Geodin_acta_2005

Geodinamica Acta 1815 (2005) 389400

Genesis of an endoreic piezometric coastal depression in sub-Sahelian Western Africa: tlie Continental Terminal Aquifer of Trarza (Mauritania).

Mjchel Lacroix *, Bakari Séméga L«ho GIS, UllilvuifCI CIC, Nîrnes, 150, Rue G. Besse, 30035 Nîrrres

Ul~il>er.\ité (le h~o~~~~!ichott . FST, B.P. 5026 - Nouokcliott. Mouritortie

Received: 30109104, accepted: 21/09/05

Abstract

Despite its capital importance. the genesis of endoreic water table depressions in Western sub-Saheliaii Afsica is not well known yet. It is the case of the Trarza Aqiiifer, which is the inain resoiirce of cIRnkable water for Nouakchott. While water needs are growing steadily, global waniiing is threateniiig this resoiirce. The geiiesis of piezoinetric depression is the object of this si-iicly, which analyses it in the light of the hydrochemistry of the westeni border of the Trarza aquifer, which is located in proxiinity of the Atlantic Ocean and its salt wedge.

The hydrocheniical facies of grouiidwater in the Idini puniping site, 60 kin Eart of Nouakchott, aiid that of a Nouakchott to the Senegal River transect, should be categorised into 3 groiips: 1) chloro-siilfated 2) sulfnto-clilorinated and chloro-bicarbonated. Such facies, which are also sodocalcic, indicnte that niiiieralizntion is of tiiixecl origin (marine and continental).

After reviewing the main niatheinatical models for piezoinetric clepression, the present stiidy atteinpts a conceptiialization of the pie- zoriietiic evolution of this phreatic aqiiifer (Coiitiriental Terininal) during the Qiiateinary. It highlights the dominant role of evaporation in the evoliition of the wnter table of the Trarza aqiiifer, whose iinpact is siiiiilar to the one of the Qiiatemary marine level increase.

O 2005 Lavoisier SAS. All rights resesved.

fiil~oi-<ls: Eiidoreic coastal aquifer; Quatemary; Sea level; Evnpotranspiration; Mauritnnia

The piezonietric depressions of Sahelian Africa stasted being wiclely stiidied in the early 1950s, and became the object of scientific papers in 1960 [l]. They cari occur close to the Atlantic Ocean, as in the cases of the Ferlo Aquifer in Senegal [2] and the Trarza one in h4auritania. as well as inland, as epitoniized by the trough of Nara, by Gondo and Azouad in Mali 131, by the depressions of Kadzell and by the siiiroundings of the Lake Tchad in Niger [4].

The different hypotheses on tlie existence of these endoreic aqiiifers [ 5 ] are: 1) overexploitntion, 2) drainage to a deeper aqiiifer 3) geological siibsidence 4) sea level changes

5) evapotranspiration. Isotopic data, piezometric shidies and rnathematical modelling underline the importance of the evapotranspiration phenomenon. D u e to Global Warming, the depression process tends to increase, and the water tables in the aquifers is dropping drastically.

In the case of coastal aquifers, the increase of the hydraulic gradient towards inland should cause the progression of the marine salt wedge and the coritarnination of these fresh water resources. Various authors ([6]; [7]; [SI; [9]; [IO]; [I I l ) have identifïed several processes, such as hydrogeochemical exchange, riiigation, cliff~ision and sediment djssolution, which govern continental fresh water and saline marine water interactions in coastal aquifers. The nuxing zone is a dynamic

* Corresponding aothor. tel: 33 4 66 70 99 72 fax: 33 4 66 70 99 89 E-m<ril oddress: nuchel.lacroix@i~nin~es.fr

O 3,005 Lavoisier SAS. All rights reseived.

390 M. Loci-oiu rt al. / Gc~o(1inuiniccr

area of ti.anspo1-t and exchange, alhere many differeiit niech- anisms occur, incliiding water-rock aiid \\,ater-water interactions, and chemical reactions between watcr aiid soil ([12]; [13], [14]). The phenoiiîena of migration aiid diff~ision occur in al1 of these 3 phases. In the transition zone, a sea alater incursion in ail aquifer results in a loss of sodiiim, and in an increase in iiiagnesiuin and calciiim ([14]; [15], [16]), with a corsesponding recluction in the NdCI ratio.

In Mauritaiiia, the socio-ecorioriiic impact of salinisation is extremely inipoi-t;iiît; the ldini piirnping field in the Trarza aqiiifer siipplies cli-iiikable water to Nouakchott, whose pop- iilation grew froii: 20000 inhabitants in the 1960s to 800,000 iiihabitaiits in 2003, and is likely IO continue srowing steadily. This paper, \szhich is drawn from BM Sérnéga's Ph.D thesis. fiiriher esploi.es the origin of this piezometric depression in the light of its Iiydrochemical characteristics, focussint on the western part of the Trarza aqiiifer, which is close to the Atlantic Ocean salt wedge.

2. Geogr:ipliical, geological and hydrogeological context

The Trarza, a region in the south-west of Mauritania, is a peneplain locatecl approxin~ately between the 16"30' and 18"30' parallels and the 14" and 16" mei-idians. Vast areas of tliis regioii are covered by parallel, fixed dunes, oriented South-West Nosth-East, and they are separated by flat, clay- hotton~ed \ialleys. The climate is hot aiid dry, Sahaiian to the no1111 of Nouakchott, Sahelian to the South of this latitude, and is characterized by a long, dry season ninning from Octo- ber to June, with a short rainy mid-season froni July to Septembcr. Rainfalls are tenuous and irregular over both time and space, with signifiant inter-annual differences. An inter- tropical front may occasionally reach the South of Mauntania and resiilt in a season of siirimer "monsoon" rains. The water balance of this region is exîremely negative. There is no per- manent hydrographie network. Average evapotraiispiration measured with a Piche evaporometer is 2359 mm.yrlin Noiiakchott and 3S9Srnrn.yri in Rosso. Daily temperatures are high, with maximum pe'aks in Jurie (monthly inter-annual averages 1960-1995: 34.5"C in Nouakchott, 40.5"C in Bou- tiliinitt and 39°C in Rosso) and in October (36.5"C at Nouakchott, 38°C in Boutilimitt and 37.50°C in Rosso). The Haimattan wind blows during the dry season; this raises tem- peratiire, decreases hurilidity, and increases evaporation. The tropical easterly trade wind, fresh and humid, moderates temperature on the coasts, while the dry continental trade wind increases it. Vegetative cover is slight to non-existent. It consists esseritially in shrub-like, thorny vegetation, with stands of tamarisk and halophilic plants at the eclge of the beach and on less-saline soil. Its dunes and bai-reii sebkhras can also present bushes paired with eiiphorbia.

The studied area is located in the coastal sedimentary basin, bordered by oiitcrops of nietamorphic and granitic craton to the east, the Atlantic Ocean to the West, and the Seriegal River to the South (Fig. 1). This basin is situated in

the depression of the Senegalese-Mailritanian Gulf at the edge of the Africaii platform, and coiitains both inarine and continental secliinents. Quaternary masine and lacustrine cleposits are heterogeneous, and present rapid lateral variation. This cover consists essentially in the saline and gypsiferous clays of the shore terrace of Aftout es Saheli, the lacustrine carbonates of R'Kiz Lake, the alluvial and clayey sands of the Senegal River valley and its tributaries, and the Aeolian Sand of the red dunes of Trarza. The Continental Teinunal, one of the riîain geological facies of the sedimen- taiy basin, is coniposed of a detrital group of clay, coarse saiid, fine clayey sailds, sandstones and niulticoloured clayey sands ailanged in lenses or locally continuous layers. The R'Kiz ridge rnaiks the limit with thin (100 m) cletrital littoral or contii~ental marine facies sedinients iri the East and thicker, clayey fonnations in the West: 480 m at Iclini and more than 5,000n1 near Nouakchott ([17]; [18]). In Idini, series of clayey sandstones, with coarse-grained saiids on the top and clays at the bottom, lay over coarse-grained sands 150 m thick. Vaiiegated sands with soft sandstones and clay at the top of the iinit, conipose the thinner layers of sediment (40 to 80 m) of the Southwest (Tiguent-Rosso-R'Kiz).

The Trarza aquifer is niultilayered. In Idini, the layers consist of weakly mineralized groundwater (1000 mg.1-l) from 10 to 40 m, of a fresh siibphreatic groundwater unit (150 to 400 rng.1-l) from 60 to 90 m, and of a deeper (150 to 170 m) saline (4000 n1g.1-') unit. In the Tigiient-Rosso area, the acluifer contains only phreatic groundwater and occa- sional saline bodies. The elevation of the Trarza groundwater table is concordant with sea level in the West, and declines moriotonically in the East, and then rises again as it approaches the crystalline craton. To the south, the piezometric sui-face is concordant with the Senegal River alluvial groiindwater. The water table is always below the sea level and reaches its maximum depression (-40 m) at Aguilal Faye. The hydraulic gradient is between 0.1 to 1%0 and 5 %O at the bank of the Senegal River, in the direction of Aguilal Faye. The recharge of the aquifer is controlled by 1) horizontal fluxes from the Ocean and from the Senegal River, 2) drainage from the Brakna and Amchetil aquifers 3) infiltration from ninoff on the craton, and from rainfall.

3. Sampling and methods of analysis

The areas of Idini and Tiguent-Rosso (Fig. 2) are adjacent to broad coastal sabkhas and correspond to the limits of manne influence on the Trarza groundwater. The saline wedge that borders these zones to the West is a diffuse band several dozen kilometres wide. In the region of Idini, at the Southeast border of the N'Drahmcha sabkha, the saline front (1000 mg.1-l) i s located about 60 km east of the Ocean, between Hassi El Bagra and Oued Naga. The pumping site used to supply Nouakchott with drinkable water is located 5 km east of this front. The Tiguent-Rosso highway axis, some twenty kilometres from the Coast and at the Eastern

M. h c r o i . ~ et r11. /Geodir~ornicn Acrn 1815 (2005) 389400 391

. . , . . . Frrsh and salt-water contact o r the s~ ih- a . O ' plireatic Continental Terminal acluifer

C\

/ 1 Qiiaternary reçeni mariiie extensioii

I / ' * \ Qiiaiernary ancient niarine exleiision

1- 'i Axis of the bnscnient dorsale

r Watcr table in m.

c Koad

0 Q~iaiernar). sediineiits

n Continei?tal Tet.iiiinal aquifer

Eocenc aquifer

I.o\ver 1Jocene aquifer

Ordovician aiid Devonian shales

E55] Stiidy area (A Idini. F3 Axis Tigueni-Rosso)

Fig. 1 The Continental Teniuiial aqiiifer of the Trarza in the coiitext of the Mauritano-Senegnlese coastal sedimentnry basin.

I 15-30

Fig. 2 Satiiple boreholes of (A) the Idini piiniping field and (B) the Tigiieiit-Rosso road axis.

border of Aftout El Saheli, is a mixing zone of fresh and saline waters, and corresponds to the hypothetical limit of the salirie front in the Southwest of Mauntania. Samples froni the Tiguent-Rosso axis were collected nionthly and binionthly at Idini from approximntely fosty sites during the anriual cycle from September 1992 to August 1993.

(Table 1) gives the analytical methods used and their accuracy.

4. Resiilts and interpretation

The pliysico-chemical cl~aracteristics of the water samples are shown in (Table 2). Their variations over the period

of study were included into the analytical essor interval. The chemical composition of sea water off the Nouakchott Coast was also stationary. The main groundwater cations are sodiuni > calcium > niagnesium >potassium, and the main anions are chloiides > sulfates > bicarbonates > nitrates. The carbonate ion is absent. Ammonium, nitrites, bromide, and fluoride are present in significant quantities between Tiguent and Rosso and within the error bar in Idini.

4.1. Hydroclzemical facies.

Representation on a triangular Piper diagram [19] estab- lishes the ionic profiles as well as the predominant features, and allows a classification of water into hydrochemical fam- ilies. In the Piper mineralization diagram (Fig. 3-a), the anion triangle shows that the most heavily mineralized waters of Idini are grouped around the median position of the chloride and siilfate poles. This reflects the mixed chloride and sulfate-bearing character of the mineralized water and the chloride and bicarbonate-bearing character of the low mineralized water. In contrast, mineralized groundwaters from the Tiguent-Rosso axis consist of mineralized waters containing mainly chlorides, and intermediate and low mineralized waters of chloiide-sulfate and clilonde-bicarbonate character. The cations triangle (Fig. 3-b) expresses water groups of mixed calco-sodic character, generally undifferen- tiated with respect to mineralization. The tendency towards calcic character indicates ionic income from the surrounding rocks dissolution.

M. Locroii. et 01. / Geodiilnmicn Acrn 1815 (2005) 389-400

Table 1. Analytical methods

Peranieter Rlethod Precision

PH hlineralization (MZ0)

Calciuiii

Mngiiesium

Sodiiini

Potassiuni

Chlorides

Carbonates Bicarbonates

Siilfates

Nitrates

Nitrites

Ammonium

Bromides

Fluorides

pHnieter WTW 196

Determined froni the specific condiictaiice at 20 "C (C,,) Conductiviii~eter WTW 196

E.D.T.A Coniplexometry

E.D.T.A Coniplexometry

Flame Photometry JENWAY PFP 7

Flame Photometry JENWAY PFP 7

Argentornetry : MOHR metliod

Titiation : Cran method

basyum cliloride iiephelonietry Spectrophotometer Spectronic 20D (Milton Roy Company) h = 530 nm

sodiuin salicylate nephelometry Spectrophotonieter Spectroiiic 20D (Milton Roy Company) h = 415 nm

Zambelli reagent nephelometry Spectrophotonieter Spectronic 20D (Milton Roy Company) h = 430 nm

Oithophenathroline nepheloriietr)~ Spectropliotomètre Spectronic 20D (Milton Roy Conipany) h = 435

HF'LC Dionex

HF'LC Dionex

Table 2. Chernicd composition of seawater off the Coast of Notiakchott (1993) and water of Trana Continental Temiinal aquifer (pumping field of Idiiii and axis Tiguent to Rosso).C20 (p.S.cm-') :M20 (nig.1-') :lonic concentration (meq.1-').

acronym T " C pH Cm MZo Ca Mg Na K NH4 CI HCOJ S 0 4 NO2 NO, Br F

M. Lncroix et nl. / Gcorlii~aniico Actn IS/5 (2005) 389400 393

Table 2. (a<ite) Clieriiicnl coiiiposition of seawnter off tlie const of Noiinkcliott (1993) aiid \vater of Trarza Coiitinental Terniinal aquifer (piiniping field of Idini and axis Tigiient to Rosso).C20 (p.S.cni-') ;M20 (nig.1-l) ;Ioiiic coricentiation (riieq.1-').

Seawater

Fig. 3 Piper diagrams of the Trarza gioundwater from (a) Idini and from (b) the south-west from Tigiient to Rosso.

Plots of dominant NalCa ion ratios as a function of S04/CI or HCO3/C1 (Fig. 421, b) [20] deterrnines two distinct groups conesponding to the more chlorinated Tiguent-Rosso waters and the more sulfate and bicarbonate-rich Idini waters. The waters belonging to the first group show low SO,/Cl (or HC03/CI) ratios near 0.2, which is the limit separating the chloiide from chloride-sulfate or chloricle-bicarbonate regions. Tliey present a predominantly chloride character and low ionic levels of mixed chloiide-sulfates and chloride-bicar- boriate. In contrast, the water belonging to the second group clisplays ratios near 1, wliich is the limit separating the chlo-

ride-sulfate and sulfate-chloride or chloride-bicarbonate and bicarbonate-chloride facies. They display significant levels of ionic mixing; chlorjde-sulfate and chloride-bicarbonate.

The NdCa ratios generally lie between 1 and 3, placing the waters from al1 zones into the sodium-calcium region, with calciiim-sodium tendencies. The sodium-calcium and calcium-sodium chloi-ide facies, however, occur in the most saline waters. These latter tendencies are more pronounced for the Tiguent-Rosso water and in some cases they present calcium-sodium facies and therefore a strong marine character. Sodium-calcium sulfate-chloride facies, rare in Southwest water, but present in water from Idini, is amongst the main differentiators, since sodium-calcium chloride-sulfate facies reveal mixed continental-marine influence.

4.2. Distribution of mineralization and chemical facies

The spatial variation in the mineralization of the Idini basin water(l90 to 850 mg.1-'), (Fig. 5-a) shows a decreas- ing gradient in the North-west to south-east direction. The most highly mineralized waters are found in the Northwest (at about 5 km from the saline wedge) and the least mineralized waters in the Southeast. The distribution of chemical

394 M. Lncroi* et nl. / Georliiiarriico Actn 184 (2005) 389400

facies (Fig. 5-a) is siinilar to mineralization distribution and iiidicates that the Noithwest zone is the most chloride-rich (C3), the Eastern and Western zones the most sulfate-rich (Cl) and the south-west zone the richest in bicarbonates (C,).

Groundwater mineralization along the Tiguent-Rosso highway axis ranges from 334 to 3367 mg.1-', and evolves heterogeneously over distance (Fig. 5-b). Miiieralization clecreases frorn West to East (fi0111 the shoreline ont0 the con-

O ldni borehde

O a . Tigmt-Rosso wdlr

. K 3

i i l

- "b O O c chlwidecbracta sulfate d ~ r x t e ! -+ - .

colciun cbrncler O 0.5 1 1.5 SOAI 2

tinent) in the Tigiient to Tigomatine zone. It also decreases in tlie North, neai the Ocean and across the central part of the Tigomatine-Tenweiatt zone. Furthest South, mineralization decreases to the Noi-theast through the zone running from Tenweratt to pklo from Rosso. The hydrochernical profile of this groundwater along this axis evolves from sodium-calcium to calcium-sodium chloride and sodiunl-calcium chloiide-sulfate facies in the North, and from sodium-calcium to calcium-

Fis. 4 NdCa as a fiinction of SO,/CI (a) and HCO,/CI (b) for Trarza groiindwater at Idini aiid Tiguent-Rosso within the hydrocheinical facies. calciuni-sodiiiiii cliloride : socliiini-calcium chlonde calciuni-sodiiiin chloride-siilfate or bicarbonate: sodiiin~-calcium chloride-siilfate or bicarbonate calciiim-sodiiim sulfate or bicnrbonate-chloride: sodiiim-calcium sulfate or bicarboiiate-chloride.

Fig. 5 Mineralization and hydrochenGca1 facies in Idini piiinpiiig site and along the Tigeni-Rosso higway axis.

IO chloride-sulfate

calcium-sodium to sodium-calcium

sodium-calcium chloride-sulfate

/ : Tiguent-Rosso road : Well or borehole

O :Total mineralization (mg.l.1)

,'- : lsosalinity curve 1000 mg.1 -1

I( : salinitygradient 16"

M. Lacivis et al. /Geodirinn7ico Acta 1815 (2005) 389400 395

sodium chloiide-sulfate facies in the Soiith, via sodium- calciiini chloride-bicarbonate facies. Froin Tiglient to Tigo- n~atine, the chloiide and calcium-rich characters are predon~inant, wliereas froni Tigoinatine to ~ k , ~ from Rosso, water is more bicarbonate-rich and siilfate-nch.

4.3. Water saturation state

The theiniodynaniic state of Trarza water in Idini and between Tiguent and Rosso, with respect to many niineral phases. Rias deterniinecl by the WATEQF conipiiter program [20] and the satiiration index values are listed in (Table 3).

The groundwaters are generally iindersaturated, with variations depending on the niineral content. The waters are nearly in ecluilibriun~ with gypsurn and anhydrite (-2.686 < 1s < -1.229) and closest to equilibrium for calcite, aragonite, dolomite aiid magnesite (-3.66 < 1s < -0.003), with which sonie of the Tiguent-Rosso waters were saturated (0.036 < 1s < 1.876). However, for sodium, chloride and bicarbonate minerals (nahcolite, natron, halite, inirabilite and thenardite), undersaturation was significant (-10.672 < 1s < -4.020). Suii~marising, undersaturation was high for sodium bicarbonate minerals (nahcolite and natron) and was low for calcium carbonates (calcite, aragonite and dolomite).

Table 3. Sntiirntion Index with rega~d to the minerals of Traiza groundwater, from Idini and from Tiguent-Rosso axis.

scroiiyin Aiiliy~lricle G j psiini Calcite Aragoiiite Doloiiiite hlagiiesite Nahcolite Natron Halite I\liral>ilite Tlieiiardite

F 1 -2.100 -1.959 -1.003 -1.141 -1.960 -1.340 -5.120 -10.271 -6.896 -7.564 -8.128

F2 -2.041 -1.897 -0.856 -0.994 -1.713 -1.240 -5.151 -10.135 4.849 -7.513 -8.090

F3 -2.213 -2.068 -0.572 -0.710 -1.1 15 -0.926 -4.954 -9.866 -6.894 -7.699 -8.281

F4 -2.028 -1.870 -0.763 -0.902 -1.506 -1.124 4 .963 -10.003 -6.750 -7.444 -8.090

F5 -1.393 -1.243 -0.632 -0.771 -1.296 -1.046 4.828 -9.703 4 .195 -6.650 -7.254

F6 -1.531 -1.382 -0.391 -0.530 -0.754 -0.745 -4.816 -9.401 4 .207 4 .727 -7.327 V1 - F7 -1.532 -1.386 4 .756 -0.894 -1.555 -1.182 -5.081 -10.077 -6.411 -7.044 -7.625 O C

2 F8 -1.376 -1.229 -0.519 -0.658 -1.072 -0.936 4.927 -9.702 -6.217 -6.748 -7.338

5i F9 -1.683 -1.542 -0.565 -0.703 -1.179 -0.997 4.957 -9.804 4 .432 -7.120 -7.678 O - .- Y, FI0 -2.207 -2.064 -0.916 -1.054 -1.887 -1.355 -5.188 -10.368 -7.004 -7.853 -8.426 M .- FI1 -1.665 -1.529 -0.701 -0.839 -1.431 -1.1 14 -4.874 -9.819 -6.081 -6.986 -7.521 2 FI2 -1.985 -1.846 -0.684 -0.822 -1.345 -1.044 -5.044 -9.923 -6.717 -7.423 -7.973 2 .- FI3 -1.842 -1.703 -0.385 -0.523 -0.731 -0.730 -5.067 -9.666 4 .725 -7.324 -7.869 C .- Z FI4 -1.492 -1.352 -0.884 -1.022 -1.806 -1.306 -5.048 -10.073 -6.306 -6.880 -7.434

FI6 -2.173 -2.037 -0.865 -1.003 -1.810 -1.329 -5.075 -10.145 -6.964 -7.656 -8.187

FI7 -2.359 -2.217 -0.885 -1.023 -1.770 -1.269 -5.185 -10.281 -7.044 -7.951 -8.515

FI8 -2.686 -2.548 4 .833 -0.971 -1.708 -1.258 -5.416 -10.552 -7.465 -8.605 -9.150

FI9 -2.410 -2.269 -0.767 -0.905 -1.616 -1.233 -5.335 -10.480 -7.206 -8.320 -8.879

F20 -1.889 -1.745 -0.223 -0.362 -0.520 -0.679 -5.228 -9.860 -6.938 -7.719 -8.296

F23 -2.507 -2.368 -0.537 -0.676 -1.254 -1.100 -5.440 -10.253 -7.345 -8.422 -8.972

396 M. Locroi.~. et cil. / Georiiiiarnicn Acta 18/5 (2005) 389400

The dissoliition process is less advanced for Idini waters than for Tigiient-Rosso waters, some samples of which are satiirated iii calcium or nlagnesium carbonates.

In that state of saturation with respect to gypsum, anhydrite, mirabilite, thenardite and halite, Trarza groundwater sliows iniich liigher siilphate and chloride concentrations than sea water off the Noiiakchott Coast (SO,/Ca = 2.42; SO,/Na = 0.125 and CIINa = 1.35).

4.4. Onic correlations

Fresli water-saline water interaction profiles are estab- lished iising logarithmic comelations between ionic species aiid cliloi-ides (Fig. 6) and compared with sea water ([22]; [33]). \Ve used chloi-ide because this anion is considered as conservative in the hydrologic cycle [24]. Various mecha- nisms (e.g., dispersion, liquid-solid interactions, and inter- element chenucal reactions) arid competition between sol- iites and between interactions (e.g., ionic exchanges, precipitation-dissolution; reactions involving ionization, coniplexion, oxidation-reduction and biocheniical activity) may occur in parallel.

Calciiim and fluoride evolution lines are fat- above the dilution fine of sea water. Sulfate developnient is analogous to calciuin development in Idini water and is heterogeneous for the Soutliwest water, which trends towards the sea water dilution trendline as chloride concentration iiicreases. Magnesium evoliition shows a tendency towards this trendline as well. This behaviour is more pronounced in the case of the Tiguent-Rosso water, most païticiilarly in the less niineralized ulater. In the case of potassium, the tendency to approach the sea water dilution tr.enclline increases with chloride concentration. This teridency is miich inore pronounced for sodium, and the most heavily niineralized waters of Idini trend progsessively towards tliis trendliiie, while the chloride-nch waters of Tigient-Rosso are generally located iight on the trendline. The bromide evolution line can be assimilated to the sea water line due to the iincertainty about the measurements. The evolution of the over- al1 nuneralization corresponds to one of the main ions, and the evolution line lies above the sea water tsendline. The Idini waters evolve further from this trendline than those of Tiguent- Rosso. These waters do not show masine behavioiirfor calcium, sulfate and fluoride, so these ions must oiiginate mainly from water-rock interaction within the aquifer. Since these trends are almost parallel for calcium and fluoride in Trarza gsoundwater and for sulfate in Idini gsoundwater, these ions must have a continental origin. The iiiagnesium arid potassium (56.13 and 44.85 n-ieq.1-') in Idini groundwater result from a process involving inaiine water. Sodium concentrations ren~ain strongly associ- ated with marine characteristics or marine effects. Chlorides (4.43 meq.1-l) and Sodium (3.52 meq.lkJ) tend to the same on- gin, as confirrned by bromide trendline. Broniide is rarely affected by hydrological and biological cycles, as these ions are fairly conseivative in aqiiifer systems, and as bromide does not vaiy when lialite goes into solution or precipitates ([22]; [25]; [SI, P l ; [241, [231).

5. Discussion

The piezonietric depression of the Ferlo aquifer in Sen- egal has been stuclied by means of a mathematical mode1 [26] and tlie authors apply the conclusions to the Trarza aquifer. They cleveloped their argument on the basis of the theoretical foimula (1) resulting from the I D mathenlatical solution for an impulse in an infinite homogeneous medium and its propagation as a time function.

(1) h(x,t) = ho erfc ( x E) avec T et S. h(x, t). x

where t is time T is transrnissivity of the aquifer S is storage coefficient of the aqiiifer h is water table at the instant t x is distance at the impulse point erfc i s complementary error fiinction

The water table variations (Table 4) are given for an average transmissivity of IO4 m2.s-' and a storage coefficient of 1 %, assurning that the aquifer is unconfined, and that the sea level increases instantaneously of 100 meters at the bound- ary of the aquifer. The equivalent theoretical hydraulic gradient iniplies that some manne water flows towards the continent if there is no counterbalancing continental fresh water gradient.

The aiithors conipare tliese theoretical water table variations obtairiecl for the 100 m head conditions equivalent to the Ogolian (ISky), where the evaporation budget is obtained by indirect isotopic methods (180, 2H) and experi- mental resiilts are obtained in Sub-Sahelian conditions ([27], PSI).

The adaptation of Bames and Allison's [29] formula (2)

where E is evaporation loss in mm.yearl ha is relative hurnidity of the air N,,, is concentration of water vapour at saturation point D, is diff~isivity of water vapour in air p is total porosity of the soi1 O is mean value of water content z is tortuosity coefficient of the soi1 p is volunuc mass of water Zef is depth of evaporation front leads to (Table 5) estimates.

Barnes and Allison conclude that tlie endoreic character of the Ferlo aquifer (and therefore that of the Trarza one) is probably subsequent to the rise of the sea level since the Ogolian, and argue that the steady state is still not in place today in spite of the excess income in the hydrological system.

o Idiiii 0 ldini

iigucnt-Rosso .

A * * .

~.og i3r (IIII~~I.I . ' ) ( 6 ) /

q ldini Tiguent-Rosso

wf +f4 "'4

Log CI (inniol 1") 1

1

O

- 1

-2

Fig. 6 Concentration (mmol.1-') of ions and total miiieralization (mg.1-') as a functioii of chlonde concentration in the Trarza groundwater. (a): water evoliition trendline; (b): seawater dilution trendline logCa = f(logC1) (1). logMg = f(logC1) (3, logNa = f(logC1) (3) , logK = f(logC1) (4). IogSO, = f(logC1) ( 5 ) , logBr = f(logC1) (6). logF = f(logC1) (7). 1 0 g M ~ ~ = f(logC1) (8).

-1 O I 7 O

Log SOA (m111ol 1") (5)

o Idini 8 Tigueni-Rosa n.0 *@* %

- o a **.**

-

Los CI (rniiiol.~") I 1

- 1

- 1

-2

-3

0 4

~ o g I' (rnino~.~.') (7) /.-,

ldini y = 0.88~ -2.3 ,,.A' - K'= 0.72 ?,-4

Tigiiciit-Rosso (?v" P*"*

~ o g M ~ O (nig.l") (8) ,' -

/'

ldini

4 Tiguetit-Rosso 3 -

y = 1.1~-0.75

Log Cl (rng.1-') 1 - 1 1

-2 " p../b I

Cb)

O I 2 I 2 3 4

-3

-4

-

Log Cl (nimol.1-') I

398 M. Lncroix et nl. / Geodinninico Acrn I8/5 (2005) 389-400

5 kA Nouûkcli«ttiaii M'et ('ltriinte cc>

I'irçci~t sc;i lcvel - Frcch'l'rarïa Aquifer

ESE

~ ~ ~ f n i i r i i a n i d e s

h\\ Fresh \vater '/A Brnchish wrer Continental terniinal wevm Eoceiic aqiiifer Lower Eocene Basement

18 k h »!y Cliinatc 1,ow sc:i Ic\.el - Sebkhas (.ienesis

(B) - - - - . __

IV:VII* ~ ~ C i ~ o ~ a i i ~ ~ i r a t i o r i . ; ESE .-.. . _ , _ - . .-

100 kA Tatariiion 3 Inchiriün Iligh sea lewl - SnIly I r a n a Aqijiler

(A)

IK'VPV FIrE

Fig. 7 Traiza Aquifer evoliitioii during the Quatemaiy, fronl (a) Tafaritian to Inchirian, @) 18 000 years (c) Nouûkchottian (d) present

Table 4. 1 Wnter table reîpoiise (hl as a fiinction of distance (x) to n niodification of (3) h = --Ln

sen level (T = IO4 in2.s-'. S = 1%, sea level increase = 100 meters). k S

where Distance to the coast

h(x,t) t is time 20 km 50 kni 100 km 150 km h is water elevation

t = 10000 years 80 50 20 z 10 k is shape factor S is storape coefficient of the aauifer

u

t = 1000000 years 90 80 70 50 Dm is rain deficit at the time t, a = (Dm-Do)/t,

Table 5. Evaporatioii balance calciilated with Bames and Allison formula [29] as a fiinction of deptli for the T r a m Aquifer in the current climatic conditions.

Depth of water table (m) 10 20 40 60 80

Evaporatioii (mrn.yeart) 1.53 0.65 0.26 0.14 0.08

Bringing forth arguments such as seasonal water table fluctuations. isotopic enrichment increasing as the water table approaches the ground level, and the lack of centripetal gradient of l4C dating, Aranyossy and Ndiaye [SI maiiitain that vertical fluxes are dominant velails lateral fluxes. This is in agreement with the seasonal deficit of infiltration due to evapotranspiration outtake.

These authors propose an analytical so l~~t ion (3) in the case of an unconfined infinite homogeneous aquifer under over evapotranspiration conditions:

After an adaptation for the transient state, this formula gives results that are sirnilar to the gradient calculated by Dieng et al. [26], to reach an hydrodynamic balance after a 2000 years period. The adjustment of the water table is calculated with a parameter called "k factor" or "shape factor", wliich is relative to the texture and structure of geological formations.

The results of this study, based on the hydrochernical characteristics of Trarza groundwater, demonstrate the dominant role played by evapotranspiration in the genesis of the water table depression.

Due to the extremely deficitarian water budget, the pres- ence of fresh water lenses in the sub-surface in spite of the continental hydraulic gradient cannot be attributed to rainfall. The isotopic correlations 180-D ([30]; [31]) show that Trarza groundwater has the same characteristics as those of the water in the medium Senegal river valley.

:a Actn IS/5 (2005) 389400 399

The mineralization of fresh groiindwater deiives froni the clissolution of the siirrounding rocks contaminated with brines. Nevertheless, their density is always lower tlian that of niarine waters.

So the « Ghyben-Herzberg » piinciple (4) can be applied:

Due to the evapotranspiration losses of fresh water near the ground sui-face. the salt wedge migrates upwards to the oiitcrop when the evapotranspintion doiiiinates, and down- wards during sains.

A 1 % storage coefficient and a weak hoiizontal peimea- bility iriiply tliat an anniial 100 n-ini infiltration wsiilts in a 10 nieter-rise of the water table per nnnum. The impact of the marine level variation shoiild be cornpensated iii less tlian 10 years, since there shoiild be no undergroiind outlet as long as the water table is depressed.

The preseiice of water that is less mineralized than iiiaiine water (>lg.l-1), 50 nleters below the sui-face, and 50 km away from the Ocean, implies that the origin of hydraulic gradient towards the continent is not a direct effect of marine elevation. However, marine oscillations coiitiibute to keep- ing a highly elevated outlet to the aquifer, thus the water table is not far from the oiitcrop where the evapotranspiration uptake is important.

This tiiiie cliniatic variation is especially important, given the length of tlie considered penod. The paleoclimatic recon- striictions during the Quatemary ([32], [33]) show that there have been sonie more or less huinid phases, with a niaximiim water table. The sea level variations cause sonie fonvard and backward shifts of the Coast line and therefore of the salt wedge towards inland.

In Mauntania the sea level oscillations in~pnnts consist of successive coiicentric lagoon deposits reaching as far as 200 kms inland [34]. The maximum of the Tafaritian transgres- sion was up to 140 kms inland at the Nouakchott latitiide with the sea level 12 to 15 meters above the present level [35].

From a hydrogeological point of view, we can extrapolate tlie salt wedge position diiiing the Quaternaiy (Fig. 9 a, b, c d). Between the Aioiijian and the Iricliirian (IOOky), the sea level was high and the clinlate was almost Sahelian (d). The hydrological situation inherited from the Tafaritian and the Aioujian being less diy rhan at present provoked some infiltration of salted water in the western part of Trarza and the apparition of fresh water lenses in the foothill formations of the Maiirita- nides, which result from precipitations over the relief.

The late Inchiriaii (18 ky) (c) is characteiized by a mini- mum sea level drop in the Quaternary [36]. The Trarza aquifer, which contained some salty water in its Western part, and fresh water in its Eastern one, was draining off and some brine precipitated in the reservoir. The upstream pai-t was iinder evapotranspiration control, drying up and showing a depressed water table shape iike the present morphology.

The Nouakchottien (5ky) (b) climate was relatively wet, with a masine sea level equivalent to the cul-sent one. The

Trarza aqiiifer was at its maximum replenishment, and pre- sented an abiwpt salt wedge due to a strong upstream gsadient.

The fresh water lens volume was up to a n~axiniiim and the regional hydraulic gradient was oriented towards the ocean. In its downstream part, the reservoir was washed out and the water lost its marine character.

At the present time (a), the evapotranspiration is, once more, tlie dominant process. The water table is depressing and tlie salt wedge is going upward. This plienomenon is aniplified by the dnnkable water piimping field of Idini, which creates a depression cone. The mineralization of fresh water is aroiiiid 500 nig.1-l and its chemical profile is influenced by tlie dissoliition of the carbonate rocks and brines inherited from the Inchirian surroiinding.

For the short tenn evolution, in Sahelian areas, between 1O0N and 30°N latitiides, the seasonal and annual rediiction of raiiifall from 1931 to 1960 and from 1961 to 1990, ranges between 20 and 50% [37]. Although soine results of atmos- phere general circulation models in transient state can be compared with the rainfall regime change acquired between 1931 and 1990, the Hadley centse [38] provided some mode1 results for Sahelian Africa on the basis of a slight increase in the rainfall and of a slight increase in atmospheric temperature.

When evapotranspiration increases, the temperature increase stops infiltration. Locally this supplementary volume of rainfall tends to increase the nin-off due to deforestation, and thus infiltration as in the kori (dry valley) of Dantiandou in Niger [39].

So in the near future, the sub-Sahelian and particiilarly the Mauiitanian hydrological conditions are expected to remain uiichanged and the infiltration deficit is likely to increase the depression of the water table of the Trarza aquifer.

The marine and continental influences on the coastal aquifer of the Trarza lead to fairly diverse hydrochemical facies. Originally pure sodium and chlorides, they evolved towards more mixed facies: chloride-sulfate to chlonde- bicarbonate and sodium-calcium to calcium-sodium, reflect- ing bot11 marine and continental impacts included in coastal areas. The evolution towards continentality is reflected in waters containing fewer and fewer chlorides and sodium and more sulfates and calcium.

The sea level oscillations during the Quatemary and the alternation of drought and rainy phases have influenced the geometry of fresh continental water lenses. The nse of the outlet level and the precipitation rate, niore abundant during the Nouakchottien than at the present time, induced the accumu- lation of fresh water in the Trarza aquifer. After this maximum replenishinent, vapotranspiration under Sub-Sahelian climate dug out the Aquilal Faye water table depression.

The medium range climatic forecast leads us to the con- clusion that there is not going to be any improvement in Mauiitania drinkable groundwater resources.

Acknomledgements

The authors would like to thank M. Rosen and B. Mahler for tlieir precious help and the A U F for its financial support in the fosm of an Awai-d of Excellence (Do~irse d'Excellence) granted to B.M. Semega.

References

Archanibault, J. Les eaux soutei~aines de ['Afrique occidentale. (Ed.) Berger-Levrault, Nancy, France, 1960, 139p. Degalier, R. Hydrogéologie du Ferlo Septentrional. Ménioires du BRGM, 2 (1954). Depagne. J. Exploitation et interpretation des mesures piézoniétri- ques effectuées de Jiiillet 1961 à Mai 1963 dans le sud-ouest h4auritanien. la Vallée (lu R'Kiz et le Lac d'Aleg. BRGM rapport DAK63 A 14, Orléans, France, 1963. Cliouret A., Foiites JC., Mathieu P. La nappe pliréatiqiie à la pénplié- rie du Lac Tchad. Rapport inteine IRD, Montpellier, France 1977. Aranyossy JF., Ndiaye B.,. Etirde et niodélisation de la fotniation des dépressions piCzométriques en Afrique Sahélienne. Revue des Sciences de 1'Lzu 6 (1993) 81-96. Mania. J., Imerzoukeiie, S., Braillon, J.-M., Poll~ition saline de la nappe côtière à l'est d'Alger. Hydrogéologie 3 (1985).213-226. Price. R.-M.. Herr~ian. J.-S., Geochemical in\~estigatioii of salt-water intiusion iiito a coastal carbonate aquifer. Mallorca, Spain. Geol. Soc. Am. Bull. 103 (1991) 1270-1279. Hovorka. S.-D., Paul Knouth. L., Stephen. Fisher. R., Gao. G., Mariiie to nonmarine facies transition in Pernlian evaporites of the Palo Duro Basiri Texas: Geochemical response. Geol. Soc. Am. Bull. 105 (1993) 1119-1 134. Roseiitlial. E., Saline groundwater in Israel: its bearing on the water crisis in the country. J. of Hydrology, 156 (1994) 389-430. Wicks. C.-M., Herman. J.-S., Randazzo A.-F., Jee. J.-L.,. Water- iock interactions in a modem coastal nuxing zone. Geol. Soc. Ani. Bull., 107 9 (1995) 1023-1032. Hiscock. K. M., Dennis P. F. Saynor. P. R., Thomas M. O... Hydro- cheniical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK, J. of Hydrology 180 (1996) 79-107. Biissaud. F, Coiichat. P.,. Interactions liquide-solide et migration des solutCs en milieu poreux saturé. Bull. BRGM. (2) III 3 (1978) 293-301. Rochon. J., Principaux mécanismes physico-chimiques causant la disparition de siibstances miscibles dans les sous-sols. Bull. BRGM. (2) III 4 (1978) 303-309. Nadler. M, Margaritz, A., Mazor, E., Chenucal reactions of seawater rocks and freshwater: experiniental aiid field observations on brack- isli waters in Israel. Geochimica et Cosniochimica Acta 44 (1980) 879-886. Lacroix. G.. Ciistodio, E., Ganoulis, J., Potie, L., Utilisation ration- nelle des ressources en eau dans les régions côtières. Bull. BRGM. (2). III. 2 (1982) 185-196. Petalas. C. P., Dianiantis 1. B., Origin and distribution of saline groundwaters in the iipper Miocene aqiiifer system, coastal Rhodope area. northeastem Greece. Hydrogeology. J., 7 (1999).305-3 16. Clouet D'oival. M., Moulaye. A., Petit. J.-P., Entre l'océan et le désert, à quel prix la capitale mauritanienne petit-elle assurer son alimentation en eau? Bull. BRGM., (2) III 4 1981) 361-368. Puyoo. S., Schwartz. J., Alimentation en eau potable de Noiiakchoa: études pour le renforcement et l'extension du champ captant d'Idini.

Rapport BRGM 87, MRT 040. Ministère de l'Hydraulique et de 1'Eiiergie-Sonelec Mauritanie, 1987. Piper. M., A graphic procedure iii the geocheinical interpretation of atater arialyses. Transactions American geophysical Union, Papers Hydrology (1944) 914-928. SéniCga. B., Interactions physico-cluriuqiies des eaux de la nappe côtière du Traiza (Mauritanie) h Idini et le long dii littoral siid. thkse UniversitC de Nice (1995) 232 p. Plumn~er. L. N., Jones. B. F., Truesdell. A. H., WATEQF-A Fortran IV version of WATEQ, a computer program for calculating chemical equilibriiini in natunl waters. U.S., Geol. Surv., Water Ressour. Invest., Pap. (1976). 76-31. Foiites. J.-C., Andrews. J.-N., Edmunds. W.-M., Guerre. A., Travi. Y.. Paleorecharge by the Niger River (Mali) deduced from ground- watei- geocheiiiistry. Water. Ress. Research 27 2 (1991). 199-214. Edniunds. W. M., Robins. N. S., Sliand. P., The saline waters of Llaiitlrindod and Biiilth, Central Wales. Journal of the Geological Society. London 155 (1998) 627-637. Andreasen. D. C., Fleck WB., Use of broiiude: chlonde ratios to dif- ferentiate potential sources of chloride in a shallow, unconfined aquifer affected by brackish-water intrusion, Hydrogeology Journal 5 2 (1997) 17-26. Sti~nîon. J., Rudolph. D., Farvolden. F., Causes of groundwater sali- nization in a low lyiiig area of Cochabamba Valley, Bolivia. Proc. Syiiip. Vienna, IAEA-SM-329126 (1993) 185-198. Dieng, B., Ledoux, E, De Marsily G., Paleohydrogeology of the Senegal sedinlentary basin: a tentative explanation of the piezometric depressions. J. of Hydrol. 118 (1990) 357-371. Allison GB., Baines CJ., Hughes MV., The distribution of deuterium aiid 180 in dry soils (expenmental). J. Hydrol. 64: (1983).377- 397. Fontes JC., Yoiisfi M., Allison GB., Estimation of long t e m diffu- sive groundwater discharge in the Northern Sahara using stable isotopes profiles in soi1 water. J. Hydrol. 86. (1986) 315-326. Barnes CJ., Allisoon GB., The distribution of deuterium and 180 in diy soils (theory). J. Hydrol. 60 (1983) 141-146. Diagaiia A., Etudes hydrogéologiques dans la vailte du Fleuve Sénégal de Podor à Bakel : relations eaux de surface et eaux souter- raines. Doctorat Université Dakar, Stnégal. (1994) 130p. Sériiéga. B.. Lacroix M., Wang HQ., Dupont JP.. Duality between salt wedge or alteration of the bedrock to assess the salinisation of the coastal aquifer of Traiza (Mauritania). In: Oliver Sililo et al. (Eds.), Balkenia (Ed.), Amsterdam, 2000, pp.611-614. Shackelton NJ., The 100 000-year ice-age cycle identified and found to lag temperature. CO2 and orbit11 eccentricity. Sciences 289 (2000) 1897- 1902. Jouzel J., Climat du passé (400000 ans) : des temps géologiques la dérive actuelle. C.R. Geosciences 335 (2003) 509-524. Elouard P., Formations sidimentaires de Mauritanie Atlantique. Notice explicative 111 000 000 de la Mauritanie. BRGM, Paris. (1975) 171-255. Giresse P., Barusseau JP., Causse C., Diouf B., Succession of sea- level changes diiring the Pleistocene in Mauritania and Senegal dis- tinguishetl by sedimentary facies study and U/Th dating. Marine Geology 170 (2000) 123-139. Michel, P., Les bassins des fleuves Sénégal et GAmbie. Etude géo- morphologique. Rapport IRD, Paris. France, 63 (1973) 752p. Hulme hl., Marsh R., Jones PP., Global changes in a hiimidity index betweeii 1993-1960 and 1961-1990. Clim. Res. 2: (1992) 1-22. Hadley Centre, Modelling Climate Change, 1860-2050. Hadley Centre, UK Met. Office, Feb. 1995, 1995, 12 p. Favreau G., Lediic C., Marlin C., Guéro A. Une dépression piézomé- trique naturelle en hausse au Sahel (Sud-Ouest du Niger). CR Geosciences 334 (2000) 395-401.

394 M. Lncroi* et nl. / Georliiiarriico Actn 184 (2005) 389400

facies (Fig. 5-a) is siinilar to mineralization distribution and iiidicates that the Noithwest zone is the most chloride-rich (C3), the Eastern and Western zones the most sulfate-rich (Cl) and the south-west zone the richest in bicarbonates (C,).

Groundwater mineralization along the Tiguent-Rosso highway axis ranges from 334 to 3367 mg.1-', and evolves heterogeneously over distance (Fig. 5-b). Miiieralization clecreases frorn West to East (fi0111 the shoreline ont0 the con-

O ldni borehde

O a . Tigmt-Rosso wdlr

. K 3

i i l

- "b O O c chlwidecbracta sulfate d ~ r x t e ! -+ - .

colciun cbrncler O 0.5 1 1.5 SOAI 2

tinent) in the Tigiient to Tigomatine zone. It also decreases in tlie North, neai the Ocean and across the central part of the Tigomatine-Tenweiatt zone. Furthest South, mineralization decreases to the Noi-theast through the zone running from Tenweratt to pklo from Rosso. The hydrochernical profile of this groundwater along this axis evolves from sodium-calcium to calcium-sodium chloride and sodiunl-calcium chloiide-sulfate facies in the North, and from sodium-calcium to calcium-

Fis. 4 NdCa as a fiinction of SO,/CI (a) and HCO,/CI (b) for Trarza groiindwater at Idini aiid Tiguent-Rosso within the hydrocheinical facies. calciuni-sodiiiiii cliloride : socliiini-calcium chlonde calciuni-sodiiiin chloride-siilfate or bicarbonate: sodiiin~-calcium chloride-siilfate or bicarbonate calciiim-sodiiim sulfate or bicnrbonate-chloride: sodiiim-calcium sulfate or bicarboiiate-chloride.

Fig. 5 Mineralization and hydrochenGca1 facies in Idini piiinpiiig site and along the Tigeni-Rosso higway axis.

IO chloride-sulfate

calcium-sodium to sodium-calcium

sodium-calcium chloride-sulfate

/ : Tiguent-Rosso road : Well or borehole

O :Total mineralization (mg.l.1)

,'- : lsosalinity curve 1000 mg.1 -1

I( : salinitygradient 16"

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