hydrogeochemical and isotopic evolution of water in the complexe terminal aquifer in the algerian...

Hydrogeochemical and isotopic evolution of waterin the Complexe Terminal aquifer in the Algerian Sahara

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

Abstract The hydrogeochemical and isotopic evolutionof groundwaters in the Mio–Pliocene sands of theComplexe Terminal (CT) aquifer in central Algeria aredescribed. The CT aquifer is located in the largesedimentary basin of the Great Oriental Erg. Down-gradient groundwater evolution is considered along themain representative aquifer cross section (south–north),from the southern recharge area (Tinrhert Plateau andGreat Oriental Erg) over about 700 km. Groundwatermineralisation increases along the flow line, from 1.5 to8 g l�1, primarily as a result of dissolution of evaporiteminerals, as shown by Br/Cl and strontium isotope ratios.Trends in both major and trace elements demonstrate aprogressive evolution along the flow path. Redox reac-tions are important and the persistence of oxidisingconditions favours the increase in some trace elements(e.g. Cr) and also NO3

�, which reaches concentrations of16.8 mg l�1 NO3-N. The range in 14C, 0–8.4 pmc in thedeeper groundwaters, corresponds with late Pleistocenerecharge, although there then follows a hiatus in the datawith no results in the range 10–20 pmc, interpreted as agap in recharge coincident with hyper-arid but coolconditions across the Sahara; groundwater in the range24.7–38.9 pmc signifies a distinct period of Holocene

recharge. All d18O compositions are enriched relative todeuterium and are considered to be derived by evapora-tive enrichment from a parent rainfall around �11‰ d18O,signifying cooler conditions in the late Pleistocene andpossibly heavy monsoon rains during the Holocene.

R�sum� Ce papier d�crit l’�volution hydrog�ochimiqueet isotopique des eaux souterraines des sables de l’aqui-f�re du Complexe Terminal, situ� dans le vaste bassins�dimentaire du Grand Erg Oriental (nord-est du Saharaalg�rien). Cette �tude a �t� r�alis�e selon la principalesection transversale nord–sud repr�sentative de l’aquif�re,qui repr�sente l’�volution selon un gradient d’�coulementvers l’aval depuis la zone de recharge m�ridionale (leplateau de Tinrhert et le Grand Erg Oriental), sur environ700 km. La min�ralisation des eaux souterraines aug-mente le long des lignes d’�coulement et est compriseentre 1.5 et 8 g l�1, avec un accroissement correspondantdes concentrations en Na+, Cl�, SO4

2�, Ca2+ et K+. Destraces de min�raux �vaporitiques pr�sents dans la matricede l’aquif�re (halite, gypse, sylvite) sont les principauxcontr�les min�ralogiques. Par ailleurs, les concentrationsen HCO3

� et Mg2+ sont relativement constantes du fait dela saturation des eaux en min�raux carbonat�s. Des�l�ments mineurs et en traces (Br�, Sr2+, Li+, B3+, F�, I� etRb+) suivent les mÞmes tendances que les ions majeurs.Les isotopes stables (18O, 2H, 13C) et le radiocarbone sonten accord avec les interpr�tations hydrochimiques d’uneaugmentation de l’�ge et d’interactions eau–roche. Lapr�sence de failles au sud de la ville d’Ouargla provoqueune remont�e par drainance d’eaux anciennes depuisl’aquif�re inf�rieur du Continental Intercalaire. Les �gesd�termin�s par le radiocarbone permettent d’identifierdeux masses d’eau diff�rentes: l’une avec des �ges de1,000–4,000 ans b.p., l’autre avec des �ges de 20,000–40,000 ans b.p.

Resumen Este art�culo describe la evoluci�n hidrogeo-qu�mica e isot�pica de las aguas subterr�neas en lasarenas Mio-Pliocenas del acu�fero Terminal Complejo,situado en una gran cuenca sedimentaria del GreatOriental Erg (Nordeste del S�hara argelino). La investi-gaci�n se ha desarrollado a lo largo de la secci�ntransversal m�s representativa del acu�fero, en direcci�nSur-Norte, que representa la evoluci�n del gradientedesde la zona meridional de recarga (plat� de Tinrhert y

Received: 27 August 2002 / Accepted: 13 March 2003Published online: 20 May 2003

Springer-Verlag 2003

A. Guendouz ())Engineering Science Faculty, Blida University,P.O. Box 270, Soum�a, Blida, Algeriae-mail: [email protected]

A. S. MoullaCentre de Recherche Nucl�aire d’Alger,P.O. Box 399, 16000 Algiers, Algeria

W. M. Edmunds · P. ShandBritish Geological Survey, Crowmarsh Gifford,Wallingford, OX10 8BB, UK

K. Zouaricole Nationale des Ing�nieurs de Sfax, Tunisia

A. MamouDirection G�n�rale des Ressources en Eau, Tunis, Tunisia

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

Great Oriental Erg) hasta una distancia superior a 700 km.Se ha determinado que la mineralizaci�n de las aguassubterr�neas aumenta a lo largo de la l�nea de corriente, yest� comprendida entre 1,500 y 8,000 mg l�1, con unaumento paralelo de iones como el sodio, cloruro, sulfato,calcio y potasio. El control mineral�gico se debe princi-palmente a trazas de minerales evapor�ticos en la matrizdel acu�fero, entre los cuales destaca la halita, los yesos yla silvita. Por otro lado, las concentraciones de bicarbo-nato y magnesio son relativamente constantes, debido a lasaturaci�n de las aguas subterr�neas con respecto a losminerales carbonatados. Los elementos menores y traza(bromuro, estroncio, litio, boro, fluoruro, yodo y rubidio)tambi�n siguen la tendencia de los iones mayoritarios.Los is�topos estables (18O, 2H, 13C) y el carbonoradioactivo son coherentes con las interpretacioneshidroqu�micas basadas en el envejecimiento e interacci�nagua-roca. La presencia de fallas hacia el Sur de la ciudadde Ouargla da lugar a un flujo vertical ascendente depaleoaguas desde el acu�fero Continental Intercalairesubyacente. La dataci�n realizada mediante el carbono

radioactivo permite identificar dos masas de agua dife-rentes: una primera en la que el agua tiene una edadcomprendida entre 1,000 y 4,000 a�os, y otra de 20 a40 a�os.

Keywords Hydrochemistry · Complexe Terminalaquifer · Palaeowaters · Stable isotopes · Trace elements ·Algeria

Introduction

The northern Sahara sedimentary basin extends over some780,000 km2 mainly in Algeria (Fig. 1). It forms part ofone of the largest and most arid deserts in the world andcontains two important aquifer systems: the ContinentalIntercalaire (CI) overlain by the Complexe Terminal(CT). The CI has its recharge source in the AtlasMountains. It is mainly confined and discharges in theChotts of Tunisia and in the Gab�s gulf (Mediterraneansea). By contrast, the CT is unconfined or semi-confined,

Fig. 1 Location and geologicalmap of the Great Oriental Ergand the study region. The CTaquifer is mainly composed ofPliocene sediments which cropout in the Tinrhert Plateau

484


with its main recharge area in the central Sahara. Thesetwo aquifers therefore present potential opportunities forthe study of palaeogroundwater evolution, since theamounts of recharge under modern climates are expectedto be small or negligible.

The CT aquifer extends over the major part of thesedimentary basin in Algeria and Tunisia. The presentstudy is limited to the eastern part of the basin, covering asurface area of 350,000 km2 of Algeria (Fig. 1). The workis focused on a 700-km south-to-north section from theTinrhert Plateau in the central Sahara to the AtlasMountains (Fig. 2), where isolated boreholes may befound in an otherwise remote area. Some of theseboreholes were drilled for the oilfields of southernAlgeria.

A number of major studies have been carried out onthe aquifer. These investigations dealt especially withmathematical models and simulations for predicting thelong-term behaviour of the aquifer. Chemical and isotopicinformation on the groundwaters was obtained as part ofseveral earlier studies (Paix 1956; Cornet 1964; UNESCO1972; Gonfiantini et al. 1974; Guendouz 1985; Guendouzand Moulla 1995; Andrews et al., unpublished data).More recently, new isotopic data for the CT and CIaquifers confirmed the existence of palaeowaters in theseaquifers (Edmunds et al. 1997; Guendouz et al. 1997;Edmunds et al. 2003a). These studies dealt especially withthe investigation of the isotopes 18O, 2H, 13C, and theradioisotopes 3H and 14C, with lesser emphasis onchemical evolution.

The focus of the present study is on the hydrogeo-chemical evolution of the groundwaters along a north–south flow line (Fig. 3), using chemical and isotopic data.The principal objectives are (1) to investigate the timingof aquifer recharge and its origins, (2) to determine therelative ages of the groundwaters, (3) to establish themain controls on the down-gradient geochemical evolu-

tion, and (4) to investigate minor and trace elements bothas indicators of the evolution and for their role inassessing the overall potability and use.

Geological and Hydrogeological Setting

The eastern basin of the CT aquifer is formed by a large,flat-lying syncline with an elongated base (Fig. 2). It isbordered in the west by early Cretaceous deposits and bythe M’zab uplift, by the early Cretaceous escarpments ofthe Tinrhert Plateau in the south, by the Cretaceousoutcrops of the Dahar hills (in Tunisia) to the east, and bythe folded Alpine chain of the Saharan Atlas in the north(Fig. 1). The lithostratigraphy of the basin has beendiscussed by several authors (Paix 1956; Cornet 1964; Beland Demargne 1966; Bel and Cuche 1970; Bel et al. 1970;UNESCO 1972).

From south to north, three different zones may bedistinguished (Fig. 2).

1. The Great Oriental Erg. In this area the Mio–Pliocenesands and the Senonian carbonates are present aroundthe city of Ouargla where they merge and areshallower than elsewhere. Their thickness does notexceed 100 m. South of 31�300N, the Mio–Pliocenesand aquifer is the only formation being exploited.

2. The Oued Rhir valley. This forms the central zone ofthe syncline. The lithological sequence is representedby the Eocene/Mio–Pliocene which forms the exploit-ed aquifer. This aquifer overlies carbonate and lacus-trine Senonian deposits containing carbonates,dolomites, clay and evaporites, including anhydrite.

3. The Chotts region. In this area, the main formations arethe Mio–Pliocene and the middle and late Eocene. TheMio–Pliocene consists of a thick continental sequence(up to 1,300 m) composed of sand and sandy-claydetrital deposits with marl/gypsum intercalations.

Fig. 2 Hydrogeological cross section of the CT aquifer in Algeria. The positions of selected sampled wells in the Mio–Pliocene sands areshown. Note the wells in the north are completed in the Senonian limestones

485


The main structural feature in this area is constitutedby the Amguid El Biod arch near the southern margin,oriented NNE–SSW (Fig. 1). This structure probablyallows the deep Continental Intercalaire water tomigrate locally upwards to the CT aquifer.

The CT formations are relatively heterogeneous andare composed of two main aquifer horizons separated bysemi-permeable to impermeable strata.

1. Mio–Pliocene sands: these constitute the roof of theaquifer 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: theSenonian limestone aquifer extends over the wholebasin, but the late Eocene only occurs north of the cityof Touggourt (Fig. 2).

The CT aquifer system is generally unconfined, anddirect recharge has taken place in the past and is possiblyoccurring at present in one or more of the four followingareas: the Saharan Atlas, the M’Zab region slopes, theTinrhert Plateau, and the dunes of the Great Oriental Erg(UNESCO 1972; Guendouz 1985). Groundwater devel-opment has mainly taken place in the sandy Mio–Plioceneformation, except to the north of the Chotts where thecarbonates have been exploited. Apart from intensive useof the groundwater around the oilfields, the sites exploitedare for isolated farms and oases; pollution of groundwaterfrom human impacts, however, is not considered to be anissue.

Methodology

Sampling and Analytical MethodsForty-six groundwater samples were collected duringfield campaigns carried out in 1994, 1995 and 1996, alongthe main flow line from the Tinrhert Plateau to the area ofthe Chotts, mainly from the sandy and sandy-clayeylayers of the Mio–Pliocene; a few samples were alsocollected from the carbonate aquifer draining towards theChotts from the north (Fig. 3). Samples were taken asclose as possible to the flow line. However, pumpedboreholes in the areas away from the oilfields are sparse,and the line selected is thus a compromise betweenselecting a flow sequence and site availability. Sampleswere taken in polyethylene bottles. On-site analysisincluded temperature, specific electrical conductance(SEC), total alkalinity (as HCO3

�) by titration, and pH.Samples for laboratory analysis were filtered through0.45- m membranes. Chloride, NO3-N, Br, F and I wereanalysed by automated colorimetry. Filtered and acidified(1% v/v HNO3) samples were collected for major cations,SO4, and a wide range of trace elements analysed eitherby ICP-OES (inductively coupled plasma optical emis-sion spectroscopy) or ICP-MS (inductively coupledplasma mass spectrometry). Calibrations for cation anal-

yses were performed using appropriately diluted stan-dards, and both laboratory and international referencematerials were used as checks for accuracy. Instrumentaldrift during ICP-MS analysis was corrected using In andPt internal standards. Samples for stable isotope analysis(18O, 2H, 13C) were measured by isotope ratio massspectrometry, and Sr isotopes by thermal ionisation massspectrometry at British Geological Survey laboratories.The 14C analyses were performed at CRNA (Centre deRecherche Nucl�aire d’Alger). A limited number of one-litre samples were also collected for 14C dating by AMS(accelerator mass spectrometry), and these analyses wereperformed at the NERC Radiocarbon Laboratory, East-Kilbride. An internal check on the quality of the data wasmade by determining the ionic balance; the balance laybelow €6% except for four samples. Precision ofmeasurement for stable isotope and radioactive analysiswas €0.1‰ for d18O and d13C, €2‰ for d2H, and €3 pmcfor 14C.

Fig. 3 Sites chosen for sampling along the considered flow line;numbers refer to sites in Table 1

486


Results and Discussion

The results are considered in relation to a generalisedsouth-to-north flow line shown in the cross section (Figs. 1and 3). All samples are projected, normal to the flowdirection, onto this line. This is a simplification to aid theinterpretation. In reality, some flow may enter this linefrom the west or southwest from the M’Zab uplift, andthere may also be several flow paths from the TinrhertPlateau (Fig. 3). Some leakage may occur from the CIaquifer in the region of the Amguid faults. The welldepths along the line of flow are very similar (100–200 m), with depth increasing in accordance with thegeological structure to the north. Despite this simplifica-tion, it will be seen that this line provides a good basis forunderstanding the progressive evolution in the isotopicand chemical compositions. The specific electrical con-ductance (SEC) increases progressively from around1,000 S cm�1 to just below 9,000 S cm�1 along theflow line (Table 1).

The approach adopted here is first to discuss theevolution of inert tracers (Cl�, d18O, d2H), together withBr. Against this background, the trend in the reactivehydrogeochemistry is interpreted.

Chloride, Stable Isotopes (d18O, d2H) and BromideChloride concentrations largely reflect the input condi-tions at the time of recharge and may be modifiedsubsequently by inputs from formation waters or evapor-ites (Herczeg and Edmunds 1999). Minimum concentra-tions near to outcrop average 200 mg l�1 and are likely torepresent the result of evaporation of rainfall solutes.Chloride increases down gradient to a maximum of2,500 mg l�1 around the Chotts (Fig. 4). The bromide/chloride ratio is used here to define the salinity sources inmore detail (Edmunds 1996). The Br/Cl ratio in seawateris 0.0035 (weight ratio), and maritime rainfall concentra-tions lie close to this value. Higher values may beexpected where organic matter is present in the sediments.In the CT aquifer, Br/Cl ratios are well below theseawater reference line and indicate a strong influence ofevaporites as the source of Cl. The relatively enriched,initial Br/Cl ratios in the up-gradient section represent arainfall input or a surface water source also enriched inevaporites, as opposed to marine aerosols. The Br/Cl ratiodecreases further along the flow line, indicating that theincrease in Cl is due to traces of halite in the claymembers of the formation (Fig. 4).

Stable isotope ratios are also used as conservativetracers of water origin. They exhibit wide variations,between �4.9 and �9.2‰ for d18O, and between �44 and�72‰ for d2H across the line of section, and show asignificant depletion in d2H in relation to the globalmeteoric water line (GMWL). The isotope ratios areshown in Fig. 5a, plotted against distance along the line ofsection, and in a delta diagram where modern rainfall(weighted mean values, 1994–1995–1996) fromAin Oussera (Edmunds et al. 1997) is also plotted(Fig. 5b). The groundwaters are related by a line with a

slope of 4.5, and with an intercept on the GMWL (forboth Holocene and Pleistocene waters) of �11‰ (this isdiscussed below). From the relationship of the data inFig. 5b, it is clear that none of the water is related tomodern rainfall recharge. The isotope distribution alongthe flow line indicates some discontinuity in the rechargesource areas or changes in past climatic conditions. Anabrupt change at 350-m distance corresponds to theboundary between waters of Holocene age, as indicatedby the profile in Fig. 6.

The overall evaporated signature of the waters corre-sponds to rainfall which has undergone evaporativeenrichment (before or during recharge) through theaeolian deposits. The variations observed along the WadiRhir valley (Fig. 5a) show (with two exceptions) homo-geneous stable isotope compositions in the section 390–610 km, with average concentrations as follows:d18O=�7.3€0.5‰ (n=13) and d2H=�59€3‰ (n=13).These more depleted signatures are considered to befrom the late Pleistocene.

In the north near the Chotts (600–680 km), Mio–Pliocene waters are evaporated, with mean d18O and d2Hequal to �5.3 and �49‰ respectively. These groundwa-ters are completely isolated from others by thick (400 m),lacustrine middle Eocene layers (Fig. 2) which are presentonly in this area. This evaporated feature exhibited bygroundwater in the Mio–Pliocene aquifer is related to themode of aquifer recharge. The aquifer is, in fact, confinedover the entire region, except on its western border where

Fig. 4 Cross section from south to north along 700 km of the CTaquifer for halogen elements (Cl, Br/Cl, I/Cl and F)

487


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489


it is replenished in the area of outcrop of the Mio–Pliocene sands. This is a former artesian discharge area ofthe aquifer where runoff from the M’zab uplift slopes wassubjected to evaporation before or while recharging theaquifer.

Timing of Groundwater RechargeRadiocarbon results of selected groundwaters along theflow line, expressed as percent modern carbon (pmc),show a smooth relationship in agreement with theevolution of the piezometric level, and thus with thedirection of flow of the aquifer (Fig. 6). South of Hassi-Messaoud (some 400 km of the profile) in the GreatOriental Erg, 14C activities were found to vary in the

range 40–20 pmc. Towards the north, from the Wadi-Rhirvalley the radiocarbon activity decreases from 8.4 pmc atOuargla to near 0 pmc around El-Megha�er in the vicinityof both the Melrhir and Merouane Chotts. The d13C oftotal dissolved inorganic carbon (TDIC) in the Mio–Pliocene sand groundwaters exhibit a general increasealong 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 rechargezone, whereas they range from �6 to �4‰ vs. PDB in thedischarge area of the Chotts. The measured overall 13Cconcentrations are all enriched relative to the stoichio-metric equilibrium value of �12.5‰ vs. PDB, indicatingexchange with the matrix which dilutes the radiocarbonconcentrations. With the exception of samples nearoutcrop, the d13C values show a progressive increasedown gradient, further supporting a smooth increase inresidence time down gradient. Absolute ages were notcalculated in the present study since, for the interpretationof residence times, reliance is placed on the raw data aspmc.

Major Ion TrendsThe major ion relationships in the aquifer are relativelystraightforward. The ratio Na/Cl weight ratio (Fig. 7)remains almost constant around 0.6 wt%, which isconsistent with stoichiometric dissolution of halite, with-out significant reaction of silicate minerals, includingcation exchange. An exception occurs near outcrop wheresome Na/Cl enrichment suggests that albite dissolutionmay be from initial weathering.

Fig. 5 a d18O and d2H plottedagainst distance along the lineof section; the waters of Holo-cene and late Pleistocene ageare distinguished. b d2H vs.d18O relationship for ground-waters in the CT aquifer inrelation to modern rainfall atAin Oussera

Fig. 6 d13C and 14C plotted against distance along the line ofsection

490


The linear increase of calcium and sulphate concen-trations from south to north indicates gypsum dissolution.The waters become gypsum-saturated at the dischargearea in the vicinity of the Chotts (Table 2). Magnesiumalso increases along the profile, as reflected throughoutmuch of the section by relatively constant Mg/Ca ratios.The latter are thought to be controlled by dissolution ofdolomite in the sands. The waters are also at or nearcalcite saturation (Table 2). Gypsum dissolution con-tributes Ca to the waters, maintaining the calcite equilib-rium and forcing some calcite precipitation due to thecommon ion effect.

Redox Relationships and Trace MetalsAlthough neither redox potential nor dissolved oxygenwere measured in this investigation, it is clear from thedistribution of redox-controlled species that oxidisingconditions are maintained within the aquifer (Fig. 8).Thus, the concentrations of total dissolved iron are low(below 0.1 mg l�1 Fe2+), signifying that at the field-measured pH (mean 7.5) the Eh would be expected to beabove 100 mV (Hem 1985). Significantly, the concentra-tions of nitrate are high and indicate that aerobic

conditions are maintained, since nitrate is rapidly reducedin the absence of dissolved oxygen (Edmunds et al. 1984).The high nitrate concentrations are typical of manygroundwaters from continental basin sediments in northAfrica and elsewhere (Edmunds and Gaye 1997). Theyare interpreted as evidence of N-fixing vegetation in therecharge areas at the time of recharge. Concentrationsrange up to 16.8 mg l�1 (Table 1), although there is nopattern relating to the flow line, suggesting that variablecontributions from soils have taken place throughout therecharge period. As elsewhere, in some locations in theContinental Terminal NO3-N concentrations thereforeexceed stated potable limits (maximum 16.8 mg l�1 atRhourde El-Baguel) due to natural processes.

The distributions of some trace metals shown in Fig. 8(and Table 1) are also controlled by the groundwaterredox status. Under the oxidising conditions throughoutthe CT aquifer, concentrations of Mn, Cr, and U aremoderately enriched throughout the aquifer, althoughthere is no distinct trend across the flow path. Theconcentrations of Cr are all above detection limits, andvary in the range 4.0–37.5 mg l�1. The uranium concen-trations show values of 2.9–7.7 g l�1. This enrichment is

Fig. 7 Cross section from south to north along 700 km of the CTaquifer for major elements

Fig. 8 Cross section from south to north along 700 km of the CTaquifer for redox parameters

491


also a feature of aerobic groundwaters in the ContinentalIntercalaire aquifer from Algeria/Tunisia where the Crconcentrations are even higher, ranging up to 74 g l�1

(Edmunds et al. 2003a). Molybdenum and nickel con-centrations, in contrast to U and Cr, increase across theflow line. Mo and Ni concentrations increase stronglyfrom >4 to 17 g l�1 and from 8 to 32 g l�1 respectively(Table 1), and this implies that the uptake on thegroundwaters is time-dependent under the oxidisingconditions, in contrast to the U and Cr which show nooverall trend. The concentrations of other metals includ-ing Cd, Pb, Cu and Co remain low (Table 2), mainlyreflecting a lower geochemical abundance and/or lowermobility under these pH–Eh conditions. Zn concentra-tions lie within a range 15–100 g l�1 and show a slighttendency for higher concentrations in the south of thearea, possibly reflecting a source control.

Controls on Non-metal Trace Element OccurrenceThe trends in other non-metal trace element concentra-tions across the aquifer provide additional insight into theevolution of the groundwater. An increase in Sr across theaquifer (Fig. 9) takes place from around 1,000 to14,000 g l�1 in the vicinity of the Chotts, well correlated

Table 2 Saturation indices forthe Complexe Terminal aquifer

Locality Calcite Dolomite Gypsum Anhydrite Celestite Barite Fluorite

Saturation index

El-Alia 0.38 0.69 �0.26 �0.46 �0.11 �0.19DASE 0.80 1.62 �0.30 �0.51 �3.35 �0.06 �0.36Blidet Omar 1964 0.09 0.14 �0.31 �0.53 �3.31 �0.08 �0.27Sidi-Slimane 0.41 0.65 �0.29 �0.51 �3.31 �0.06 �0.11Touggourt Ville 0.30 0.52 �0.27 �0.50 �3.27 �0.12 �0.20Djam�a (Ain Zerrouk) 0.43 0.78 �0.10 �0.33 �3.10 �0.01 0.04Sidi-Khellil 0.51 0.88 �0.23 �0.45 �3.27 �0.02 0.00El-Megha�er (El Alia) 1987 0.49 0.83 �0.42 �0.65 �3.46 �0.11 0.00El Megha�er 0.48 0.81 �0.39 �0.61 �3.42 0.02 0.13Khchem Er’rih 0.17 0.36 �0.49 �0.71 �3.51 �0.15 �0.43Sidi-Belkheir 0.36 0.72 �0.46 �0.68 �3.54 �0.10 �0.54El-Bekrat 0.02 0.07 �0.48 �0.70 �3.56 �0.15 �0.55Istikama

(Hassi Ben Abdellah)0.16 0.31 �0.49 �0.70 �3.59 �0.04 �0.64

Ain-Djrad 0.32 0.53 �0.55 �0.76 �3.64 �0.13 �0.62Gassi-Touil, GT3 �0.89 �1.93 �0.69 �0.90 �3.82 �0.17 �1.39Gassi-Touil, HT4 �0.77 �1.69 �0.61 �0.81 �3.72 �0.12 �1.32Gassi-Touil, GT2 �0.20 �0.54 �0.55 �0.74 �3.63 �0.14 �1.34Gassi-Touil, M3 �0.49 �1.12 �0.54 �0.74 �3.62 �0.13 �1.25Gassi-Touil, P 0.04 �0.04 �0.66 �0.86 �3.80 �0.14 �1.44Rhourde El Baguel, MP103 �0.49 �1.03 �0.73 �0.94 �3.52 �0.07 �0.66Rhourde El Baguel, MP106 �0.40 �0.85 �0.74 �0.95 �3.56 �0.07 �0.61Rhourde El Baguel, P1 �0.31 �0.83 �0.64 �0.84 �3.45 0.12 �0.47Rhourde El Baguel, MP105 �0.14 �0.31 �0.62 �0.82 �3.41 0.02 �0.65Hassi-Messaoud Sagra, S1 0.39 0.67 �0.25 �0.45 �3.30 0.03 �0.32Hassi-Messaoud, H2 0.71 1.30 �0.44 �0.31 �3.21 �0.52 �0.69Djam�a Sidi-Yahia, MP5 �0.17 �0.02 �0.20 �0.42 �3.21 �0.07 �0.11Hamra�a, HAM6 0.05 0.10 �0.52 �0.70 �3.44 �0.13 �0.23Hamra�a, HAM4 �0.06 �0.15 �0.50 �0.68 �3.43 �0.13 �0.21M’Guebra, GUEB 0.20 0.32 �0.34 �0.54 �3.33 �0.13 �0.07Rhourde Nouss, RN15 �0.28 �0.70 �0.70 �0.92 �3.56 0.02Rhourde Nouss, RN17 0.22 0.38 �0.64 �0.85 �3.47 �0.04Rhourde Nouss, ALCIM 0.12 0.14 �1.24 �1.46 �4.22 �0.10El-Hamra, HRA 0.14 0.22 �1.19 �1.39 �4.44 �0.13El-Hamra, HRA1 0.31 0.53 �1.03 �1.24 �4.26 �0.10Rhourde Nouss, St. Pompage 0.30 0.40 �0.24 �0.47 �3.46 0.00

Fig. 9 Cross section from south to north along 700 km of the CTaquifer for Sr, Sr/Ca and 87Sr/86Sr

492


with Ca (as well as with sulphate) which indicates acontribution from intra-formational evaporites. Muchhigher Sr/Ca ratios are found in the south of the oilfieldarea of El-Hamra (60 km downstream of the flow line),and in the region around Ouargla (350–400 km) wheresalinity is still relatively low, and suggest that an initialstrontium-enriched source is present. This is also sug-gested by the high 87Sr/86Sr ratios which indicate that amore radiogenic Sr source is involved. In the south, theMio–Pliocene formations are composed of sands with thinlayers of gravels and white yellowish limestone anddolomite (Sonatrach 1992). In addition, the Mio–Pliocenesands lie directly over the dolomitic Senonian formationsin the vicinity of Ouargla (Bel and Cuche 1970). It isconsidered that the enriched 87Sr/86Sr ratios indicateweathering of silicate minerals derived from basementrocks to the south and their weathering products. Themaximum concentrations of Sr are closer to saturationwith respect to celestite (Table 2).

There is a progressive increase in fluoride concentra-tions across the aquifer, from 0.6 to 2.75 mg l�1 F (Fig. 4).This smooth increase indicates an uptake of fluoride,probably from traces of carbonate in the aquifer. It alsohelps to confirm the age relationships as well as the flowrelationships, suggesting that there are no major sourcesof external groundwater. At the higher concentrationstowards the Chotts, the upper limits are controlled bysaturation with fluorite, as calculated with PHREEQC.The concentrations of iodide vary in the range 30–200 g l�1 across the aquifer (Table 1), and the increase ismainly related to an increase in Cl (as expressed as I/Cl).The I/Cl ratio is very low and at least one order ofmagnitude lower than seawater, signifying a non-marineevaporite source.

Barium concentrations are low (Table 1), and aremaintained at levels not exceeding 28 g l�1 by baritesaturation (Table 2). The concentrations of lithium (also Band Rb) are found, in the absence of any solubility limits,

to increase progressively across the aquifer along its flowline (Table 1; Fig. 10). Lithium is usually a good indicatorof lithofacies and of the extent of water–rock interaction,and has been used to indicate groundwater residencetimes (Edmunds and Smedley 2000). Concentrations of Lirange from 40 g l�1 up-gradient to 180 g l�1 in theChotts region. In Fig. 10, the concentrations of Li and alsothe Li/Cl ratios are plotted along the flow line. Nearoutcrop the Li/Cl ratios are relatively high (correspondingto the weathering of silicate minerals), but the overallincrease in Li along the flow line is masked by the overallsalinity increase. This contrasts with Li behaviour in theunderlying CI aquifer (Edmunds et al. 2003b), where theLi/Cl ratio increases linearly from 0.0002 to 0.0006 andmay be used as a supplementary residence time indicator.

Discussion and Conclusions

The evolution of groundwater in the CT aquifer has beendescribed for a transect approximately along the modernflow direction over a distance of some 700 km, from anarea which today is completely arid but which is knownfrom past climatic records to have been wetter in the latePleistocene and Holocene (Petit Maire and Riser 1982;Gasse 2000; Maley 2000). The groundwaters in the CTand other aquifers also contain sequential evidence of pastclimate changes. The radiocarbon data show a smoothtrend, from 38.9 to 0% modern carbon, from the formerrecharge area in the south to the discharge area in the areaof the Chotts.

The results from the CT may be compared with theregional trends recorded in the stable isotope record indated groundwaters across northern Africa (Sonntag et al.1979; Edmunds et al. 2003b). Air mass circulation overAfrica during the late Pleistocene was significantlydifferent from the present day, with evidence of areinforcement and southward shift of the Atlantic west-erly flow across the present Sahara during the period. Acorresponding decline of monsoon rains also occurred atthis time. Evidence then is found for a northwardextension of the African monsoon, with increased rainfallintensity notably during the early to mid-Holocene,coinciding with a retreat of the Atlantic system to thenorth. The extent of cooling at the LGM recorded in thenoble gas ratios was up to 7 �C (Guendouz et al. 1997).The groundwater isotopic evidence in different placesrecords strong variations in humidity of the air massessupplying moisture across the continent at different timesover the past 30,000 years.

The range in pmc (0–8.4) in the deeper groundwaterscorresponds with late Pleistocene recharge, although therethen follows a hiatus in the data (Table 1, Fig. 11), withno results in the range 10–20 pmc. This hiatus has beeninterpreted in data from across the Sahara as a gap in therecharge conditions coincident with the last glacialmaximum when hyperarid but cool conditions extendedover the Sahara. The evidence of groundwater in therange 24.7–38.9 pmc signifies Holocene recharge, possi-

Fig. 10 Cross section from south to north along 700 km of the CTaquifer for Li, Li/Cl

493


bly connected with the northward extension of monsoonrains from the south.

It is apparent from the limited 14C data that waters ofHolocene age are found in the profile to a distance ofsome 400 km from outcrop, with late Pleistocene watersin the deeper aquifer (compare Figs. 6 and 8). Mostgroundwaters in the first 400 km are isotopically enriched(d18O values around �6‰), in contrast with most watersin the late Pleistocene which have lighter compositions(d18O around �7.5‰). Some anomalies are likely to bedue to mixing of shallow and deep waters, but it isunlikely that upward leakage from the CI aquifer isunimportant.

These conclusions are in line with the general obser-vations across north Africa which show that mostHolocene groundwaters are enriched isotopically com-pared with the late Pleistocene. It remains difficult,however, to explain the relative oxygen enrichment(relative to deuterium) of all the groundwaters in theCT aquifer. In Fig. 6 it can be seen that the watersextrapolate towards an intercept of near �11‰ d18O withthe GMWL. This is consistent with regional observations,especially in the NE Sahara (Edmunds et al. 2003b). Insouthern Libya, for example, values as low as �11.5‰d18O are found in late Pleistocene waters (possiblyassociated with runoff from the Tibesti mountains), andstrongly depleted waters also are found in Sudan andEgypt, associated with the monsoon advance northwardsduring the Holocene. Thus, depleted values close to theGMWL can be explained by rainfall or runoff which mayhave then been subjected to evaporation prior to or duringrecharge. A similar evaporative explanation has beenproposed for the CT aquifer in the Great Occidental Erg(western Algeria) by other authors (Conrad and Fontes1970; Gonfiantini et al. 1974; Fontes et al. 1986).

Geochemical trends in the groundwater along the flowpath indicate that significant water–rock interaction istaking place within the basin. There is an increase inmineralisation along the direction of flow. The salinity,expressed as Cl, increases from 200 to around 2,000 mg l�1

from south to north, and the trace element concentrationsreinforce major element trends to help interpret the mainprocesses taking place in the aquifer. At outcrop the low-salinity waters retain some properties of evaporated rains(supporting the concept of isotopic enrichment), but thelow Br/Cl ratios indicate that dissolution of halite isimportant, firstly, as a component of atmospheric inputsand, secondly, through some dissolution of halite inevaporites within the Mio–Pliocene sequence. Gypsumdissolution is also important in controlling the watercomposition.

The smooth increase in elements such as F and Li(which show conservative behaviour at least as far assolubility limits are reached) suggests that the influenceof sources of water external to the flow line is minimal;oscillations in chemistry along the flow line can mainlybe explained by mixing of stratified waters. Slightly moreradiogenic strontium isotope ratios indicate some inputfrom silicate weathering in the recharge areas, butotherwise reflect the dissolution of gypsum or calcite.

Redox is an important control on the groundwaterchemistry, and oxidising conditions prevail throughoutthe CT aquifer. Nitrate concentrations are consistentlyhigh, sometimes exceeding potability limits, and are inline with concentrations found elsewhere in north Africangroundwaters. These high-nitrate groundwaters are con-sidered to result from N-fixing vegetation existing in theformer recharge areas.

These findings have direct impacts on the groundwaterresource development of the CT in the Great Oriental Erg.The groundwater resources are shown to be non-renew-

Fig. 11 Plot of d18O against 14C(pmc) for all groundwaters forwhich age information is avail-able

494


able and therefore their development, as in several otherbasins of northern Africa, must be conducted on the basisof mining. The quality is only marginal for development,unlike the underlying CI aquifer which has lower salinity.The CI and CT represent a vast resource which needscareful protection and conservation. It is importantespecially that the huge oilfield and other developmentsin this region also recognise the vulnerability of theunconfined CT aquifer.

Acknowledgements The work was carried out within the frame-work of a project partly supported by the European Commissionunder the programme Avicenne (contract no. CT 93AVI0015). Wethank Sonatrach for hosting us while in the field for sampling. Wethank Fiona Darbyshire (NIGL Keyworth) for conducting strontiumisotope analyses, and the staff of the Centre de Recherche Nucl�aired’Alger (CRNA) for other isotope analyses. Additional radiocarbonanalysis was conducted through the NERC Radiocarbon Labora-tory, East Kilbride, under research allocation 779/0119. This paperis published with the permission of the directors of the CRNA andthe British Geological Survey (Natural Environment ResearchCouncil).

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