (historical) high (+314 m) level of Lake Abhè was probably correlated to the southern migration of polar-front rains rather than to exceptional events of monsoon rains which would have had a high 180 content owing to the vicinity of the sea. At the same time, the thermal spring should have had a higher discharge and a stable isotope content lower than at present (some litres per second, 6 180 = 2 .8 %o).

3.4. Conclusions

From a review of this set of isotope data, one can draw hydrogeological and methodological conclusions. General information is obtained on regional ground­ water systems characterized by severe aridity and only sporadic heavy rain 2 6 0 FONTES et al. episodes, high hydraulic gradients, internal drainage conditions, and circulations in fractured and porous media. (i) Recent recharge is significant through fractured rocks, whereas evaporation counteracts vertical infiltration in the porous media in the centre of the closed basins; (ii) Infiltration is due to major episodes of rains and floods which, without evaporation, give rise to direct seepage through the cracks and probably through the upper bed of the flood channels which are not filled by thin sediments; (iii) The sporadic recharge conditions are generally preserved within the aquifers where low mixing actually occurs; (iv) Recent recharge of hypothermal and mesothermal waters, including the confined aquifer of Wadi Ambouli, is confirmed by radiocarbon analyses which show also that storage is generally low because of the low porosity of the fractured lava; and (v) Some water from geothermal circulations can be some tens of years old, and one of them was about 12 centuries old.

Methodological considerations are:

(i) The consistency of the results given by an estimation of 14C initial content indicates that unconfined aquifers can behave as closed systems with respect to carbon chemistry; (ii) The comparison between 14C and 3H data shows that waters with a 3H content as low as 5 or 6 TU can be entirely recent in inter-tropical maritime regions of monsoon climate where condensing vapour has extensively exchanged with ocean masses and lost much of its tritium. Since the radiocarbon excess of the atmosphere is frequently submitted to this kind of decrease by exchange with the oceanic reservoir, it appears that, in these areas as well as in the southern hemisphere where 3H fallout is low, 14C analyses could represent a more suitable tool than 3H for evaluating recent recharge provided that 13C, hydrochemistry, and field pH and alkalinity are available; and (iii) An attempt at interpreting the stable isotope contents of inorganic carbonate in terms of palaeo-environmental indicators shows that such an approach is promising provided it is possible to determine that the carbonates were precipitated in equilibrium with the atmosphere. IAEA-AG-158/17 261

REFERENCES

FONTES, J.Ch., MOUSSIE, C., POUCHAN, P., WEIDMANN, M., Phases humides au Pléistocène Supérieur et à l’Holocène dans le sud de l’afar (T.F.A.I.), C.R. Hebd. Séances Acad. Sei. 277, Ser. D (1973). GASSE, F., “L’évolution des lacs de l’afar central (Ethiopie et T.F.A.I.) du Plio — Pléistocène à l’actuel”, Reconstitution des paléomilieux lacustres à partir de l’étude des Diatomées, Thèse Doctorat, Paris (1975) 406. FONTES, J.Ch., FLORKOWSKI, T., POUCHAN, P., ZUPPI, G.M., “Preliminary isotopic study of Lake Asal system (Republic of Djibouti)”, Isotopes in Lake Studies (Proc. Advisory Group Meeting, Vienna 1977), IAEA, Vienna (1979) 163. GONFIANTINI, R., BORSI, S., FERRARA, G., PANICHI, C., Isotopic composition of waters from the Danakil Depression (Ethiopia), Earth Planet. Sei. Lett. 18 (1973) 13. SCHOELL, M., FABER, E., Survey on the isotopic composition of waters from northeast Africa, Geol. Jahrb. D17 (1976) 197. INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No.l : World Survey of Isotope Concentrations in Precipitation 1953—1963, Tech. Rep. Ser. No. 96, IAEA, Vienna (1969). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 2: World Survey of Isotope Concentrations in Precipitation 1964—1965, Tech. Rep. Ser. No. 117, IAEA, Vienna (1970). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 3: World Survey of Isotope Concentrations in Precipitation 1966—1967, Tech. Rep. Ser. No. 129, IAEA, Vienna (1971). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 4: World Survey of Isotope Concentrations in Precipitation 1968—1969, Tech. Rep. Ser. No. 147, IAEA, Vienna (1973). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 5: World Survey of Isotope Concentrations in Precipitation 1970-1971, Tech. Rep. Ser. No. 165, IAEA, Vienna (1975). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Isotope Data No. 6: World Survey of Isotope Concentrations in Precipitation 1972—1975, Tech. Rep. Ser. No. 192, IAEA, Vienna (1979). FONTES, J.Ch., GARNIER, J.M., “Determination of the initial 14C activity of the total dissolved carbon: Age estimation of waters in confined aquifers”, Proc. 2nd Int. Symp. on Water-Rock Interaction, (H. PAQUET, Y. TARDY, Eds) 1 (1977) 363. FONTES, J.Ch., GARNIER, J.M., “Determination of the initial 14C activity of the total dissolved carbon: A review of the existing models and a new approach, Water Resour. Res., in press. TAMERS, M.A., “Surface water infiltration and groundwater movement in arid zones of Venezuela”, Isotopes in Hydrology (Proc. Symp. Vienna 1967), IAEA, Vienna (1967) 339. PEARSON, “Use of C-13/C-12 ratios to correct radiocarbon ages of material initially diluted by limestone”, Proc. 6th Int. Conf. on Radiocarbon and Tritium Dating, Pulmann (1965) 357. MOOK, W.G., “The dissolution-exchange model for dating groundwater with 14C”, Interpretation of Environmental Isotope and Hydrochemical Data in Groundwater Hydrology, (Proc. Advisory Group Meeting, Vienna 1975), IAEA, Vienna (1976) 213. 2 6 2 FONTES et al.

[17] NYDAL, R., LOVSETH, K., SYRSTAD, D., Bomb C-14 in the human population, Nature 232 (1971) 418. [18] TRUESDELL, A.H., JONES, B.F., WATEQ, a computer program for calculating chemical equilibria of natural waters, J. Res. US Geol. Surv. 2 (1974) 233. [19] FONTES, J.Ch., POUCHAN, P., “Les cheminées du lac Abhè (T.F.A.I.): stations hydroclimatiques de l’Holocène”, C.R. Hebd. Séances Acad. Sei. 280 Ser. D (1975) 383. [20] FONTES, J.Ch., LEPVRIER, Cl., MELIERES, F., PIERRE, C., “Isotopes stables dans les carbonates évaporitiques du Miocène Supérieur, de Méditerranée occidentale”, Proc. Messinian Events in the Mediterranean 1973, Konkndljke Nederlands Adad. Van Wetenschappen, Amsterdam (1978) 91. [21] BOTTINGA, Y., Calculation of fractionation factors for carbon and oxygen isotopic exchange in the system calcite-carbon dioxide-water, J. Phys. Chem. 72 (1968) 800. [22] CRAIG, H., “The measurement of oxygen isotope paleotemperatures”, Stable Isotopes in Oceanographic Studies and Paleotemperatures (TONGIOGI, E., Ed.), Pisa, Consiglio Nazionale delle Ricerche, Laboratorio di Geologia Nucleare (1965) 161. [23] CRAIG, A., KEELING, C.D., The efforts of atmospheric N20 on the measured composition of atmospheric C 02, Geoch. Cosm. Acta 27 (1963) 549, LIST OF PARTICIPANTS

AUSTRALIA

Airey, P.L. Australian Atomic Energy Commission, PO Box 41, Coogee 2034, New South Wales

AUSTRIA

Zötl, J.G. Technische Universität Graz, Abteilung für Hydrogeologie, Rechbauerstrasse 12, 8010 Graz

CANADA

Fritz, P. University of Waterloo, Department of Earth Sciences, Waterloo, Ontario N2L 3G1

FRANCE

Fontes, J.Ch. Laboratoire d’hydrologie et de géochimie isotopique, Université de Paris-Sud, Bât. 504, F-91405 Orsay Cedex

Guizerix, J. > Département de chimie appliquée du Commissariat à l’énergie atomique, B.P. 85, Centre de Tri, F-38041 Grenoble Cedex

Olive, Ph. Université Pierre et Marie Curie, Centre de recherches géodynamiques, Avenue de Corzent, F-74203 Thonon-les-Bains

GERMANY, FEDERAL REPUBLIC OF

Geyh, M.A. Niedersächsisches Landesamt für Bodenforschung, Postfach 510153, D-3000 Hanover 51

Moser, H. Gesellschaft für Strahlen- und Umweltforschung mbH, Institut für Radiohydrometrie, Ingolstädter Landstrasse 1, D-8042 Neuherberg

Münnich, K.O. Institut für Umweltphysik der Universität Heidelberg, Im Neuenheimer Feld 366, D-6900 Heidelberg

263 2 6 4

INDIA

R ao, S.M. Bhabha Atomic Research Centre, Isotope Division, Bombay 400 085

IRAN

Movahed, M.A. Iranian Ministry of Energy, Department of Pollution and Quality Control of Water and Radioisotopes, Teheran

Shahlayi, A.K. Pahlavi University, Irrigation Department of the Agriculture College, Shiraz

ISRAEL

Gat, J.R . The Weizmann Institute of Science, Isotope Department, PO Box 26, Rehovot

Issar, A. Ben Gurion University of the Negev, Institute for Desert Research, Kiriat Sdeh-Boqer

Levin, M. Ben Gurion University of the Negev, Institute for Desert Research, Kiriat Sdeh-Boqer

PAKISTAN

Sajjad, M.I. Pakistan Atomic Energy Commission, PO Box 114, Islamabad

TURKEY

Dinçer, T., Deli Huseyinpasa Cad. Ozyurt, Yapi Koop. Blok 2 N.14, Bahcelievlei, Istanbul

UNITED KINGDOM

Bath, A.H. Institute of Geological Sciences, Hydrogeology Unit, MacLean Building, Crowmarsh Gifford, Wallingford, Oxfordshire 0X 10 8BB 2 6 5

UNITED KINGDOM (cont.)

Edmunds, W.M. Institute of Geological Sciences, Hydrogeology Unit, MacLean Building, Crowmarsh Gifford, Wallingford, Oxfordshire 0 X 10 8BB

Lloyd, J.W. The University of Birmingham, Department of Geological Sciences, PO Box 363, Birmingham B15 2TT

UNITED STATES OF AMERICA

Pearson, F.J., Jr. US Geological Survey, National Center, MS 432, Reston, Virginia 22092

INTERNATIONAL ORGANIZATIONS

FOOD AND AGRICULTURE ORGANIZATION (FAO)

Underhill, H. Land and Water Development Division, Via delle Terme di Caracalla, 00100 Rome, Italy

INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA)

Kakihana, H. Deputy Director General, Department of Research and Isotopes

D ray,M . Section of Isotope Hydrology, Division of Research and Laboratories

Drost, W. Section of Isotope Hydrology, Division of Research and Laboratories

Florkowski, T. Section of Isotope Hydrology, Division of Research and Laboratories

Payne, B.R. Section of Isotope Hydrology, Division of Research and Laboratories

Quijano, L. Section of Isotope Hydrology, Division of Research and Laboratories

Zuppi, G.M. Section of Isotope Hydrology, Division of Research and Laboratories

HOWTO ORDER IAEA PUBLICATIONS

■ An exclusive sales agent for IAEA publications, to whom all orders and inquiries should be addressed, has been appointed in the following country:

UNITED STATES OF AMERICA UNIPUB, 345 Park Avenue South, New York, NY 10010

■ In the following countries IAEA publications may be purchased from the sales agents or booksellers listed or through your major local booksellers. Payment can be made in local currency or with UNESCO coupons.

ARGENTINA Comisión Nacional de Energía Atómica, Avenida del Libertador 8250, RA-1429 Buenos Aíres AUSTRALIA Hunter Publications, 58 A Gipps Street, Collingwood, Victoria 3066 BELGIUM Service Courrier UNESCO, 202, Avenue du Roi, B-1060 Brussels CZECHOSLOVAKIA S.N.T.L., Spálená 51, CS-113 02 Prague 1 Alfa, Publishers, Hurbanovo námestie 6, CS-893 31 Bratislava FRANCE Office International de Documentation et Librairie, 48, rue Gay-Lussac, F-75240 Paris Cedex 05 HUNGARY Kultura, Hungarian Foreign Trading Company P.O. Box 149, H-1389 Budapest 62 INDIA Oxford Book and Stationery Co., 17, Park Street, Calcutta-700 016 Oxford Book and Stationery Co., Scindia House, New Delhi-110 001 ISRAEL Heiliger and Co., Ltd., Scientific and Medical Books, 3, Nathan Strauss Street, Jerusalem 94227 ITALY Librería Scientifica, Dott. Lucio de Biasio "aeiou". Via Meravigli 16, 1-20123 Milan JAPAN Maruzen Company, Ltd., P.O. Box 5050, 100-31 Tokyo Internationa) NETHERLANDS Martinus Nijhoff B.V., Booksellers, Lange Voorhout 9-11, P.O. Box 269, N L-2501 The Hague PAKISTAN Mirza Book Agency, 65, Shahrah Quaid-e-Azam, P.O. Box 729, Lahore 3 POLAND Ars Polona-Ruch, Céntrala Handlu Zagranicznego, Krakowskie Przedmiescie 7, PL-00-068 Warsaw ROMANIA llexim, P.O. Box 136-137, Bucarest SOUTH AFRICA Van Schaik's Bookstore (Pty) Ltd., Libri Building, Church Street, P.O. Box 724, Pretoria 0001 SPAIN Diaz de Santos, Lagasca 95, Madrid-6 Diaz de Santos, Balmes 417, Barcelona-6 SWEDEN AB C.E. Fritzes Kungl. Hovbokhandel, Fredsgatan 2, P.O. Box 16356, S-103 27 Stockholm UNITED KINGDOM Her Majesty's Stationery Office, Agency Section PDIB, P.O.Box 569, London SE1 9NH (j U .S.S.R . Mezhdunarodnaya Kniga, Smolenskaya-Sennaya 32-34, Moscow G-200 YUGOSLAVIA Jugoslovenska Knjiga, Terazije 27, P.O. Box 36, YU-11001 Belgrade

Orders from countries where sales agents have not yet been appointed and requests for information should be addressed directly to: ^ Division of Publications $ International Atomic Energy Agency & Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria О) CNо О 00Ó

INTERNATIONAL SUBJECT GROUP: IV ATOMIC ENERGY AGENCY Chemistry, Geology and Raw Materials/Hydrology VIENNA, 1980 PRICE: Austrian Schillings 390,—