Seventh Conference of Nuclear Sciences & Applications 6-10 February 2000. , En-10 Geochemical Characteristics and Environmental Isotopes of Groundwater Resources in some Oases in the Western , Egypt EG0100098 M.S.Hamza, M.A.Awad, S.A.El-Gamal and M.A.Sadek Siting and Environmental Department, National Center for Nuclear Safety and Radiation Control, Atomic Energy Authority, 3 Ahmed El-Zomor St., Nasr City-11762, B.O.BOX 7551, Cairo-Egypt.

ABSTRACT A study has been conducted using hydrochemistry and environmental isotopes (deuterium, oxygen-18 & carbon-14) on the Nubian Sandstone aquifer which underlies the oases. The concerned three oases (El-Farafra, El-Dakhla and El-Kharga) covers an area about 8000 km2 from the total area of the Western Desert. Seventy one water samples were collected from these three oases and subjected to both chemical and isotopic analysis to evaluate their groundwater resources. The hydrochemical data of these water samples reveals that thier salinity doesn't exceed 500 mg/I as well as the presence of marine and meteoric water types with different percentage. The mineralization of the investigated groundwater may be evoluated under flusing the original marine water entrapped between the pores of the aquifer matrix by meteoric water which is furthely modified through leaching, dissolution, cation exchange and oxidation-reduction processes. The investigated groundwater indicates some sort of quality hazards for drinking and domestic purposes due to the high concentration of both iron (agverage 6 mg/1) and hydrogen sulphide (average 2.5 nig/I ) relative to WHO standard. This water can be used safety for all kinds of livestocks. About 95% of the these groundwater have low to medium salinity hazards and low sodium content and can be used for irrigation. The mean geothermal gradient with depth is measured in two wells in El-Kharga and found to be slightly higher than the natural one, it is equal to about 1.4° C/l 00 m. The base tempetature was calculated by two cation geothermometers (K-Mg) and (Na-K-Ca) that indicate a generally higher temperature due to continuous rock-water interaction and/or mixing with old connate water which having assembly acquiring higher cationic temperatures. The corrosivity which noticed in some wells may be related to combined effect of high temperature of water, high concentration.of II2S and CO2 and low pH. The environmental isotopes data of the investigated groundwater samples reveals that theniain recharge source for the Nubian Sandstone aquifer is in situ precipitation in much cooler and humid climate thousands of years ago, since the carbon-14 age for the shallow and deep zones of six wells collected from these oases is > 20,000 years.

Key Words : Environmental Isotopes/Geochemistry / The Western Desert Oases/ Nubian Sandstone Aquifer

INTRODUCTION

The River is considered the main source of fresh water that covers the water demands for Egypt's population. Due to rapidly growth of population groundwater became an important integral part of the national policy of Egypt.

981 The Western desert oases have been inhabited since pharonic time (2000 B.C.)- The development of Nubian Sandstone aquifer in these oases has made to cultivate intensive large low land areas depending on the natural flowing water of this aquifer. This flowing water has ceased in many wells in the area due to continuous exploitation over the past years and thus pumpage and water lifting started in some of these oases as a result of declines of the piozometric pressure with time. So, it is necessary to identify the replenishment, flow dynamics, recharge mechnisms and the mean residence time of the groundwater in this vital aquifer using hydrochemical and environmental isotopes (I8O, D,1^) techniques.

LOCATION

The investigated area extends for about 8000 Kin2 between latitudes 24° 15 and 27° 30 N and between longitudes 27° 31 and 30° 27 E. It comprises three of the main oases in the Western Desert ( El-Kharga, El-Dakhla and El-Farafra ),as shown in Fig. (1). The climate of the study area is generally described as torrid and practically rainless.

GEOLOGICAL AND HYDROLOGICAL ASPECTS

The surface exposures throughout the whole area of study are entirely composed of sedimentary rocks belonging to Upper Cretaceous to Quaternary. The Basement rocks which only crops out at the southern part of El- is unconformably overlained by the Jurassic to Upper Cretaceous Nubian Sandstone followed br Pre-Maestrichtian variegated shale, Maestrichtian Duwi Phosphate Formation, Maestrichtian to Danian Dakhla shale Formation, Landanion Tarawan chalky limestone Formation, Upper Paleocene-Lower Eocene shale, Lower Eocene Thebes Formation and the Pleistocene and Recent sediments. The major homoclinal structure in the study area is generally interrupt by a number of dominantly oriented NE-SW and NW-SE upfols and downfolds. A great number of E-W faults are recognized mainly in the southern portion together with N-S and NW-SE trends/0 The Nubian Sandstone aquifer in Egypt is a part of a regional hydrogeological system with extension into Libya, and Chad. It contains a vast amount of water for agricultural and domestic uses ( 3x 10M million cubic meters ). Nubian Sandstone is the main groundwater bearing horizon outside the Nile Valley and Delta, it underlies the whole Western Desert and extends to the Eastern Desert and Sinai Peninsula. It consist mainly of alterning beds of sandstone and clay, the clay beds are laterally discontinuous and separate sandstone into muti-Iayer aquifer system bounded by impervious basement rocks. The thickness of the sediments varies between a few hundred meters in the south to 4,000 meters west of Abu Mongar. In the southern portion, the Nubian Sandstone aquifer is unconfined and depth of groundwater varies from 25-40 meters. Further north where the aquifer is overlain by Tertian.- shales groundwater is naturally confined, although springs are present originating from fissures in the confioning layers. In the erosional depressions where the shales have been removed groundwater is discharged at the surface forming the inhabited oases *\ The piezometric pressure ranges from 350 m (a.s.l., above sea level) at the southern corner of Egypt ( Uweinat) to about 100 m (a.s.l.) at the Kharga and Dakhla oases with a general flow direction from S.W to N.E and N as shown in Fig.(l). The huge depressions in the Western Desert below sea level constitute the natural discharge areas of groundwatcr contained in the Nubian Sandstone aquifer, thus considerable amounts of water reserves normalh- under pressure is expected to be lost by evaporation* '.

METHODOLOGY AND TECHNIQUES

A total number of seventy one groundwater samples were collected from the different bearing horizons of the Nubian Sandstone aquifer system in the concerned oases (El-Dakhla, El-Kharga

982 B

2000- I00O cr, Cr, 00 -WOO Pcm -2000

PRE-EOCENE (Duwi-Fm-Dakhla Fm-Kurkur Fm-Trawan Fm)

JURASSIC-EARLY CRETCEOUS (Abu Simbil Fm-Abu Ballas Fm-El Borg Fm-Bahariya Fm)

PRE-JURASSIC (Gilf Fm-and undiffcrentiatcd Paleozoic rocks)

PRECAMBRIAN oo (Crystalline Basement Rocks)

PLIOCENE-QUATERNARY (Terraces-Unconsolidated-Surficial deposits)

PRE-PLIOCENE (Qattara Fm-Basalls-Moghra Fm-Marmarica Fm)

EOCENE (Garra Fm-Dungul Fm-Thebes Fm-Minya Fm- Fm)

Fig. (1) : Location map, sampling sites and generalized geological north-south cross section of the western desert and El-Farafra) in 1995. Sample identification, locality, pH, E.C., water's temp., H2S concentration were performed in situ. Chemical analysis were carried out according to standard methods <4\ The water samples were also analysed for oxygen-18 and deuterium using isotopic ratio mass spectrometer. The (3) ISQ/I&Q ratjQ wag measurecj after equlibrium between 5 ml of water sample and standard CO2 gas , while the 2HJ 'H ratio was determined after reduction reaction between 0.8 u ml from water sample and 25 mg Zn under vaccum at a temperature 450°C <6). The isotopic composition of a water samples is usually reporated in delta per mille (%o) deviation from the Vienna Standard Mean Ocean Water (V.SMOW) with uncertainty level of ± 0.1 %o for 5!SO and ± 1 %0 5 D. Benzene synthesis and liquid scintillation counting have been used for 14C measurements. The sampling was done in the field using about 100 to 120 liter containers to precipitate BaCO3 for laboratory analysis based on a modified IAEA technique<7).

RESULTS AND DISCUSSION

A- General chemical characteristics

The results of the chemical analysis could be outlined as follows :-

A. 1 - Salinity, major ion concentrations and water type

The groundwater salinity and major ions concentration in the investigated oases vary within a narrow rang indicatig a high homogeneity in chemical composition due to the dominance of clastic facies. The values of total dissolved salts (TDS) does not exceed 500 mg/1 and increase generally with the decrease in depth of well due to the following factors : 1- The continuous rock-water interaction during water flow upward as a result of the high piezometric pressure and due to the long period of exploitation of shallow zone of the aquifer, which characterized by shally materials. 2- The mixing with irrigation water return and/or evaporation. The correlation coefficients between TDS and each of the major ions, Table (1), show that the most significant ions which control the TDS are Cl and Na followed by Ca and Mg. The low correlation coefficient between TDS and K could be attributed to the adsorpance of K on the shale content dominating in the highly saline water in shallow7 zone of the aquifer. The reduction of SO4 is indicated by the high H:S content, may explain the low correction coefficient between SO4 and TDS. The low correlation between TDS and HCO3 could be attributed to the association of freshness process by bicarbonate salts .

Table ( 1 ): Correlation coefficients between TDS and major ions

Variable K Na Mg Ca Cl SO4 HCO3

Correlation 0.44 0.81 0.51 0.55 0.92 0.45 0.34 Coefficients

The water types distribution. Table (2), shows that both the marine water types ( Mg-Cl & Ca-Cl) and meteoric one ( Na-HCOj & Mg-HCO.?) are encountered with different degree in the three oases. This could be attributed to the different portion of meteoric water invasion to the original marine solution entraped between rock pores under different flow rates and infiltration velocties due to facies changes laterally.

984 Table ( 2 : Water types distribution in the investigated oases

Water Na-Cl Mg-CI Ca-Cl Mg-SO4 Na-HCO3 Mg-HCO3 Ca-HCO3 type

Oasis

El-Kharga 65% 8% 4% 15% 4% 4%

El-Dakhla 42% 10% 24% 10% 10% 4%

El-Farafra 16% 23% 38% 8% 15%

A.2 - Graphical Representation

The different sequances of ion dominance are illustrated in the grid graph, Fig.(2). This graph reflect that all the possible cationic combinations are recorded in the investigated groundwater with only three anionic groups, this may reveal that mineralization is affected by the processes involving cationic changes as leaching, adsorption and cation exchange.

Sulin graph of genetic chemical classification was used to represent the chemical composition of the examined groundwater samples in the study areas as shown in Fig.(3). This diagram reflect the presence of marine character in form of Mg Cl2 genetic salt in the direction from El- Kharga (20%) to El-Dakhla (35%) to El-Farafra (50%) as the basin of deposition becomes thicker and less permeable strata are prevails. The meteoric character is indicated by appearance of either NaSO4 or NaHCOs salts in the rest of water samples.

A.3 - Groundwater Temperature and Geothermal Gradient

Potentially the interior of the earth is an almost infinite reservoir of thermal energy which is generally burried too deep. Groundwater temperature increases with depth because of this hot interior of the earth. This increase is averaged normally to one degree per hundred meter depth [1°C/100 mj('. The depth-temperature relationships, Figs.(4,5) are constructed based on the available data derived from the geophysical temperature logging carried out by EGDDO on two wells, El-Khaga-1 obsevation and El-Kharga-1 production. The mean geothermal gradient is found to be slightly higher than that indicated by White*1" and equal to (1.4°C/100m ). It is consistent with the temperature measured for the groundwater samples collected from the three oases at 1995( \

The results of applying two different cationic geothermometers {[K-MgJ }and{[Na-K- Ca|(13)} on twenty two of the examined groundwater samples representig the important prcdommet water types. Table (3), give generally high tempreturc than that measured in situ. The K-Mg geothermometer is relatively more applicable but still giving higher temperatures than the actual measurements. This may be due to continuous rock-water interaction and/or mixing with old connate, water that has assembly acquiring higher cationic geothermometers.

985 Mg- Na 0 Ca o D

Na-Mg .

Mg a a a

Na- Ca D a a D Ca - Mg • a \° D O Mg -Ca

u i o • EL-Kharga o 10 I/) I • EL-DakhlG O CD ° EL-Farafra o X

l'ii>. (2) : Grid diagram Tor chemical representation of the investigated groundwater samples in the three oases

986 El-Kharga Oasis

Fig. (3) : Sulin's genciic chemical clnssillcalioii diagnun for the investigated groitndwnler Samples in (lie three OJISCS

987 ( El-Kharga Production well-1 )

350-

Grodient 2°C / 150 m 250 - (i.ej 13.3°C/Km a Q 150-

2 °C 50 -

25 26 28 29 30 31 32 33 Temperature °C

Fig. (4): Depth-Temprature relationship, ( El-Kharga Production well-1 )

500- ( El-Kharga Observation well-1 )

400- Gradient 2Cf 150 rn 300- C <-e) f3.3 / Km Q. 150cm Q 200-

J00-

I ) I 1 I T 1G 20 24 28 3 2 36 Temperature C Fig. (5) : Depth-Teinprature relationship, ( El-Kharga Observation well-1 )

988 Table ( 3 ) : Calculated cationic geothermometers Sample Calculated Temperatures (°C) No. Geothermometers K-Mg Na-K-Ca 1 87 142 3 85 133 5 85 131 10 94 142 12 74 114 15 82 147 17 42 70 23 43 103 27 54 112 29 81 121 33 66 36 35 87 106 36 54 72 38 74 89 41 85 116 57 56 74 58 60 75 59 84 103 63 58.5 127 69 58 63 70 90 153 71 70 105 A. 4 - Groundwater mhriralization

The evolution of groundwater mineralization in the concerned oases may be affected by the following geochemical processes:

A.4.1 - leaching and dissolution

Several ions are released in water through leaching of the adsorption surface of many sediments such as shales, claves and marls. The solubility processes are highly affected by pH, EH, temperature, gas pressure, ionic strength etc. The effect of leaching on the chemical composition of the investigated groundwater can be followed from the ratio of different ions ( K, Na, Mg, Ca and SO4 ) to chloride which has a conservative nature. The average ratios in epm for 71 groundwater samples are listed in Table (4);

Table ( 4 ) : The ion ratios of the investigated groundvvater samples Ratio The average in the investigated samples Sea Water(I4) K/Cl 0.34 0.02 Na/Cl 0.84 0.86 M«/C1 0.68 0.19 Ca/Cl 0.56 0.04 SOVCl 0.50 0.10 Na/K 5.40 47.4

989 and attain higher values than sea water due to the relative increase of different ions under the action of leaching process. Also, the ratio Na/K aquircs generally low values due to the active leaching on the shal ley deposits rich in adsorbed potassium.

The dissolution of the different terrestrial salts participating in the chemical composition of the investigated groundvvater can be noticed from positive correlation coefficient between the following ion pair ; Na:Cl = + 0.85 & Mg:HCO3 = + 0.66 and Ca : HCO3 = + 0.54.

A. 4.2- Mixing

Mixing of two intermingling water bodies of different genetic origins gives rise to a transitional zone of mixed water. This zone is called zone of diffusion. Examining the main hypothetical salts encounted in the three oases as disscussed previously in Sulin graph could be reflect a mixing case between marine water and meteoric water with different ratios based on flow rates and diffusion intensities. The old marine salt CaCI2 is appeared in one sample. The existance of marine salt MgCb reflect a primary' stage of evolution in 30% of the collected groundwater samples with more prevalence in El-Farafra and El-Dakhla than in El-Kharga. The meteoric Na2SC>4 salt of continental origin is revealed in the rest of samples ( 70% ) with more domination in El-Kliarga oasis. The NaHC03 salt which corresponds to the final stage of meteoric invasion is identified only at the nothern part of EL- Kliarga oasis and very rarely in El-Farafra.

A.4.3- Base exchange

The base exchange causes a disequilibrium between Cl and (Na+K) when water circulates in sediment. A considerable difference between the concentration of Cl and Na+K indicates a base exchange. The ion exchange index calculated using the formula; {Cl-(Na+K) /Cl }; where the concentration of the ions are expressed in epm as shown in Table (5). This index acquries negative values for about 70% of the investigated samples and gave positive values for the rest of these samples. This means that the ion exchange process is mildly active in the area of study.

A.4.4- Oxidation-Reduction Reactions The oxidation-reduction process is expected to be playing a role in modifying the groundwater mineralization in the study area, where high concentration of H2S gas with its specific odour is detected in some wells with concentration ranges from 0.5 to 3.6 mg/I, indicating the sulphate reduction which occured under anaerobic conditions and high temperatures. The high iron content determined in the investigated groundwater samples ( average 6 ing/1 ) is provides an evidence of pyrite oxdidation. The reduced SO4 ma}- be partially substituted from rock-water interaction or from pyrite oxdiation. The SO4 reduction may also leads to HCO3 formation in the presence of organic media and could be precipitated as CO3 at high pH or changed to SO4 due to recation with sulphoric acid which result from pyrite oxdiation.

A.4.5- Corrosion Potential This paragraph deals with a serious socio-economic water quality problem in utilizing the Nubian Sandstone groundwater wells in the Western Desert oases. This problem is the high aggressivity and corrosivity of this water against metallic components of the pumping equipments ( pipes, casings, Screens and pumps) which leads to a decrease in the economical expolitation of these wells. A water saturated with CaCO3 tends to precipitate a scale which may protect the pipes of the groundwater wells from corresion. Langelier index [IL] can help to indicate the potentiality of the scale formation. The calculated (IL) index for the investigated groundwater arc listed in Table (5). This

990 Table (5): Base exchange and corrosivity indexes for the investigated groundwater samples

Ser. i ! Corrosivity indexes Ser. Corrosivity indexes I Well's location Base exchange Langgelier Ryzner index Weil's location Base exchange Langgelier Ryzner index No. ] index (IL) (TR) No. index (IL) (TR) ; El-Kharga Oasis I 36 Mut 3A 0.41 | -ve 9.18 1 i Mahariq 2 -0.10 -ve 3.48 37 Mut 23 0.09 I •ve 10.25 2 ! Sharka 3 -0.72 -ve 9.40 38 Mut 24 0.14 ! +VB 3.48 3 I Kharga 40 0.24 -ve 10.14 39 Mut 25 0.72 -ve 10.80 4 Kharga 23 1.22 -ve 10.75 40 E!-aasr El-Balad 0.51 .ve 9.74 5 ! Kharga Gomhoriya -0.3 0 -ve 3.71 41 Ei-aasr10 0.94 I •ve 12.06 s Ganah 0.4S -ve 9.24 42 El-Qaer 3 0.73 -ve 11.56 7 Nasser drinking 0.35 -ve 9.2S 43 E!-Gedida 12 0.32 •ve - 3 Nasser 1 0.70 •ve 10.3S 44 El-Qalamon 12 0.20 •ve 10.26 9 Sulak 5 0.S1 -ve 9.S4 45 :i-Qalamon 2 0.21 -ve 8.91 10 Bulak 5A 0.62 -ve 9.48 46 EI-Qasr3A 0.82 -ve 11.00 11 Bulak 17 0.53 + ve S.84 47 Masara 12 0.22 -ve 10.14 12 Gannachin 3 0.50 -ve 9.90 48 ^lasara 20 0.93 -ve 10.99 13 Garmachin 2S 1.27 -ve 3.88 49 Masara 3 0.99 -ve 11.65 14 Bans 37 0.42 -ve 10.10 50 Sheikh wall 0.83 -ve 9.23 15 Dosti Subs -0.14 -ve 10.30 51 Mushia 3 0.85 -ve 9.95 16 Bans (32) Baqhdad 0.34 -ve 9.70 52 Mushia 4 0.87 -ve 9.92 17 Bans (47) 0.02 -ve _ 53 Mushia El-Balad 0.42 -ve I 6.40 18 Bans El-Fakhura -0.05 -ve - 54 Asmant dinking 0.06 -ve 10.92 19 Bans (16) Subs 1.80 -ve - 55 Masara 12 0.76 «ve 9.35 20 Bans El-Salad 0.01 •ve _ 56 Qalamon 5 drinking 0.52 -ve 8.38 21 Bans E]-Ououn 0.07 .ve — 57 El-Qasr 5 0.35 -ve 11.05 22 Garmacrt/n -0.15 -ve - Farafra Oasis 23 El-Monira El-Balad -0.32 .ve _ 58 Farafra 1 0.72 -ve 10.20 24 Um Said 0.13 -ve _ 59 Farafra 6 1.01 -ve 9.58 25 Garmachin 13 -0.01 -ve - 60 Farafra Balad 1.04 -ve - 26 Bans 2 0.25 -ve 9.50 S1 Farafra 4A 0.S7 I .ve - -ve i El- ! 62 Farafra drinking -0.98 9.80 27 Tenada (8) 0.95 I -ve 10.12 63 Farafra 11 I 0.43 I -ve 9.30 28 Tenada (12) 0.48 -ve 10.28 64 Farafra 13 -1.00 I -ve 9.20 29 Tenada (3) 0.40 i -ve 10.46 55 Farafra 14 -0.8$ I .ve 9.58 30 Tenada drinking I 0.57 | -ve 9.42 SS Farafra 15 .0.34 I -ve - 31 Balat 40 I 0.49 ! -ve 10.56 67 Farafra 7 41.77 I -ve - 32 Balat 23 ! 0.43 I -ve 10.95 63 Farafra 8 0.38 I -ve - 33 !8alat 2A I 0.42 ! -ve | 9.90 S9 Ain Dalla 0.76 I -ve 7.90 34 Balat (4) I 0.72 i -ve 10.65 70 Farafra (4) Obs. 1.10 ! -ve - 35 ! Mut 3 I 0.93 ! -ve F 11.06 71 Farafra 3C | 0.36 ! -ve 9.30 The values of both IL and IR was derived from the chemical analysis data that mentioned in Sadek's Ph.D (Table 23. page 141-144). factor shows negative values for approximately all samples indicating high corrosivity. Another factor called Ryzner index [1RJ( was used to detect the corrosivity in the examined groundwater samples and also gives high values which means high corrosion. The high corrosivity which noticed in some groundwater wells is due to the combined effect of the following factors: • • *

1- The high tempreture ( average 39° C ) 2- The genrally low pH (i.e in the range of 6.38 - 7.5 for about 90% of the collected samples )

3- The dissolved CO: gas( average 12 mg/1) and the dissolved H2S gas ( average 2.5 mg/1) 4- The less potentially of CaCCb scale formation against corrosion (-ve longgclier index, and high values of Ryzner index )

EVALUATION OF THE INVESTIGATED GROUNDWATER QUALITY FOR DIFFERENT PURPOSES a. Drinking and Domastic uses Comparing the results of the chemical analysis with the critria published by WHO(I7), a doughtfi.il suitability could be admitted to the investigated groundwater for both drinking and domestic uses. The quality hazards arise principally from the high iron content (average 6 mg/1) and the high dissolved hydrogen sulphide (average 2.5 mg/1). The total hardness of the examined groundwater varies in the range from 41 to 248 and classified as soft to moderatly hard based on hardness classification'18' . b. Agricultral use

According to U.S. irrigation laboratory staff classification . the investigated groundwater are located on two classses CpSi and Q-Si reflecting low to medium salinity hazard and low sodium content, as shown in Fig.(6), this means good quality and suitability for irrigation uses.

ENVIRONMENTAL ISOTOPES

The conventional 5D Vs. 8ISO diagram is drawn to represent the isotopic content of the investigated samples. Fig.(7). This diagram is also include two reference lines {(Golobal meteoric water line(20). 5D=85lsO+10) & Palaeo water line'2", 5D=85l8O+5 } and two water points representing the isotopic composition of the Nile water at cairo (5lsO=+3.76 & 8D=+28.9) and the Mediterranean sea at (5IbO=+i.87 & 5D=+9.8) for comparison. The isotopic content of the investigated groiindwater is very depleted and scattered around the palaco water line. The isotopic content of these samples is comparable to the isotopic composition of the Central European winter precipitation which are recorded in the IAEA precipitation network'"*', and significally lower than the Nile water and local rain values. This give an indices that there is no active potential current recharge either from the Nile or from local precipitation.

The relationship between the total dissolved solids (TDS in mg/I) and oxygen-18 of the investigated groundwater samples, Fig.(8), reflect a modification in groiindwater salinity either by a capillary evaporation in the shallow zone or due to mixing with evaporated recycled irrigation water return. This behavior is more prominent in El-Kharga than El-Dakhla and EI-Farafra in accordance to the scale of exploitation and the amount of irrigation water return. The increase in the TDS relative to

992 6D°/oo 100 / 1000 2000 3000 5000

80

60

60 • //* Nile water 2 0 -//

1 . • 1 . , // ! , I . 1 i -12 -8 -L // U 8 12 18 '_ 5 o •/.. m O /:

VT5/-6O

-80 100 250 750 2250 5000 Conductivity Micromnos/cm(S;Cxl0Jcl 25&C ) -100 /

Fig. (6) : US. laboratory staff diagram for the classification p. . 60 n.18 Vs. 5Deuterium plot for the of .rngat.on water m the concerned three oases investigated water samples in the three oases B e/j Fig. (8):Relationshipbetween5O.\ygcn-l8andTD S

DC TDSi TDS(mg/1) 250 100 200 300 --12 150 50 0 250 200 300 l« 200 150 100 too SO 0 500 60 -12 70 - - - — • - - for thecollectedgrounUwatcinthreoases - - El-Farafra El-Kharg El-Dakhla Oasis i 9. a Oasis «o. ; -II ../> «• 14*26 Oasis. 50xygen-18(%o) 1 2 51* -11 31 28 * 994 I * 13• 59 35 SO • 30, .CO Cc 15 *25 .57 9 -10 23 0 I .» the increase in oxygen-18 is higher than that expected in the case of evaporation, this may reflect the effect of leaching process on the salinity.

The relationship between the screen's depth of the investigated wells and their oxygen-18 content, Fig.(9), reveals the abscence of any defined pattern. This implies that there is no sharp'distinction between the isotopic composition of both shallow and deep horizon due to interconnection of the aquifer system.

The carbon-14 age was determined for six groundwater samples, collected from both shallow and deep zones in the Nubian aquifer and shows values ranging from 18700 to 31800 years B.P., which is related to late Pleistocene and Early Holocene. During this time ( Pluvial time ), the North Africa's received sufficient amounts of the winter rain from western drift wet air masses which earring water from the Atlantic ocean producing huge groundwater reservour( \ So, the potential recharging source for the Nubian Sandstone aquifer in the New Valley is in-situ rainwater of palaeometeoric origin during the last Pluvial period.

The relationship between carbon-14 age and the screen's depth, Fig.(10), shows a reverse relation (e.g. increasing of the age with decreasing of the screen's depth ), this could be attributed to mixing through the upward flow under artesien conditions.

CONCLUSIONS

The study area covers about 8000 Km2 from the Western Desert and include El-Farafra, El- Dakhla and El-Kharga oases. The Nubian Sandstone aquifer underlies the New Valley'oases represent the main groundwater bearing formation that contain • a huge amount of water for agricultural and domestic uses ( 3xl0M million cubic meters). The environmental isotope data reveals that the mean recharge source for this aquifer is in situ precipitation in much cooler and humid period thousands of years ago since the I4C age is > 20,000 years. The hydrochemical data revealed that the meteoric water invasion is less recognized in El-Farafra oasis than El-Dakhla and El-Kharga, this could be attributed to higher percentage of less permeable beds like shales which acts as retarding factor for meteoric water percolation. The mineralization of the investigated groundwater is controlled by leaching, cation exchange and oxidation-reduction processes. Corrosion potential is high due to the effect of high water temperature, high content of both H2S and CO2 and low pH values of the investigated groundwater samples.

995 Depth 1000 1200 200 400 600 800 -12 0 1 — Fig. (9):Relationshipbetweenthescreen'sdept h and 5Oxygen-18 fortheinvestigatedwells a -11 0 X • 1 0 * CO • °o Oxygen-18(5 o • a ° a D o • 1 „ • EL-Dakhla D EL-Kharga o EL-Farafra o o a a o a -9 1 ° 600 i SZ 800 a. 1400 1000 1200 200 400 Fig. (10):RelationshipbetweenCarbon-14andscreen' s depth 0 18 202U63 2 34 - 61• ~ •43 - , I.1-• for someselectedgroundwatesamples 3 C-U Age(yearsx10) \. 5 1,1,' *40 ^^\ REFERENCES

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