Hydrogeochemical and isotopic evolution of water in the Complexe Terminal aquifer in the Algerian

A. Guendouz · A. S. Moulla · W. M. Edmunds · K. Zouari · P. Shand · A. Mamou

Abstract The hydrogeochemical and isotopic evolution recharge. All d18O compositions are enriched relative to of groundwaters in the Mio–Pliocene sands of the deuterium and are considered to be derived by evapora- Complexe Terminal (CT) aquifer in central are tive enrichment from a parent rainfall around 11‰ d18O, described. The CT aquifer is located in the large signifying cooler conditions in the late Pleistocene and sedimentary basin of the Great Oriental Erg. Down- possibly heavy monsoon rains during the Holocene. gradient groundwater evolution is considered along the main representative aquifer cross section (south–north), Rsum Ce papier dcrit l’volution hydrogochimique from the southern recharge area (Tinrhert Plateau and et isotopique des eaux souterraines des sables de l’aqui- Great Oriental Erg) over about 700 km. Groundwater fre du Complexe Terminal, situ dans le vaste bassin mineralisation increases along the flow line, from 1.5 to sdimentaire du (nord-est du Sahara 8gl1, primarily as a result of dissolution of evaporite algrien). Cette tude a t ralise selon la principale minerals, as shown by Br/Cl and strontium isotope ratios. section transversale nord–sud reprsentative de l’aquifre, Trends in both major and trace elements demonstrate a qui reprsente l’volution selon un gradient d’coulement progressive evolution along the flow path. Redox reac- vers l’aval depuis la zone de recharge mridionale (le tions are important and the persistence of oxidising plateau de Tinrhert et le Grand Erg Oriental), sur environ conditions favours the increase in some trace elements 700 km. La minralisation des eaux souterraines aug- (e.g. Cr) and also NO3 , which reaches concentrations of mente le long des lignes d’coulement et est comprise 1 14 1 16.8 mg l NO3-N. The range in C, 0–8.4 pmc in the entre 1.5 et 8 g l , avec un accroissement correspondant + 2 2+ + deeper groundwaters, corresponds with late Pleistocene des concentrations en Na ,Cl,SO4 ,Ca et K . Des recharge, although there then follows a hiatus in the data traces de minraux vaporitiques prsents dans la matrice with no results in the range 10–20 pmc, interpreted as a de l’aquifre (halite, gypse, sylvite) sont les principaux gap in recharge coincident with hyper-arid but cool contrles minralogiques. Par ailleurs, les concentrations 2+ conditions across the Sahara; groundwater in the range en HCO3 et Mg sont relativement constantes du fait de 24.7–38.9 pmc signifies a distinct period of Holocene la saturation des eaux en minraux carbonats. Des lments mineurs et en traces (Br,Sr2+,Li+,B3+,F,I et Rb+) suivent les mÞmes tendances que les ions majeurs. Les isotopes stables (18O, 2H, 13C) et le radiocarbone sont Received: 27 August 2002 / Accepted: 13 March 2003 en accord avec les interprtations hydrochimiques d’une Published online: 20 May 2003 augmentation de l’ge et d’interactions eau–roche. La Springer-Verlag 2003 prsence de failles au sud de la ville d’ provoque une remonte par drainance d’eaux anciennes depuis l’aquifre infrieur du Continental Intercalaire. Les ges A. Guendouz ()) dtermins par le radiocarbone permettent d’identifier Engineering Science Faculty, Blida University, deux masses d’eau diffrentes: l’une avec des ges de P.O. Box 270, Souma, Blida, Algeria b.p. e-mail: [email protected] 1,000–4,000 ans , l’autre avec des ges de 20,000– 40,000 ans b.p. A. S. Moulla Centre de Recherche Nuclaire d’Alger, P.O. Box 399, 16000 Algiers, Algeria Resumen Este artculo describe la evolucin hidrogeo- qumica e isotpica de las aguas subterrneas en las W. M. Edmunds · P. Shand arenas Mio-Pliocenas del acufero Terminal Complejo, British Geological Survey, Crowmarsh Gifford, situado en una gran cuenca sedimentaria del Great Wallingford, OX10 8BB, UK Oriental Erg (Nordeste del Shara argelino). La investi- K. Zouari gacin se ha desarrollado a lo largo de la seccin cole Nationale des Ingnieurs de Sfax, Tunisia transversal ms representativa del acufero, en direccin A. Mamou Sur-Norte, que representa la evolucin del gradiente Direction Gnrale des Ressources en Eau, Tunis, Tunisia desde la zona meridional de recarga (plat de Tinrhert y

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 484 Great Oriental Erg) hasta una distancia superior a 700 km. radioactivo permite identificar dos masas de agua dife- Se ha determinado que la mineralizacin de las aguas rentes: una primera en la que el agua tiene una edad subterrneas aumenta a lo largo de la lnea de corriente, y comprendida entre 1,000 y 4,000 aos, y otra de 20 a est comprendida entre 1,500 y 8,000 mg l1, con un 40 aos. aumento paralelo de iones como el sodio, cloruro, sulfato, calcio y potasio. El control mineralgico se debe princi- Keywords Hydrochemistry · Complexe Terminal palmente a trazas de minerales evaporticos en la matriz aquifer · Palaeowaters · Stable isotopes · Trace elements · del acufero, entre los cuales destaca la halita, los yesos y Algeria la silvita. Por otro lado, las concentraciones de bicarbo- nato y magnesio son relativamente constantes, debido a la saturacin de las aguas subterrneas con respecto a los Introduction minerales carbonatados. Los elementos menores y traza (bromuro, estroncio, litio, boro, fluoruro, yodo y rubidio) The northern Sahara sedimentary basin extends over some tambin siguen la tendencia de los iones mayoritarios. 780,000 km2 mainly in Algeria (Fig. 1). It forms part of Los istopos estables (18O, 2H, 13C) y el carbono one of the largest and most arid deserts in the world and radioactivo son coherentes con las interpretaciones contains two important aquifer systems: the Continental hidroqumicas basadas en el envejecimiento e interaccin Intercalaire (CI) overlain by the Complexe Terminal agua-roca. La presencia de fallas hacia el Sur de la ciudad (CT). The CI has its recharge source in the Atlas de Ouargla da lugar a un flujo vertical ascendente de Mountains. It is mainly confined and discharges in the paleoaguas desde el acufero Continental Intercalaire Chotts of Tunisia and in the Gabs gulf (Mediterranean subyacente. La datacin realizada mediante el carbono sea). By contrast, the CT is unconfined or semi-confined,

Fig. 1 Location and geological map of the Great Oriental Erg and the study region. The CT aquifer is mainly composed of Pliocene sediments which crop out in the Tinrhert Plateau

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 485

Fig. 2 Hydrogeological cross section of the CT aquifer in Algeria. The positions of selected sampled wells in the Mio–Pliocene sands are shown. Note the wells in the north are completed in the Senonian with its main recharge area in the central Sahara. These tion, and (4) to investigate minor and trace elements both two aquifers therefore present potential opportunities for as indicators of the evolution and for their role in the study of palaeogroundwater evolution, since the assessing the overall potability and use. amounts of recharge under modern climates are expected to be small or negligible. The CT aquifer extends over the major part of the Geological and Hydrogeological Setting sedimentary basin in Algeria and Tunisia. The present study is limited to the eastern part of the basin, covering a The eastern basin of the CT aquifer is formed by a large, surface area of 350,000 km2 of Algeria (Fig. 1). The work flat-lying syncline with an elongated base (Fig. 2). It is is focused on a 700-km south-to-north section from the bordered in the west by early Cretaceous deposits and by Tinrhert Plateau in the central Sahara to the Atlas the M’zab uplift, by the early Cretaceous escarpments of Mountains (Fig. 2), where isolated boreholes may be the Tinrhert Plateau in the south, by the Cretaceous found in an otherwise remote area. Some of these outcrops of the Dahar hills (in Tunisia) to the east, and by boreholes were drilled for the oilfields of southern the folded Alpine chain of the Saharan Atlas in the north Algeria. (Fig. 1). The lithostratigraphy of the basin has been A number of major studies have been carried out on discussed by several authors (Paix 1956; Cornet 1964; Bel the aquifer. These investigations dealt especially with and Demargne 1966; Bel and Cuche 1970; Bel et al. 1970; mathematical models and simulations for predicting the UNESCO 1972). long-term behaviour of the aquifer. Chemical and isotopic From south to north, three different zones may be information on the groundwaters was obtained as part of distinguished (Fig. 2). several earlier studies (Paix 1956; Cornet 1964; UNESCO 1972; Gonfiantini et al. 1974; Guendouz 1985; Guendouz 1. The Great Oriental Erg. In this area the Mio–Pliocene and Moulla 1995; Andrews et al., unpublished data). sands and the Senonian carbonates are present around More recently, new isotopic data for the CT and CI the city of Ouargla where they merge and are aquifers confirmed the existence of palaeowaters in these shallower than elsewhere. Their thickness does not aquifers (Edmunds et al. 1997; Guendouz et al. 1997; exceed 100 m. South of 31300N, the Mio–Pliocene Edmunds et al. 2003a). These studies dealt especially with sand aquifer is the only formation being exploited. the investigation of the isotopes 18O, 2H, 13C, and the 2. The Oued Rhir valley. This forms the central zone of radioisotopes 3H and 14C, with lesser emphasis on the syncline. The lithological sequence is represented chemical evolution. by the Eocene/Mio–Pliocene which forms the exploit- The focus of the present study is on the hydrogeo- ed aquifer. This aquifer overlies carbonate and lacus- chemical evolution of the groundwaters along a north– trine Senonian deposits containing carbonates, south flow line (Fig. 3), using chemical and isotopic data. dolomites, clay and evaporites, including anhydrite. The principal objectives are (1) to investigate the timing 3. The Chotts region. In this area, the main formations are of aquifer recharge and its origins, (2) to determine the the Mio–Pliocene and the middle and late Eocene. The relative ages of the groundwaters, (3) to establish the Mio–Pliocene consists of a thick continental sequence main controls on the down-gradient geochemical evolu- (up to 1,300 m) composed of sand and sandy-clay detrital deposits with marl/gypsum intercalations.

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 486 The main structural feature in this area is constituted by the Amguid El Biod arch near the southern margin, oriented NNE–SSW (Fig. 1). This structure probably allows the deep Continental Intercalaire water to migrate locally upwards to the CT aquifer.

The CT formations are relatively heterogeneous and are composed of two main aquifer horizons separated by semi-permeable to impermeable strata.

1. Mio–Pliocene sands: these constitute the roof of the aquifer and are also termed the “Continental Termi- nal”. They cover unconformably nearly the whole area, but thin out on the slopes of the M’Zab uplift. 2. Late Eocene and early Senonian carbonates: the Senonian aquifer extends over the whole basin, but the late Eocene only occurs north of the city of (Fig. 2).

The CT aquifer system is generally unconfined, and direct recharge has taken place in the past and is possibly occurring at present in one or more of the four following areas: the Saharan Atlas, the M’Zab region slopes, the Tinrhert Plateau, and the dunes of the Great Oriental Erg (UNESCO 1972; Guendouz 1985). Groundwater devel- opment has mainly taken place in the sandy Mio–Pliocene formation, except to the north of the Chotts where the carbonates have been exploited. Apart from intensive use of the groundwater around the oilfields, the sites exploited are for isolated farms and oases; pollution of groundwater from human impacts, however, is not considered to be an issue.

Methodology Fig. 3 Sites chosen for sampling along the considered flow line; numbers refer to sites in Table 1 Sampling and Analytical Methods Forty-six groundwater samples were collected during yses were performed using appropriately diluted stan- field campaigns carried out in 1994, 1995 and 1996, along dards, and both laboratory and international reference the main flow line from the Tinrhert Plateau to the area of materials were used as checks for accuracy. Instrumental the Chotts, mainly from the sandy and sandy-clayey drift during ICP-MS analysis was corrected using In and layers of the Mio–Pliocene; a few samples were also Pt internal standards. Samples for stable isotope analysis collected from the carbonate aquifer draining towards the (18O, 2H, 13C) were measured by isotope ratio mass Chotts from the north (Fig. 3). Samples were taken as spectrometry, and Sr isotopes by thermal ionisation mass close as possible to the flow line. However, pumped spectrometry at British Geological Survey laboratories. boreholes in the areas away from the oilfields are sparse, The 14C analyses were performed at CRNA (Centre de and the line selected is thus a compromise between Recherche Nuclaire d’Alger). A limited number of one- selecting a flow sequence and site availability. Samples litre samples were also collected for 14C dating by AMS were taken in polyethylene bottles. On-site analysis (accelerator mass spectrometry), and these analyses were included temperature, specific electrical conductance performed at the NERC Radiocarbon Laboratory, East- (SEC), total alkalinity (as HCO3 ) by titration, and pH. Kilbride. An internal check on the quality of the data was Samples for laboratory analysis were filtered through made by determining the ionic balance; the balance lay 0.45-m membranes. Chloride, NO3-N, Br, F and I were below €6% except for four samples. Precision of analysed by automated colorimetry. Filtered and acidified measurement for stable isotope and radioactive analysis (1% v/v HNO3) samples were collected for major cations, was €0.1‰ for d18O and d13C, €2‰ for d2H, and €3 pmc SO4, and a wide range of trace elements analysed either for 14C. by ICP-OES (inductively coupled plasma optical emis- sion spectroscopy) or ICP-MS (inductively coupled plasma mass spectrometry). Calibrations for cation anal-

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 487 Results and Discussion

The results are considered in relation to a generalised south-to-north flow line shown in the cross section (Figs. 1 and 3). All samples are projected, normal to the flow direction, onto this line. This is a simplification to aid the interpretation. In reality, some flow may enter this line from the west or southwest from the M’Zab uplift, and there may also be several flow paths from the Tinrhert Plateau (Fig. 3). Some leakage may occur from the CI aquifer in the region of the Amguid faults. The well depths along the line of flow are very similar (100– 200 m), with depth increasing in accordance with the geological structure to the north. Despite this simplifica- tion, it will be seen that this line provides a good basis for understanding the progressive evolution in the isotopic and chemical compositions. The specific electrical con- ductance (SEC) increases progressively from around 1,000 S cm1 to just below 9,000 S cm1 along the flow line (Table 1). The approach adopted here is first to discuss the evolution of inert tracers (Cl, d18O, d2H), together with Br. Against this background, the trend in the reactive hydrogeochemistry is interpreted. Chloride, Stable Isotopes (d18O, d2H) and Bromide Chloride concentrations largely reflect the input condi- Fig. 4 Cross section from south to north along 700 km of the CT tions at the time of recharge and may be modified aquifer for halogen elements (Cl, Br/Cl, I/Cl and F) subsequently by inputs from formation waters or evapor- ites (Herczeg and Edmunds 1999). Minimum concentra- tions near to outcrop average 200 mg l1 and are likely to slope of 4.5, and with an intercept on the GMWL (for represent the result of evaporation of rainfall solutes. both Holocene and Pleistocene waters) of 11‰ (this is Chloride increases down gradient to a maximum of discussed below). From the relationship of the data in 2,500 mg l1 around the Chotts (Fig. 4). The bromide/ Fig. 5b, it is clear that none of the water is related to chloride ratio is used here to define the salinity sources in modern rainfall recharge. The isotope distribution along more detail (Edmunds 1996). The Br/Cl ratio in seawater the flow line indicates some discontinuity in the recharge is 0.0035 (weight ratio), and maritime rainfall concentra- source areas or changes in past climatic conditions. An tions lie close to this value. Higher values may be abrupt change at 350-m distance corresponds to the expected where organic matter is present in the sediments. boundary between waters of Holocene age, as indicated In the CT aquifer, Br/Cl ratios are well below the by the profile in Fig. 6. seawater reference line and indicate a strong influence of The overall evaporated signature of the waters corre- evaporites as the source of Cl. The relatively enriched, sponds to rainfall which has undergone evaporative initial Br/Cl ratios in the up-gradient section represent a enrichment (before or during recharge) through the rainfall input or a surface water source also enriched in aeolian deposits. The variations observed along the Wadi evaporites, as opposed to marine aerosols. The Br/Cl ratio Rhir valley (Fig. 5a) show (with two exceptions) homo- decreases further along the flow line, indicating that the geneous stable isotope compositions in the section 390– increase in Cl is due to traces of halite in the clay 610 km, with average concentrations as follows: members of the formation (Fig. 4). d18O=7.3€0.5‰ (n=13) and d2H=59€3‰ (n=13). Stable isotope ratios are also used as conservative These more depleted signatures are considered to be tracers of water origin. They exhibit wide variations, from the late Pleistocene. between 4.9 and 9.2‰ for d18O, and between 44 and In the north near the Chotts (600–680 km), Mio– 72‰ for d2H across the line of section, and show a Pliocene waters are evaporated, with mean d18O and d2H significant depletion in d2H in relation to the global equal to 5.3 and 49‰ respectively. These groundwa- meteoric water line (GMWL). The isotope ratios are ters are completely isolated from others by thick (400 m), shown in Fig. 5a, plotted against distance along the line of lacustrine middle Eocene layers (Fig. 2) which are present section, and in a delta diagram where modern rainfall only in this area. This evaporated feature exhibited by (weighted mean values, 1994–1995–1996) from groundwater in the Mio–Pliocene aquifer is related to the Ain Oussera (Edmunds et al. 1997) is also plotted mode of aquifer recharge. The aquifer is, in fact, confined (Fig. 5b). The groundwaters are related by a line with a over the entire region, except on its western border where

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 yrgooyJunl(03 14345DI10.1007/s10040-003-0263-7 DOI 11:483–495 (2003) Journal Hydrogeology 488

Table 1 Field and analytical data from the Complexe Terminal aquifer

2 18 13 14 + + 2+ 2+ Locality Site Distance to Depth Lat. Long. d H d O C C T pH SEC Na K Ca Mg HCO3 SO4 Cl NO3 -N Si no. recharge zone (m) (pmc) (C) (S cm1) (km) (‰) (mg l1)

El-Alia 11 520 160 324102000 52202000 72 9.2 29.4 7.49 4,320 365 14.0 394 145 126 1,550 618 7.4 7.4 DASE 13 500 100 323303500 53801200 60 7.1 27.0 8.00 4,900 396 29.2 381 177 121 1,510 824 6.6 5.5 Blidet Omar 1964 15 560 168 325501000 60101500 59 7.3 24.4 7.25 5,190 450 28.1 404 171 125 1,380 897 5.5 6.3 Sidi-Slimane 19 600 180 331202500 60503500 50 5.1 4.70 2.4€0.7 24.9 7.56 5,660 660 26.1 430 135 122 1,400 1,072 8.6 7.6 Touggourt Ville 21 570 170 330401500 60303000 58 6.7 5.1 4.0€0.5 23.4 7.45 6,730 851 40.5 467 186 124 1,500 1,404 6.5 6.7 Djama (Ain Zerrouk) 23 610 180 333400000 60000000 55 7.1 23.3 7.38 8,590 873 23.6 719 285 141 1,790 1,795 6.9 8.0 Sidi-Khellil 24 620 230 333802000 55801500 50 7.6 24.7 7.56 6,160 651 27.2 528 174 131 1,430 1,115 8.5 7.7 El-Meghaer (El Alia) 1987 25 630 271 335401600 55604000 47 4.9 23.9 7.59 4,700 553 24.2 382 125 143 987 798 7.5 7.3 El Meghaer 26 640 278 335901500 55803000 48 5.6 6.44 0.0€1.5 24.5 7.58 4,810 548 24.1 386 123 143 1,090 813 7.6 7.2 Khchem Er’rih 41 480 120 322504000 54404000 60 7.7 25.7 7.36 5,080 583 25.4 338 159 123 991 1,005 7.5 6.6 Sidi-Belkheir 43 470 125 321705500 53408800 66 7.2 24.7 7.55 6,690 746 28.3 358 165 128 1,080 1,299 6.9 6.7 El-Bekrat 44 465 115 315205000 53104500 65 7.3 24.7 7.21 6,830 878 34.8 366 176 127 1,070 1,465 6.7 6.4 Istikama () 46 460 110 320301500 53702500 70 7.5 7.6 8.4€0.8 25.6 7.31 6,130 791 33.6 371 161 130 993 1,470 6.6 6.9 Ain-Djrad 48 450 112 320802000 54001500 64 7.4 25.6 7.53 5,600 647 27.8 310 110 126 887 1,140 7.1 6.3 Gassi-Touil, GT3 51 210 160 303103000 52803000 61 6.9 27.0 6.52 3,170 402 14.3 198 62.5 100 735 440 6.3 7.2 Gassi-Touil, HT4 52 190 150 303101100 62800000 56 6.8 7.0 24.7€1.0 27.7 6.66 3,650 433 15.9 239 74.1 82 813 550 7.0 6.9 Gassi-Touil, GT2 53 205 170 303105000 62805400 58 6.7 9.5 31.4€3.7 30.8 7.15 4,030 468 17.0 276 83.7 81 883 660 8.0 6.7 Gassi-Touil, M3 60 220 165 303302400 62804600 57 6.4 30.1 6.99 3,820 474 18.7 282 85.8 60 884 680 7.9 6.5 Gassi-Touil, P 61 215 165 303304800 62804200 59 6.6 29.7 7.22 3,080 408 14.1 205 65.7 159 800 430 5.7 7.2 Rhourde El Baguel, MP103 62 330 100 313205500 65703200 53 5.3 6.56 28.0€0.8 28.5 7.04 3,270 338 27.5 215 80.8 65 617 530 15.0 7.2 Rhourde El Baguel, MP106 63 335 95 312203700 65702500 53 5.3 28.6 7.19 3,250 340 27.4 210 78.8 58 607 530 14.9 7.1 Rhourde El Baguel, P1 70 340 95 312802000 72101900 48 5.1 27.9 7.17 3,280 379 26.8 230 61.2 73 734 480 12.9 8.2 Rhourde El Baguel, MP105 71 350 100 312302000 65700700 52 5.3 29.1 7.34 3,970 433 29.9 274 108 62 743 760 16.8 7.1 Hassi-Messaoud Sagra, S1 74 400 90 313001500 62001000 59 7.4 29.0 7.63 10,300 1,450 50.3 597 191 74 1,530 2,500 1.8 7.2 Hassi-Messaoud, H2 75 380 95 313100200 61101800 62 7.6 9.0 38.9€2 31.8 7.54 4,520 512 24.4 327 112 87 1,050 770 1.8 10.4 Djama Sidi-Yahia, MP5 77 550 170 333100400 55700500 53 7.3 24.1 7.01 9,860 1,230 24.3 662 630 96 1,990 2,300 7.1 8.1 Hamraa, HAM6 80 630 584 340503600 62000500 49 4.9 33.6 7.32 4,660 539 23.0 298 113 90 986 800 5.9 7.0 Hamraa, HAM4 81 625 517 340603200 61303300 47 5.9 32.5 7.25 4,770 552 22.6 304 114 83 1,020 830 5.8 7.0 M’Guebra, GUEB 82 650 600 341502300 60004600 56 6.9 31.0 7.49 5,110 624 18.7 352 118 89 1,420 730 2.5 7.2 Rhourde Nouss, RN15 84 100 283501900 70500500 55 5.5 7.4 24.1€0.8 23.8 7.11 2,820 275 17.1 205 71.2 108 658 406 14.6 7.5 Rhourde Nouss, RN17 86 90 283905700 63004100 53 5.4 7.5 22.7€0.7 28.4 7.54 3,210 320 17.7 230 86.3 106 751 502 14.8 7.8 Rhourde Nouss, ALCIM 87 80 293700500 64205000 58 5.6 27.2 7.83 1,806 214 15.4 67.9 23.5 114 357 164 13.8 6.3 El-Hamra, HRA 88 70 294105000 60300600 64 6.5 29.1 7.65 1,750 172 9.3 87.6 31.7 130 321 180 8.1 7.6 El-Hamra, HRA1 89 60 294400600 64201100 61 6.8 6.6 42.5€1.5 29.3 7.71 1,555 185 9.8 114 39.5 135 397 208 6.3 7.4 Rhourde Nouss, St. Pompage 90 110 291303000 62904700 69 8.6 22.5 7.71 4,670 532 17.4 407 124 78 1,570 660 6.3 6.2 yrgooyJunl(03 14345DI10.1007/s10040-003-0263-7 DOI 11:483–495 (2003) Journal Hydrogeology

Table 1 (continued)

Fe total Mn F Br I P total Li B Al Cr Co Ni Cu Zn Ge Rb Sr Mo Cd Ba Pb U

(g l1)

El-Alia 0.08 0.007 2.13 0.74 0.147 <0.5 91 380 4.6 37.5 0.9 32.2 <5.5 30.4 <0.6 6.3 8,700 12.6 0.9 13.0 1.0 5.2 DASE 0.12 0.006 1.76 0.72 0.0928 <0.5 117 360 2.0 26.4 0.8 28.0 <5.5 26.1 <0.6 9.3 6,490 16.4 0.5 13.8 1.3 3.8 Blidet Omar 1964 0.07 0.007 1.8 0.9 0.0747 <0.5 121 350 <1.6 26.4 0.8 25.0 <5.5 30.1 <0.6 8.4 7,760 17.3 <0.4 13.4 0.9 3.6 Sidi-Slimane 0.05 <0.004 2.1 1.13 0.0934 <0.5 142 460 <1.6 28.8 0.9 25.8 <5.5 20.4 <0.6 9.2 7,740 15.8 0.4 14.4 1.0 3.4 Touggourt Ville 0.04 <0.005 1.91 1.4 0.0955 <0.5 172 460 2.5 20.5 0.9 24.5 <5.5 15.0 <0.6 9.8 8,940 15.4 <0.4 12.1 0.4 5.0 Djama (Ain Zerrouk) 0.07 <0.007 2.26 1.78 0.163 <0.5 174 530 <1.6 19.4 1.1 31.8 <5.5 21.7 <0.6 8.5 14,000 14.8 <0.4 15.7 1.2 7.7 Sidi-Khellil 0.09 <0.004 2.23 1.25 0.111 <0.5 158 530 <1.6 22.7 1.0 27.9 <5.5 18.6 <0.6 10.3 9,180 15.7 <0.4 15.6 0.5 5.4 El-Meghaer (El Alia) 1987 0.07 0.008 2.33 0.88 0.0891 <0.5 141 510 6.8 24.8 0.6 21.6 <5.5 25.4 <0.6 8.3 6,820 16.4 0.7 13.7 0.9 4.2 El Meghaer 0.05 <0.004 2.75 0.51 0.0902 <0.5 140 510 <1.6 17.3 0.4 13.1 <5.5 16.2 <0.6 2.2 6,890 <10 <0.4 18.3 0.6 5.6 Khchem Er’rih <0.02 0.005 1.6 0.84 0.177 <0.5 89 370 <1.6 11.8 0.6 19.3 <5.5 100.1 <0.6 8.2 6,220 <10 0.7 14.5 0.2 3.2 Sidi-Belkheir 0.03 <0.003 1.4 1.86 0.198 <0.5 102 430 <1.6 16.7 0.6 20.3 <5.5 18.6 <0.6 7.9 5,790 <10 0.7 15.3 0.5 4.0 El-Bekrat 0.03 <0.003 1.4 0.99 0.122 <0.5 127 490 <1.6 14.1 0.8 20.8 <5.5 22.1 <0.6 8.8 5,730 <10 <0.4 14.4 0.4 4.0 Istikama (Hassi Ben Abdellah) 0.06 <0.003 1.25 0.74 0.119 <0.5 133 520 9.7 10.8 0.7 16.6 <5.5 17.4 <0.6 8.7 5,560 <10 <0.4 19.2 <0.2 4.2 Ain-Djrad 0.04 0.004 1.28 1.76 0.124 <0.5 110 430 <1.6 20.8 0.6 20.8 <5.5 27.2 <0.6 6.9 4,790 <10 <0.4 14.5 <0.2 4.0 Gassi-Touil, GT3 0.04 <0.003 0.59 0.7 0.092 <0.5 78 260 <2.4 8.4 0.6 8.9 2.5 77.7 <0.2 3.2 2,820 4.6 <0.5 13.1 <0.6 6.1 Gassi-Touil, HT4 0.12 0.004 0.61 0.8 0.0955 <0.5 82 280 14.4 11.0 0.7 11.1 3.2 54.9 <0.2 3.4 3,500 4.3 <0.5 14.7 0.6 4.9 Gassi-Touil, GT2 0.04 0.005 0.6 0.93 0.104 <0.5 87 310 <2.4 7.7 0.6 11.9 <0.5 66.0 <0.2 4.8 4,260 4.7 <0.5 16.5 <0.6 4.8 Gassi-Touil, M3 0.16 0.018 0.66 1.05 0.098 <0.5 92 330 <2.4 4.5 0.8 14.2 <0.5 69.5 <0.2 5.0 4,390 4.2 <0.5 15.8 <0.6 4.2 Gassi-Touil, P 0.08 0.004 0.58 0.68 0.098 <0.5 82 230 <2.4 8.2 0.6 9.4 1.6 45.3 <0.2 3.2 2,790 3.6 <0.5 14.5 <0.6 6.4 Rhourde El Baguel, MP103 <0.02 <0.003 1.35 0.62 0.065 <0.5 84 320 <2.4 11.6 0.5 9.9 0.8 37.8 <0.2 7.4 6,550 7.1 <0.5 20.0 <0.6 3.1 Rhourde El Baguel, MP106 0.002?? 0.003 1.44 0.595 0.065 <0.5 81 330 <2.4 4.0 0.6 8.8 0.9 32.8 <0.2 5.8 6,080 7.0 <0.5 20.2 <0.6 3.5 Rhourde El Baguel, P1 0.09 0.004 1.6 0.77 0.046 <0.5 60 370 18.3 13.5 0.8 11.1 2.0 59.6 <0.2 8.5 6,640 17.5 <0.5 27.5 <0.6 2.9 Rhourde El Baguel, MP105 <0.02 <0.003 1.32 0.91 0.075 <0.5 94 370 <2.4 11.4 0.7 12.9 1.0 33.8 <0.2 7.1 8,420 6.6 1.1 25.5 <0.6 3.6 Hassi-Messaoud Sagra, S1 <0.02 0.018 1.7 2.96 0.11 <0.5 217 700 <2.4 2.3 1.7 31.7 5.8 39.2 0.2 16.5 9,920 5.4 <0.5 24.0 <0.6 5.0 Hassi-Messaoud, H2 0.03 <0.003 1.95 0.113 0.0585 <0.5 99 350 10.0 2.5 1.0 14.9 3.0 51.6 0.2 7.9 8,560 12.0 <0.5 15.6 <0.6 4.1 Djama Sidi-Yahia, MP5 <0.02 <0.003 2.5 1.9 0.16 <0.5 175 530 92.6 4.1 2.1 30.6 2.2 41.1 <0.2 8.2 12,500 9.0 <0.5 16.1 <0.6 7.7 Hamraa, HAM6 0.06 <0.003 2.3 0.12 0.0735 <0.5 111 460 <2.4 7.3 0.8 14.6 1.0 36.6 <0.2 7.3 6,580 12.0 <0.5 18.2 <0.6 3.0 Hamraa, HAM4 0.08 0.004 2.3 0.121 0.0735 <0.5 115 460 <2.4 8.5 0.7 17.9 1.0 39.7 <0.2 7.1 6,600 9.9 <0.5 16.6 <0.6 3.3 M’Guebra, GUEB 0.05 <0.003 2.6 0.131 0.0975 <0.5 142 720 <2.4 7.9 0.8 15.5 1.4 35.5 <0.2 7.4 6,710 13.0 <0.5 14.0 <0.6 4.1 Rhourde Nouss, RN15 <0.02 <0.003 0.96 0.41 0.0612 <0.5 60 230 5,430 19 Rhourde Nouss, RN17 <0.02 <0.003 1.08 0.49 0.0604 <0.5 70 230 6,540 19 Rhourde Nouss, ALCIM <0.02 <0.003 0.85 0.26 0.061 <0.5 40 240 1,370 18 El-Hamra, HRA <0.02 <0.003 0.54 0.25 0.0373 <0.5 50 160 933 20 El-Hamra, HRA1 <0.02 <0.003 0.55 0.29 0.0371 <0.5 40 150 1,280 20 Rhourde Nouss, St. Pompage <0.02 <0.003 0.41 1.06 0.0828 <0.5 60 110 4,730 13 489 490 Fig. 5 a d18O and d2H plotted against distance along the line of section; the waters of Holo- cene and late Pleistocene age are distinguished. b d2H vs. d18O relationship for ground- waters in the CT aquifer in relation to modern rainfall at Ain Oussera

range 40–20 pmc. Towards the north, from the Wadi-Rhir valley the radiocarbon activity decreases from 8.4 pmc at Ouargla to near 0 pmc around El-Meghaer in the vicinity of both the Melrhir and Merouane Chotts. The d13Cof total dissolved inorganic carbon (TDIC) in the Mio– Pliocene sand groundwaters exhibit a general increase along the flow path, ranging from 10.5 to 4.70‰ vs. PDB (Fig. 6; Table 1). These concentrations vary from 6.6 to 7‰ vs. PDB at the aquifer’s southern recharge zone, whereas they range from 6 to 4‰ vs. PDB in the discharge area of the Chotts. The measured overall 13C concentrations are all enriched relative to the stoichio- metric equilibrium value of 12.5‰ vs. PDB, indicating exchange with the matrix which dilutes the radiocarbon

13 14 concentrations. With the exception of samples near Fig. 6 d C and C plotted against distance along the line of 13 section outcrop, the d C values show a progressive increase down gradient, further supporting a smooth increase in residence time down gradient. Absolute ages were not it is replenished in the area of outcrop of the Mio– calculated in the present study since, for the interpretation Pliocene sands. This is a former artesian discharge area of of residence times, reliance is placed on the raw data as the aquifer where runoff from the M’zab uplift slopes was pmc. subjected to evaporation before or while recharging the aquifer. Major Ion Trends The major ion relationships in the aquifer are relatively Timing of Groundwater Recharge straightforward. The ratio Na/Cl weight ratio (Fig. 7) Radiocarbon results of selected groundwaters along the remains almost constant around 0.6 wt%, which is flow line, expressed as percent modern carbon (pmc), consistent with stoichiometric dissolution of halite, with- show a smooth relationship in agreement with the out significant reaction of silicate minerals, including evolution of the piezometric level, and thus with the cation exchange. An exception occurs near outcrop where direction of flow of the aquifer (Fig. 6). South of Hassi- some Na/Cl enrichment suggests that albite dissolution Messaoud (some 400 km of the profile) in the Great may be from initial weathering. Oriental Erg, 14C activities were found to vary in the

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 491

Fig. 7 Cross section from south to north along 700 km of the CT Fig. 8 Cross section from south to north along 700 km of the CT aquifer for major elements aquifer for redox parameters

The linear increase of calcium and sulphate concen- conditions are maintained, since nitrate is rapidly reduced trations from south to north indicates gypsum dissolution. in the absence of dissolved oxygen (Edmunds et al. 1984). The waters become gypsum-saturated at the discharge The high nitrate concentrations are typical of many area in the vicinity of the Chotts (Table 2). Magnesium groundwaters from continental basin sediments in north also increases along the profile, as reflected throughout Africa and elsewhere (Edmunds and Gaye 1997). They much of the section by relatively constant Mg/Ca ratios. are interpreted as evidence of N-fixing vegetation in the The latter are thought to be controlled by dissolution of recharge areas at the time of recharge. Concentrations dolomite in the sands. The waters are also at or near range up to 16.8 mg l1 (Table 1), although there is no calcite saturation (Table 2). Gypsum dissolution con- pattern relating to the flow line, suggesting that variable tributes Ca to the waters, maintaining the calcite equilib- contributions from soils have taken place throughout the rium and forcing some calcite precipitation due to the recharge period. As elsewhere, in some locations in the common ion effect. Continental Terminal NO3-N concentrations therefore exceed stated potable limits (maximum 16.8 mg l1 at Redox Relationships and Trace Metals Rhourde El-Baguel) due to natural processes. Although neither redox potential nor dissolved oxygen The distributions of some trace metals shown in Fig. 8 were measured in this investigation, it is clear from the (and Table 1) are also controlled by the groundwater distribution of redox-controlled species that oxidising redox status. Under the oxidising conditions throughout conditions are maintained within the aquifer (Fig. 8). the CT aquifer, concentrations of Mn, Cr, and U are Thus, the concentrations of total dissolved iron are low 1 2+ moderately enriched throughout the aquifer, although (below 0.1 mg l Fe ), signifying that at the field- there is no distinct trend across the flow path. The measured pH (mean 7.5) the Eh would be expected to be concentrations of Cr are all above detection limits, and above 100 mV (Hem 1985). Significantly, the concentra- vary in the range 4.0–37.5 mgl1. The uranium concen- tions of nitrate are high and indicate that aerobic trations show values of 2.9–7.7 g l1. This enrichment is

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 492

Table 2 Saturation indices for Locality Calcite Dolomite Gypsum Anhydrite Celestite Barite Fluorite the Complexe Terminal aquifer Saturation index El-Alia 0.38 0.69 0.26 0.46 0.11 0.19 DASE 0.80 1.62 0.30 0.51 3.35 0.06 0.36 Blidet Omar 1964 0.09 0.14 0.31 0.53 3.31 0.08 0.27 Sidi-Slimane 0.41 0.65 0.29 0.51 3.31 0.06 0.11 Touggourt Ville 0.30 0.52 0.27 0.50 3.27 0.12 0.20 Djama (Ain Zerrouk) 0.43 0.78 0.10 0.33 3.10 0.01 0.04 Sidi-Khellil 0.51 0.88 0.23 0.45 3.27 0.02 0.00 El-Meghaer (El Alia) 1987 0.49 0.83 0.42 0.65 3.46 0.11 0.00 El Meghaer 0.48 0.81 0.39 0.61 3.42 0.02 0.13 Khchem Er’rih 0.17 0.36 0.49 0.71 3.51 0.15 0.43 Sidi-Belkheir 0.36 0.72 0.46 0.68 3.54 0.10 0.54 El-Bekrat 0.02 0.07 0.48 0.70 3.56 0.15 0.55 Istikama 0.16 0.31 0.49 0.70 3.59 0.04 0.64 (Hassi Ben Abdellah) Ain-Djrad 0.32 0.53 0.55 0.76 3.64 0.13 0.62 Gassi-Touil, GT3 0.89 1.93 0.69 0.90 3.82 0.17 1.39 Gassi-Touil, HT4 0.77 1.69 0.61 0.81 3.72 0.12 1.32 Gassi-Touil, GT2 0.20 0.54 0.55 0.74 3.63 0.14 1.34 Gassi-Touil, M3 0.49 1.12 0.54 0.74 3.62 0.13 1.25 Gassi-Touil, P 0.04 0.04 0.66 0.86 3.80 0.14 1.44 Rhourde El Baguel, MP103 0.49 1.03 0.73 0.94 3.52 0.07 0.66 Rhourde El Baguel, MP106 0.40 0.85 0.74 0.95 3.56 0.07 0.61 Rhourde El Baguel, P1 0.31 0.83 0.64 0.84 3.45 0.12 0.47 Rhourde El Baguel, MP105 0.14 0.31 0.62 0.82 3.41 0.02 0.65 Hassi-Messaoud Sagra, S1 0.39 0.67 0.25 0.45 3.30 0.03 0.32 Hassi-Messaoud, H2 0.71 1.30 0.44 0.31 3.21 0.52 0.69 Djama Sidi-Yahia, MP5 0.17 0.02 0.20 0.42 3.21 0.07 0.11 Hamraa, HAM6 0.05 0.10 0.52 0.70 3.44 0.13 0.23 Hamraa, HAM4 0.06 0.15 0.50 0.68 3.43 0.13 0.21 M’Guebra, GUEB 0.20 0.32 0.34 0.54 3.33 0.13 0.07 Rhourde Nouss, RN15 0.28 0.70 0.70 0.92 3.56 0.02 Rhourde Nouss, RN17 0.22 0.38 0.64 0.85 3.47 0.04 Rhourde Nouss, ALCIM 0.12 0.14 1.24 1.46 4.22 0.10 El-Hamra, HRA 0.14 0.22 1.19 1.39 4.44 0.13 El-Hamra, HRA1 0.31 0.53 1.03 1.24 4.26 0.10 Rhourde Nouss, St. Pompage 0.30 0.40 0.24 0.47 3.46 0.00 also a feature of aerobic groundwaters in the Continental Intercalaire aquifer from Algeria/Tunisia where the Cr concentrations are even higher, ranging up to 74 g l1 (Edmunds et al. 2003a). Molybdenum and nickel con- centrations, in contrast to U and Cr, increase across the flow line. Mo and Ni concentrations increase strongly from >4 to 17 g l1 and from 8 to 32 g l1 respectively (Table 1), and this implies that the uptake on the groundwaters is time-dependent under the oxidising conditions, in contrast to the U and Cr which show no overall trend. The concentrations of other metals includ- ing Cd, Pb, Cu and Co remain low (Table 2), mainly reflecting a lower geochemical abundance and/or lower mobility under these pH–Eh conditions. Zn concentra- tions lie within a range 15–100 g l1 and show a slight tendency for higher concentrations in the south of the area, possibly reflecting a source control. Controls on Non-metal Trace Element Occurrence The trends in other non-metal trace element concentra- tions across the aquifer provide additional insight into the Fig. 9 Cross section from south to north along 700 km of the CT 87 86 evolution of the groundwater. An increase in Sr across the aquifer for Sr, Sr/Ca and Sr/ Sr aquifer (Fig. 9) takes place from around 1,000 to 14,000 g l1 in the vicinity of the Chotts, well correlated

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 493 to increase progressively across the aquifer along its flow line (Table 1; Fig. 10). Lithium is usually a good indicator of lithofacies and of the extent of water–rock interaction, and has been used to indicate groundwater residence times (Edmunds and Smedley 2000). Concentrations of Li range from 40 g l1 up-gradient to 180 g l1 in the Chotts region. In Fig. 10, the concentrations of Li and also the Li/Cl ratios are plotted along the flow line. Near outcrop the Li/Cl ratios are relatively high (corresponding to the weathering of silicate minerals), but the overall increase in Li along the flow line is masked by the overall salinity increase. This contrasts with Li behaviour in the underlying CI aquifer (Edmunds et al. 2003b), where the Li/Cl ratio increases linearly from 0.0002 to 0.0006 and may be used as a supplementary residence time indicator.

Fig. 10 Cross section from south to north along 700 km of the CT Discussion and Conclusions aquifer for Li, Li/Cl The evolution of groundwater in the CT aquifer has been described for a transect approximately along the modern with Ca (as well as with sulphate) which indicates a flow direction over a distance of some 700 km, from an contribution from intra-formational evaporites. Much area which today is completely arid but which is known higher Sr/Ca ratios are found in the south of the oilfield from past climatic records to have been wetter in the late area of El-Hamra (60 km downstream of the flow line), Pleistocene and Holocene (Petit Maire and Riser 1982; and in the region around Ouargla (350–400 km) where Gasse 2000; Maley 2000). The groundwaters in the CT salinity is still relatively low, and suggest that an initial and other aquifers also contain sequential evidence of past strontium-enriched source is present. This is also sug- climate changes. The radiocarbon data show a smooth gested by the high 87Sr/86Sr ratios which indicate that a trend, from 38.9 to 0% modern carbon, from the former more radiogenic Sr source is involved. In the south, the recharge area in the south to the discharge area in the area Mio–Pliocene formations are composed of sands with thin of the Chotts. layers of gravels and white yellowish limestone and The results from the CT may be compared with the dolomite ( 1992). In addition, the Mio–Pliocene regional trends recorded in the stable isotope record in sands lie directly over the dolomitic Senonian formations dated groundwaters across northern Africa (Sonntag et al. in the vicinity of Ouargla (Bel and Cuche 1970). It is 1979; Edmunds et al. 2003b). Air mass circulation over considered that the enriched 87Sr/86Sr ratios indicate Africa during the late Pleistocene was significantly weathering of silicate minerals derived from basement different from the present day, with evidence of a rocks to the south and their weathering products. The reinforcement and southward shift of the Atlantic west- maximum concentrations of Sr are closer to saturation erly flow across the present Sahara during the period. A with respect to celestite (Table 2). corresponding decline of monsoon rains also occurred at There is a progressive increase in fluoride concentra- this time. Evidence then is found for a northward tions across the aquifer, from 0.6 to 2.75 mg l1 F (Fig. 4). extension of the African monsoon, with increased rainfall This smooth increase indicates an uptake of fluoride, intensity notably during the early to mid-Holocene, probably from traces of carbonate in the aquifer. It also coinciding with a retreat of the Atlantic system to the helps to confirm the age relationships as well as the flow north. The extent of cooling at the LGM recorded in the relationships, suggesting that there are no major sources noble gas ratios was up to 7 C (Guendouz et al. 1997). of external groundwater. At the higher concentrations The groundwater isotopic evidence in different places towards the Chotts, the upper limits are controlled by records strong variations in humidity of the air masses saturation with fluorite, as calculated with PHREEQC. supplying moisture across the continent at different times The concentrations of iodide vary in the range 30– over the past 30,000 years. 200 g l1 across the aquifer (Table 1), and the increase is The range in pmc (0–8.4) in the deeper groundwaters mainly related to an increase in Cl (as expressed as I/Cl). corresponds with late Pleistocene recharge, although there The I/Cl ratio is very low and at least one order of then follows a hiatus in the data (Table 1, Fig. 11), with magnitude lower than seawater, signifying a non-marine no results in the range 10–20 pmc. This hiatus has been evaporite source. interpreted in data from across the Sahara as a gap in the Barium concentrations are low (Table 1), and are recharge conditions coincident with the last glacial maintained at levels not exceeding 28 g l1 by barite maximum when hyperarid but cool conditions extended saturation (Table 2). The concentrations of lithium (also B over the Sahara. The evidence of groundwater in the and Rb) are found, in the absence of any solubility limits, range 24.7–38.9 pmc signifies Holocene recharge, possi-

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 494 Fig. 11 Plot of d18O against 14C (pmc) for all groundwaters for which age information is avail- able

bly connected with the northward extension of monsoon Geochemical trends in the groundwater along the flow rains from the south. path indicate that significant water–rock interaction is It is apparent from the limited 14C data that waters of taking place within the basin. There is an increase in Holocene age are found in the profile to a distance of mineralisation along the direction of flow. The salinity, some 400 km from outcrop, with late Pleistocene waters expressed as Cl, increases from 200 to around 2,000 mg l1 in the deeper aquifer (compare Figs. 6 and 8). Most from south to north, and the trace element concentrations groundwaters in the first 400 km are isotopically enriched reinforce major element trends to help interpret the main (d18O values around 6‰), in contrast with most waters processes taking place in the aquifer. At outcrop the low- in the late Pleistocene which have lighter compositions salinity waters retain some properties of evaporated rains (d18O around 7.5‰). Some anomalies are likely to be (supporting the concept of isotopic enrichment), but the due to mixing of shallow and deep waters, but it is low Br/Cl ratios indicate that dissolution of halite is unlikely that upward leakage from the CI aquifer is important, firstly, as a component of atmospheric inputs unimportant. and, secondly, through some dissolution of halite in These conclusions are in line with the general obser- evaporites within the Mio–Pliocene sequence. Gypsum vations across north Africa which show that most dissolution is also important in controlling the water Holocene groundwaters are enriched isotopically com- composition. pared with the late Pleistocene. It remains difficult, The smooth increase in elements such as F and Li however, to explain the relative oxygen enrichment (which show conservative behaviour at least as far as (relative to deuterium) of all the groundwaters in the solubility limits are reached) suggests that the influence CT aquifer. In Fig. 6 it can be seen that the waters of sources of water external to the flow line is minimal; extrapolate towards an intercept of near 11‰ d18O with oscillations in chemistry along the flow line can mainly the GMWL. This is consistent with regional observations, be explained by mixing of stratified waters. Slightly more especially in the NE Sahara (Edmunds et al. 2003b). In radiogenic strontium isotope ratios indicate some input southern Libya, for example, values as low as 11.5‰ from silicate weathering in the recharge areas, but d18O are found in late Pleistocene waters (possibly otherwise reflect the dissolution of gypsum or calcite. associated with runoff from the Tibesti mountains), and Redox is an important control on the groundwater strongly depleted waters also are found in Sudan and chemistry, and oxidising conditions prevail throughout Egypt, associated with the monsoon advance northwards the CT aquifer. Nitrate concentrations are consistently during the Holocene. Thus, depleted values close to the high, sometimes exceeding potability limits, and are in GMWL can be explained by rainfall or runoff which may line with concentrations found elsewhere in north African have then been subjected to evaporation prior to or during groundwaters. These high-nitrate groundwaters are con- recharge. A similar evaporative explanation has been sidered to result from N-fixing vegetation existing in the proposed for the CT aquifer in the Great Occidental Erg former recharge areas. (western Algeria) by other authors (Conrad and Fontes These findings have direct impacts on the groundwater 1970; Gonfiantini et al. 1974; Fontes et al. 1986). resource development of the CT in the Great Oriental Erg. The groundwater resources are shown to be non-renew-

Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7 495 able and therefore their development, as in several other Intercalaire aquifer of southern Algeria and Tunisia: trace basins of northern Africa, must be conducted on the basis element and isotopic indicators. Appl Geochem 18(6):805–822 Edmunds WM, Dodo A, Djoret D, Gasse F, Gaye CB, Goni IB, of mining. The quality is only marginal for development, Travi Y, Zouari K, Zuppi GM (2003b) Groundwater as an unlike the underlying CI aquifer which has lower salinity. archive of climatic and environmental change. The PEP-III The CI and CT represent a vast resource which needs traverse. In: Battarbee RW, Gasse F, Stickley CE (eds) Past careful protection and conservation. It is important climate variability through Europe and Africa. Kluwer, Dor- drecht, Developments in Palaeoenvironmental Research Series especially that the huge oilfield and other developments (in press) in this region also recognise the vulnerability of the Fontes JCH, Yousfi M, Allison GB (1986) Estimation of long-term, unconfined CT aquifer. diffuse groundwater discharge in the northern Sahara using stable isotope profiles in soil water. J Hydrol 86:315–327 Acknowledgements The work was carried out within the frame- Gasse F (2000) Hydrological changes in the African tropics since work of a project partly supported by the European Commission the Last Glacial Maximum. Quat Sci Rev 19:189–211 under the programme Avicenne (contract no. CT 93AVI0015). We Gonfiantini R, Conrad G, Fontes, JCh, Sauzay G, Payne BR (1974) thank Sonatrach for hosting us while in the field for sampling. We tude isotopique de la nappe du Continental Intercalaire et de thank Fiona Darbyshire (NIGL Keyworth) for conducting strontium ses relations avec les autres nappes du Sahara septentrional isotope analyses, and the staff of the Centre de Recherche Nuclaire (Isotopic investigation of the Continental Intercalaire aquifer d’Alger (CRNA) for other isotope analyses. Additional radiocarbon and its relationship with other aquifers in the northern Sahara). analysis was conducted through the NERC Radiocarbon Labora- In: Isotope Techniques in Groundwater Hydrology, Vienna. tory, East Kilbride, under research allocation 779/0119. This paper IAEA SM-182/25, vol 1, pp 227–241 is published with the permission of the directors of the CRNA and Guendouz A (1985) Contribution l’tude hydrochimique et the British Geological Survey (Natural Environment Research isotopique des nappes profondes du Sahara septentrional, Council). 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Hydrogeology Journal (2003) 11:483–495 DOI 10.1007/s10040-003-0263-7