Identification of dominant hydrogeochemical processes for groundwaters in the Algerian Sahara ...

23 Pages·2017·7.61 MB·English

Hydrol.EarthSyst.Sci.,21,1669–1691,2017
www.hydrol-earth-syst-sci.net/21/1669/2017/
doi:10.5194/hess-21-1669-2017
©Author(s)2017.CCAttribution3.0License.
Identification of dominant hydrogeochemical processes for
groundwaters in the Algerian Sahara supported by inverse
modeling of chemical and isotopic data
RabiaSlimani1,AbdelhamidGuendouz2,FabienneTrolard3,AdnaneSouffiMoulla4,BelhadjHamdi-Aïssa1,and
GuilhemBourrié3
1OuarglaUniversity,Fac.desSciencesdelaNatureetdelaVie,Lab.BiochimiedesMilieuxDésertiques,
30000Ouargla,Algeria
2BlidaUniversity,ScienceandEngineeringFaculty,P.O.Box270,Soumaâ,Blida,Algeria
3INRA–UMR1114EMMAH,Avignon,France
4AlgiersNuclearResearchCentre,P.O.Box399,Alger-RP,16000Algiers,Algeria
Correspondenceto:RabiaSlimani([email protected])
Received:8September2015–Discussionstarted:1February2016
Revised:31October2016–Accepted:20November2016–Published:21March2017
Abstract. Unpublished chemical and isotopic data taken 1 Introduction
in November 1992 from the three major Saharan aquifers,
namelytheContinentalIntercalaire(CI),theComplexeTer- Ascientificstudypublishedin2008(OECD,2008)showed
minal (CT) and the phreatic aquifer (Phr), were integrated that 85% of the world population lives in the driest half
withoriginalsamplesinordertochemicallyandisotopically of the earth. More than 1 billion people residing in arid
characterize the largest Saharan aquifer system and investi- and semi-arid areas of the world have access to only few
gatetheprocessesthroughwhichgroundwatersacquiretheir or nonrenewable water resources. The North-Western Sa-
mineralization.InsteadofclassicalDebye–Hückelextended haraAquiferSystem(NWSAS)isoneofthelargestconfined
law, a specific interaction theory (SIT) model, recently in- reservoirs in the world and its huge water reserves are es-
corporatedinPHREEQC3.0,wasused.Inversemodelingof sentiallycomposedofanoldcomponent.Itisrepresentedby
hydrochemical data constrained by isotopic data was used twomaindeepaquifers,theContinentalIntercalaireandthe
here to quantitatively assess the influence of geochemical Complexe Terminal. This system covers a surface of more
processes: at depth, the dissolution of salts from the geo- than 1millionkm2 (700000km2 in Algeria, 80000km2 in
logical formations during upward leakage without evapora- Tunisiaand250000km2 inLibya).Duetotheclimaticcon-
tionexplainsthetransitionsfromCItoCTandtoafirstend ditionsofSahara,theseformationsarepoorlyrenewed:about
member,aclusterofPhr(clusterI);nearthesurface,thedis- 1billionm3yr−1essentiallyinfiltratedinthepiedmontofthe
solutionofsaltsfromsabkhasbyrainwaterexplainsanother Saharan Atlas in Algeria, as well as in the Jebel Dahar and
cluster of Phr (cluster II). In every case, secondary precip- Jebel Nafusa in Tunisia and Libya, respectively. However,
itation of calcite occurs during dissolution. All Phr waters the very large extension of the system as well as the great
result from the mixing of these two clusters together with thickness of the aquifer layers has favored the accumula-
calciteprecipitationandionexchangeprocesses.Thesepro- tion of huge water reserves. Ouargla basin is located in the
cessesarequantitativelyassessedbythePHREEQCmodel. middle of the NWSAS and thus benefits from groundwa-
Globally, gypsum dissolution and calcite precipitation were ter resources (Fig. 1) which are contained in the following
foundtoactasacarbonsink. three main reservoirs (UNESCO, 1972; Eckstein and Eck-
stein,2003;OSS,2003,2008):
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
1670R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara
Figure1.LocationandschematicrelationsofaquifersinOuargla.Bluelinesrepresentlimitsbetweenaquifers,andthenamesofaquifers
aregiveninboldletters;asthelimitbetweenSenonianandMio-Plioceneaquifersisnotwelldefined,adashedbluelineisused.Namesof
villagesandcitiesaregiveninroman(Bamendil,Ouargla,SidiKhouiled),whilegeological/geomorphologicalfeaturesareinitalic(glacis,
sabkha,chott,dunes).Depthsarerelativetothegroundsurface.Letters“a”and“b”refertothecrosssection(Fig.2)andtothelocalization
map(Fig.3).
– At the top, the phreatic aquifer (Phr), in the Quater- aquifers. In particular, such studies focused on the recharge
nary sandy gypsum permeable formations of Quater- of the deep CI aquifer system. These investigations espe-
nary, is almost unexploited, due to its extreme salinity ciallydealtwithwaterchemicalfacies,mappediso-contents
(50gL−1). of various parameters and reported typical geochemical ra-
tios([SO2−]/[Cl−],[Mg2+]/[Ca2+])aswellasothercorre-
– In the middle, the Complexe Terminal (CT) (Cornet 4
lations.Minerals–solutionsequilibriawerecheckedbycom-
and Gouscov, 1952; UNESCO, 1972) is the most ex-
putingsaturationindiceswithrespecttocalcite,gypsum,an-
ploited and includes several aquifers in different ge-
hydrite and halite, but processes were only qualitatively as-
ological formations. Groundwater circulates in one or
sessed.Thepresentstudyaimsatapplyingforthefirsttime
thetwolithostratigraphicformationsoftheEoceneand
everinAlgeria,inversemodelingtoanextremeenvironment
SenoniancarbonatesorintheMio-Pliocenesands.
featuringalackofdataonascarcenaturalresource(ground-
water). New data were hence collected in order to charac-
– Atthebottom,theContinentalIntercalaire(CI),hosted
terizethehydrochemicalandtheisotopiccompositionofthe
in the lower Cretaceous continental formations (Bar-
major aquifers in the Saharan region of Ouargla. New pos-
remian and Albian), mainly composed of sandstones,
sibilitiesofferedbyprogressingeochemicalmodelingwere
sands and clays. It is only partially exploited because
used.Theobjectivewasalsotoidentifytheoriginofthemin-
ofitssignificantdepth.
eralization and the water-rock interactions that occur along
The integrated management of these groundwaters is theflowpath.Morespecifically,inversemodelingofchemi-
presently a serious issue for local water resources man- calreactionsallowsonetoselectthebestconceptualmodel
agers due to the large extension of the aquifers and the for the interpretation of the geochemical evolution of Ouar-
complexity of the relations between them. Several stud- glaaquifersystem.Thestepwiseinversionstrategyinvolves
ies (Guendouz, 1985; Fontes et al., 1986; Guendouz and designingalistofscenarios(hypotheses)thattakeintocon-
Moulla,1996;Edmundsetal.,2003;Guendouzetal.,2003; sideration the most plausible combinations of geochemical
Hamdi-Aïssa et al., 2004; Foster et al., 2006; OSS, 2008) processes that may occur within the studied medium. After
started from chemical and isotopic information (2H, 18O, resolving the scenarios in a stepwise manner, the one that
234U, 238U, 36Cl) to characterize the relationships between provides the best conceptual geochemical model is then se-
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R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara1671
Figure2.GeologiccrosssectionintheregionofOuargla.Thebluepatternusedforchottandsabkhacorrespondtothelimitofthesaturated
zone.
lected, which allowed Dai et al. (2006) to optimize simul- 2 Methodology
taneously transmissivities and geochemical transformations
inaconfinedaquifer.InversemodelingwithPHREEQC3.0
2.1 Presentationofthestudyarea
wasusedhereinadifferentway(onlyongeochemicaldata
but for several aquifers) to account for the modifications of
ThestudyareaislocatedinthenortheasterndesertofAlgeria
the composition of water along the flow path. At least two
(lowerSahara)(LeHouérou,2009)nearthecityofOuargla
chemical analyses of groundwater at different points of the
(Fig.1),31◦54(cid:48)to32◦1(cid:48)Nand5◦15(cid:48)to5◦27(cid:48)E,withamean
flowpath,andasetofphases(mineralsand/orgases)which
elevationof134(ma.s.l.).Itislocatedinthequaternaryval-
potentiallyreactwhilewatercirculates,areneededtooperate
ley of Oued Mya basin. The present climate belongs to the
theprogram(Charltonetal.,1996).
arid Mediterranean type (Dubief, 1963; Le Houérou, 2009;
Anumberofassumptionsareinherenttotheapplicationof
ONM, 1975/2013), as it is characterized by a mean annual
inversegeochemicalmodeling:(i)thetwogroundwateranal-
temperatureof22.5◦C,ayearlyrainfallof43.6mmyr−1and
ysesfromtheinitialandthefinalboreholesshouldrepresent
averyhighevaporationrateof2138mmyr−1.
groundwater that flows along the same flow path; (ii) dis-
Ouargla’s region and the entire lower Sahara has expe-
persionanddiffusiondonotsignificantlyaffectgroundwater
rienced during its long geological history alternating ma-
chemistry;(iii)achemicalsteadystateprevailsintheground-
rineandcontinentalsedimentationphases.DuringSecondary
watersystemduringthetimeconsidered;and(iv)themineral
era,verticalmovementsaffectedthePrecambrianbasement,
phases used in the inverse calculation are or were present
causing, in particular, collapse of its central part, along an
in the aquifer (Zhu and Anderson, 2002). The soundness or
axis passing approximately through the Oued Righ Valley
the validity of the results depends on a valid conceptualiza-
and the upper portion of the Oued Mya Valley. According
tion of the groundwater system, on the validity of the ba-
to Furon (1960), a epicontinental sea spread to the Lower
sic hydrogeochemical concepts and principles, on the accu-
EoceneofnorthernSahara.AftertheOligocene,theseagrad-
racyofmodelinputdataandonthelevelofunderstandingof
ually withdrew. It is estimated at present that this sea did
the geochemical processes occurring in the area (Güler and
not reach Ouargla and transgression stopped at the edge of
Thyne,2004;Sharifetal.,2008).Theserequirementsareful-
thebowl(Furon,1960;Lelièvre,1969).Thebasiniscarved
filled in the region of Ouargla, which can be considered as
into Mio-Pliocene (MP) deposits, which alternate with red
a “window” to the largest Saharan aquifer, and thus one of
sands, clays and sometimes marls; gypsum is not abundant
thelargestaquifersintheworldinasemi-aridtohyper-arid
anddatedfromthePontianera(duringtheMP)(Cornetand
regionsubjecttobothglobalchanges:urbansprawlandcli-
Gouscov, 1952; Dubief, 1953; Ould Baba Sy and Besbes,
matechange.Themethodologydevelopedhereandthedata
2006).ThecontinentalPlioceneconsistsofalocallimestone
collectedcaneasilybeintegratedinthePRECOSframework
crust with puddingstone or lacustrine limestone (Fig. 2),
proposed for the management of environmental resources
shaped by eolian erosion into flat areas (regs). The Quater-
(Trolardetal.,2016).
nary formations are lithologically composed of alternating
layers of permeable sand and relatively impermeable marl
(Aumassipetal.,1972;Chellatetal.,2014).
TheexploitationofMio-Plioceneaquiferisancientandat
theoriginofthecreationoftheoasis(Lelièvre,1969;Mou-
lias, 1927). The piezometric level was higher (145ma.s.l.)
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1672R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara
but overexploitation at the end of the 19th century led to a solving the inverse problem under two alternate scenarios:
catastrophic decrease of the resource, with presently more (1) considering a geochemical system in which calcite is
than900boreholes(ANRH,2011). present and (2) considering a geochemical system without
The exploitation of the Senonian aquifer dates back calcite.Aftersimulatingthetwoscenarios,itisusuallypossi-
to1953atadepthbetween140and200m,withasmallini- bletoselectthesetupthatgivesthebestresultsasthesolution
tial rate of approximately 9Ls−1; two boreholes have been totheinversemodelingaccordingtothefitbetweenthemod-
exploited since 1965 and 1969, with a total flow rate of ap- eled and observed values. Then one can conclude whether
proximately42Ls−1,fordrinkingwaterandirrigation. calcitedissolution/precipitationisrelevantornot.Thisstep-
TheexploitationoftheAlbianaquiferdatesbackto1956; wise strategy allows us to identify the relevance of a given
presently,twoboreholesareexploited: chemical process by inversely solving the problem through
alternate scenarios in which the process is either participat-
– ElHedebI,1335mdeep,withaflowrateof141Ls−1;
ingornot(Daietal.,2006).
Inthegeochemicalmodelinginverse,soundnessofresults
– ElHedebII,1400mdeep,withaflowrateof68Ls−1.
isdependentuponvalidconceptualizationofthesystem,va-
lidityofbasicconceptsandprinciples,accuracyofinputdata
2.2 Samplingandanalyticalmethods
andlevelofunderstandingofthegeochemicalprocesses.We
usetheinformationfromthelithology,generalhydrochemi-
The sampling programme consisted of collecting samples
calevolutionpatterns,saturationindicesandmineralstability
along transects corresponding to directions of flow for both
diagramstoconstraintheinversemodels.
thePhrandCTaquifers,whileitwaspossibletocollectonly
eightsamplesfromtheCI.Atotalof107sampleswerecol-
lected during a field campaign in 2013 along the main flow
3 Resultsanddiscussion
pathofOuedMya.Ofthese,67werefrompiezometerstap-
ping the phreatic aquifer, 32 from CT wells and the last 8 Tables1to4illustratetheresultsofthechemicalandtheiso-
from boreholes tapping the CI aquifer (Fig. 3). Analyses of topicanalyses.Samplesareorderedaccordingtoanincreas-
Na+, K+, Ca2+, Mg2+, Cl−, SO24− and HCO−3 were per- ingelectricconductivity(EC),andthisisassumedtoprovide
formedbyionchromatographyatAlgiersNuclearResearch anorderforincreasingsaltcontent.Inboththephreaticand
Center (CRNA). Previous and yet unpublished data (Guen- CT aquifers, temperature is close to 25◦C, while for the CI
douzandMoulla,1996)sampledin1992areusedheretoo: aquifer, temperature is close to 50◦C. The values presented
59samplesforthePhraquifer,15samplesfortheCTaquifer inTables1to5arerawanalyticaldatathatwerecorrectedfor
and 3 samples for the CI aquifer for chemical analyses and defects of charge balance before computing activities with
dataof18Oand3H(GuendouzandMoulla,1996). PHREEQC. As analytical errors could not be ascribed to
a specific analyte, the correction was made proportionally.
2.3 Geochemicalmethod
Thecorrectionsdonotaffecttheanion-to-anionmoleratios,
such as for [HCO−]/([Cl−]+2[SO2−]) or [SO2−]/[Cl−],
PHREEQCwasusedtocheckminerals–solutionsequilibria 3 4 4
whereas they affect the cation-to-anion ratio, such as for
usingthespecificinteractiontheory(SIT),i.e.,theextension
[Na+]/[Cl−].
ofDebye–HückellawbyScatchardandGuggenheimincor-
porated recently in PHREEQC 3.0 (Parkhurst and Appelo,
3.1 Characterizationofchemicalfaciesofthe
2013).Inversemodelingwasusedtocalculatethenumberof
groundwater
minerals and gases’ moles that must, respectively, dissolve
or precipitate/degas to account for the difference in compo- Piper diagrams drawn for the studied groundwaters (Fig. 4)
sition between initial and final water end members (Plum- broadly show a scatter plot dominated by a sodium chlo-
mer and Back, 1980; Kenoyer and Bowser, 1992; Deutsch, ride facies. However, when going into small details, the
1997;PlummerandSprinckle,2001;GülerandThyne,2004; widespread chemical facies of the Phr aquifer are closer to
Parkhurst and Appelo, 2013). This mass balance technique the NaCl cluster than those of the CI and CT aquifers. Re-
hasbeenusedtoquantifyreactionscontrollingwaterchem- spectively,CaSO ,Na SO ,MgSO andNaClarethemost
4 2 4 4
istryalongflowpaths(Thomasetal.,1989).Itisalsousedto dominantchemicalspecies(minerals)thatarepresentinthe
quantifythemixingproportionsofend-membercomponents phreatic waters. This sequential order of solutes is compa-
in a flow system (Kuells et al., 2000; Belkhiri et al., 2010, rabletothatofothergroundwateroccurringinnorthAfrica
2012). andespeciallyintheneighboringareaofthechotts(depres-
Inverse modeling involves designing a list of scenarios sionswheresaltsconcentratebyevaporation)Merouaneand
(modeling setups) that take into account the most plausible Melrhir(Vallèsetal.,1997;Hamdi-Aïssaetal.,2004).
combinationsofgeochemicalprocessesthatarelikelytooc-
curinoursystem.Forexample,thewaytoidentifywhether
calcitedissolution/precipitationisrelevantornotconsistsof
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R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara1673
Figure3.Locationmapofsamplingpoints.
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1674R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara
Theyare,however,allundersaturatedwithrespecttogypsum
(Fig.6).
Moreover,theyareoversaturatedwithrespecttodolomite
and undersaturated with respect to anhydrite and halite
(Fig.7).
Waters from the CT and phreatic aquifers show the same
pattern,butsomeofthemaremorelargelyoversaturatedwith
respecttocalcite,at25◦C.
However, several phreatic waters (P031, P566, PLX4,
PL18,P002,P023,P116,P066,P162andP036)thatarelo-
cated in the sabkhas of Safioune, Oum er Raneb, Bamendil
andAïnelBeïda’schottaresaturatedwithgypsumandanhy-
drite.Thisisinaccordancewithhighlyevaporativeenviron-
mentsfoundelsewhere(UNESCO,1972;Hamdi-Aïssaetal.,
2004;Slimani,2006).
NosignificanttrendofSIfromsouthtonorthupstreamand
downstreamofOuedMya(Fig.7)isobserved.Thissuggests
that the acquisition of mineralization is due to geochemical
processes that have already reached equilibrium or steady
Figure 4. Piper diagram for the Continental Intercalaire (filled stateintheupstreamareasofOuargla.
squares),ComplexeTerminal(opencircles)andphreatic(opentri-
angles)aquifers. 3.4 Changeoffaciesfromthecarbonatedclustertothe
evaporites’cluster
3.2 Spatialdistributionofthemineralization
The facies shifts progressively from the carbonated cluster
(CIandCTaquifers)totheevaporites’cluster(Phraquifer)
The salinity of the phreatic aquifer varies considerably de-
with an increase in sulfates and chlorides at the expense of
pendingonthelocation(namely,thedistancefromwellsor
carbonates (SI of gypsum, anhydrite and halite). This is il-
drains)andtime(duetotheinfluenceofirrigation)(Fig.5a). lustrated by a decrease of the [HCO−]/([Cl−]+2[SO2−])
Itssalinityislowaroundirrigatedandfairlywell-drained 3 4
ratio (Fig. 8) from 0.2 to 0 and of the [SO2−]/[Cl−] ratio
areas,suchasthepalmgrovesofHassiMiloud,justnorthof 4
from 0.8 to values smaller than 0.3 (Fig. 9) while salinity
Ouargla(Fig.3),thatbenefitfromfreshwaterandaredrained
increases.Carbonateconcentrationstendtowardsverysmall
tothesabkhaOumElRaneb.However,thethreelowestsalin-
values,whileitisnotthecaseforsulfates.Thisisduetoboth
ityvaluesareobservedinthewellsoftheOuarglapalmgrove
gypsum dissolution and calcite precipitation. Chlorides in
itself,wherethePhraquiferwatertableisdeeperthan2m.
groundwatermaycomefromthreedifferentsources:(i)an-
Conversely,thehighestsalinitywatersarefoundinwells
cient sea water entrapped in sediments, (ii) dissolution of
drilledinthechottsandsabkhas(asabkhaisthecentralpart
halite and related minerals that are present in evaporite de-
ofachottwheresalinityisthelargest)(SafiouneandOumer
positsand(iii)dissolutionofdryfalloutfromtheatmosphere,
Raneb)wheretheaquiferisoftenshallowerthan50cm.
particularlyinthesearidregions(Matiatosetal.,2014;Hadj-
ThesalinityoftheCT(Mio-Pliocene)aquifer(Fig.5b)is
Ammaretal.,2014).
muchlowerthanthatofthePhraquiferandrangesfrom1to
2gL−1;however,itshardnessislargeranditcontainsmore The[Na+]/[Cl−]ratiorangesfrom0.85to1.26fortheCI
aquifer,from0.40to1.02fortheCTaquiferandfrom0.13
sulfate, chloride and sodium than the waters of the Senon-
to 2.15 for the Phr aquifer. The measured points from the
ian formations and those of the CI aquifer. The salinity of
theSenonianaquiferrangesfrom1.1to1.7gL−1,whilethe threeconsideredaquifersarelinearlyscatteredwithgoodap-
averagesalinityoftheCIaquiferis0.7gL−1(Fig.5c). proximation around the unity slope straight line that stands
forhalitedissolution(Fig.10).Thelatterappearsasthemost
A likely contamination of the Mio-Pliocene aquifer by
dominantreactionoccurringinthemedium.However,atvery
phreatic groundwaters through casing leakage in an area
highsalinity,Na+ seemstoswervefromthestraightlineto-
where water is heavily loaded with salt, and therefore par-
wardssmallervalues.
ticularlyaggressive,cannotbeexcluded.
A further scrutiny of Fig. 10 shows that CI waters are
3.3 Saturationindices very close to the 1:1 line. CT waters are enriched in both
Na+ and Cl− but slightly lower than the 1:1 line while
The calculated saturation indices (SIs) reveal that waters phreatic waters are largely enriched and much more scat-
fromCIat50◦Careclosetoequilibriumwithrespecttocal- tered.CTwatersareclosertotheseawatermoleratio(0.858),
cite,exceptforthreesamplesthatareslightlyoversaturated. butsomelowervaluesimplyacontributionfromasourceof
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R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara1675
chloride other than halite or from entrapped seawater. Con-
versely, a [Na+]/[Cl−] ratio larger than 1 is observed for
phreatic waters, which implies the contribution of another
sourceofsodium,mostlikelysodiumsulfate,thatispresent
asmirabiliteorthenarditeinthechottsandthesabkhaareas.
The[Br−]/[Cl−]ratiorangesfrom2×10−3 to3×10−3.
− 34 33 Thevalueofthismolarratioforhaliteisaround2.5×10−3,
Br 0.0 0.0 which matches the aforementioned range and confirms that
halitedissolutionisthemostdominantreactiontakingplace
+ inthestudiedmedium.
2Ca 3.30.50.81.21.32.31.44.65.24.91.4 In the CI, CT and Phr aquifers, calcium originates both
fromcarbonateandsulfate(Figs.11and12).Threesamples
[phone] from the CI aquifer are close to the [Ca2+]/[HCO−] 1:2
Mg 2.0.1.1.5.3.4.3.5.5.4. line, while calcium sulfate dissolution explains the3excess
) ofcalcium.However,ninesamplesfromthePhraquiferare
1
+K −L 0.60.20.20.20.70.70.50.80.50.60.7 depletedincalcium,andplottedunderthe[Ca2+]/[HCO−]
ol 1:2 line. This cannot be explained by precipitation of ca3l-
m
+a m 0.75.75.15.66.30.73.90.81.82.20.6 cite,assomeareundersaturatedwithrespecttothatmineral,
N ( 1 111112
whileothersareoversaturated.
− Inthiscase,acationexchangeprocessseemstooccurand
24 81299652782
O 6.1.1.1.3.4.5.5.7.7.6. lead to a preferential adsorption of divalent cations, with a
S
release of Na+. This is confirmed by the inverse modeling
−Cl 5.85.86.26.59.812.413.114.315.115.318.6 tNhaa+tisanddevKel+opreedlebaseelosw. andwhichimpliesMg2+fixationand
k. 54632919536 Larger sulfate values observed in the phreatic aquifer
Al 3.0.0.1.3.1.2.2.3.3.2. (Fig.12)with[Ca2+]/[SO2−]<1canbeattributedtoaNa-
4
Mgsulfatedissolutionfromamineralbearingsuchelements.
H 63655368663
p 7.7.7.7.7.7.7.6.7.7.7. Thisis,forinstance,thecaseofbloedite.
T ◦(C) 46.549.347.448.948.949.347.450.546.550.154.5 3.5 Isotopegeochemistry
) The CT and CI aquifers exhibit depleted and homogeneous
1
C −m 09022224029 18Ocontents,rangingfrom−8.32to−7.85‰.Thiswasal-
E Sc 2.1.2.2.2.2.2.2.2.2.2. readypreviouslyreportedbymanyauthors(Edmundsetal.,
quifer. (m 2o0th0e3r;hGaunedn,d1o8uOzveatluael.s, 2fo0r03th;eMpohurlelaatiectaaql.u,if2e0r1a2r)e. Ownidethlye
Intercalairea Date 9Nov201219921992199222Feb201311Dec201011Dec201024Feb201327Feb20139Nov201222Feb2013 dWiemnisarapittceeehrrlsseye,dldoHicnaaansthesdeidaMvvnayoirlryiotshuobdtoeotftpwotehesSeeeanbvnkid−rhtau8era.te8lS4tlahinfiauenosducmno3oen.,r4nea2erec‰evtiafnopg(uoT,nraaadbptelpmedr.oo4xIr)ne-.
ontinental Elev. 134134146132132134146110134110160 toihrfraibtgoaattrhieoaCn,IitshaenndownCaetTxeirsgttreaonbutl.neCdiwosnacvtleoerrssseewltyoit,htwhpeahtesruersarftlaioccceoatnaeendsdstomhuriotxhuingoghf
eC g. 8686907676869036863666 HassiMilouduptoOuarglacityshowdepletedvalues.This
h n 99200926963
fort Lo (m) 723723724721721723724721723721720 ifsrotmhethceleuarndfienrglyeirnpgriCntIoafndaCcoTnatrqibuuifteiorsn(tGootnhfieanPthinriweattaelr.s,
a
dat 5050109696501059505964 1975; Guendouz, 1985; Fontes et al., 1986; Guendouz and
cal at. 4747438888474316471602 Moulla,1996).
yti L 5353535454535359535956 Phreaticwatersresultfromamixingoftwoendmembers.
nal 33333333333 Evidenceforthisisgivenbyconsideringthe[Cl−]and18O
a
d aa relationship(Fig.13).Thetwoclustersare(i)afirstclusterof
n ss
able1.Fielda Locality HedebIHedebIHedebIIAouinetMousAouinetMousHedebIHedebIIHassiKhfifHedebIHassiKhfifElBour 1osin8afOlt1ihn8-edOiteyn-p.eolnTretrhthieceedhrlneagdtrptoegaurrrntoisdouwfncdotahwmteeaprstoe(tuFsrediwgdy.iotr1hef4gpp)io,ohsnarien.taidvtie(civiw)aalautneeorstshaoencrdccaulurhrsiitngeghr
T
www.hydrol-earth-syst-sci.net/21/1669/2017/ Hydrol.EarthSyst.Sci.,21,1669–1691,2017
1676R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara
T
FraneGaretChemiaAbazatBourelAïchaChottPalmeraieStationdePompRouissatIRouissatIElBourAïnMoussaVItasElKoumElKoumN’GoussaElHoH.MiloudElBourH.MiloudAïnN’SaraAïnMoussaIIGhâzaletA.HA.LouiseAïnN’SaraSidiKouiledSARMekhadmaOglatLarbaâSaidOtbaIfriRouissatIIISaidOtbaIRouissatIIIDebicheSaidOtbaIElBourOglatLarbaâSaidOtbaIfriBamendilBamendil Locality Fieldanable2.
ag u d
e a
cho naly
tt tic
a
DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD S l
6F61F12F61F15F75F83F13F14F99F11F16F66F66F51F16F91F16F59F36F74F76F59F11F96F62F61F13F12F73F16F62F74F96F62F61F17F47F4 ite data
2139347088435077135735009302146510101144651 fo
r
A the
MSMMSSMMMMMSSSMSMSSMSSSSMSSSSSMSMMSSMM qu C
ife om
r p
le
33333333333333333333333333333333333333 x
55555555555555555555555555555555555555 e
73543433333775444539354364335345364366 L T
06258155686336707978790660857577660800 a e
1717505321655656244018696925559355328175523285755025890641065541245025897575 t. rm
54439644596446767340335717182872517199 in
a
l
7171717272727272727171727271717171717172727172717272727271727171727272727272 (m L aq
71336808278603915541352124982498264186807046163916398979706758167067686896650356190468689055782293690085106023528272235270678272264193690085106005860586 ) ong. uifer.
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16721313786243224808010021093143143198173169173255220119310255329221177216204248212248173211100177216204296296 lev.
27283554 26 821 2131 25322226252433131272026 26 27 20
Jan2013Jan2013Feb2013Feb2013Feb2013Feb20131992Jan20131992Feb2013Jan20131992Jan2013Jan20131992Jan2013Feb2013Feb2013Feb2013Feb2013Jan2013Jan2013Jan2013Feb2013Jan2013Jan2013Jan2013Jan2013Jan20131992Jan20131992Jan20131992199219921992Jan2013 Date
(m
S
3.84.13.53.43.43.32.03.12.32.43.72.53.12.92.12.62.82.67.52.82.63.42.62.52.42.42.42.35.63.12.22.33.12.32.32.72.02.0 cm EC
−
1
)
24.228.024.622.224.624.520.023.021.225.323.925.022.922.922.719.921.623.823.922.524.025.721.325.823.724.922.918.925.126.123.924.226.218.024.023.521.120.1 (C)◦ T
77777878777787887777778777787778778787 p
.9.3.6.3.5.2.8.1.9.2.5.6.1.5.1.0.5.6.5.5.5.4.1.7.6.9.8.0.3.3.7.2.4.9.0.0.2.9 H
22243313121132223223224322112211111101 A
.3.2.2.1.3.9.7.2.6.3.5.5.5.0.8.1.3.1.4.5.0.0.6.4.3.2.7.6.4.4.8.5.6.4.4.3.9.6 lk
.
22222222211111111111111111111111111111 C
5.95.94.73.22.32.11.71.20.19.48.88.88.48.48.17.97.97.77.57.47.46.96.86.56.36.15.45.24.34.34.23.52.81.41.00.70.60.1 l−
S
1 1111 1 O
39222181797799599989998888886685664235 42
.5.5.7.2.1.9.5.1.2.4.1.2.7.6.7.3.2.2.2.3.1.7.8.5.6.6.3.6.9.9.4.7.8.8.7.7.5.8 −
22.625.421.121.220.919.917.719.612.118.810.110.217.917.116.615.816.515.517.316.613.915.916.116.113.616.513.712.613.113.112.615.05.211.611.58.010.69.9 (mm Na+
o
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00111210203300011100200000010000120000 L K
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1
)
M
8.93.68.58.68.37.65.17.15.83.35.05.06.56.23.65.86.26.13.16.25.83.46.25.35.94.95.25.83.43.45.43.31.62.02.12.32.33.9 g2
+
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7.27.26.56.05.86.36.06.05.27.65.85.85.15.04.34.74.94.76.54.95.17.95.04.95.04.34.84.35.45.44.42.69.14.63.32.11.82.5 a2+
0.03 0.03 0.03 0.03 0.03 Br−
7 4 3 3 4
Hydrol.EarthSyst.Sci.,21,1669–1691,2017 www.hydrol-earth-syst-sci.net/21/1669/2017/
R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara1677
ClusterIrepresentsthewatersfromCIandCTwhoseiso-
− 35 37 topic composition is depleted in 18O (average value around
Br 0.0 0.0 −8.2‰)(Fig.13).Theycorrespondtoanoldwaterrecharge
(paleo-recharge),whoseage,estimatedbymeansof14C,ex-
+
2Ca 8.08.07.17.77.76.16.16.59.7 ceeds15000yrBP(Guendouz,1985;GuendouzandMiche-
lot, 2006). Thus, it is not a water body that is recharged by
+2Mg 4.44.510.05.05.06.26.28.47.4 raencdenptarptrlyecoipfitpahtiroenat.icItwcoatnesrisstasnodfcCaInabnedaCscTribgerodutnodawnatueprs-
) wardleakagefavoredbytheextensionoffaultsnearAmguid
+K −1L 0.60.61.70.50.51.21.20.91.1 ElBioddorsal.
ol ClusterII,observedinSebkhetSafioune,canbeascribed
m
+Na (m 22.923.123.923.923.922.822.924.036.8 to the direct dissolution of surficial evaporitic deposits con-
veyedbyevaporatedrainwater.
−24 8.78.64.29.09.18.38.38.93.5 Evaporation alone cannot explain the distribution of data
O 1 1
S that is observed (Fig. 13). Evidence for this is given in a
semi-logarithmic plot (Fig. 14), as classically obtained ac-
− 944998872
Cl 27.28.28.28.28.29.29.34.42. cording to the simple approximation of the Rayleigh equa-
tion(cf.Appendix):
k. 664227763
Al 2.2.2.2.2.1.1.1.2.
δ18O≈1000×(1−α)log(cid:2)Cl−(cid:3)+k,
H 006355247
p 7.8.7.7.7.7.7.7.7. ≈−(cid:15)log(cid:2)Cl−(cid:3)+k, (1)
T ◦(C) 24.123.225.223.722.825.425.325.125.4 where α is the fractionation factor during evaporation,
) (cid:15)≡−1000×(1−α)istheenrichmentfactorandkisacon-
1
−
C m 218994217 stant (Ma et al., 2010; Chkir et al., 2009). CI and CT wa-
E c 4.3.3.3.2.4.6.5.3.
S tersarebetterseparatedinthesemi-logarithmicplotbecause
m
( they are differentiated by their chloride content. According
Date Jan20131992Jan2013Jan20131992Feb2013199219921992 tiionseEFqiqug.a.(l11t4)o,).s−iTm7h3pe.l5ev.aelvuaepoofra(cid:15)tiounsegdivisesthaesvtraaliugehtatli2n5e◦(sCo,liwdhliinche
25 2828 3 P115istheonlysamplethatappearsonthestraightevap-
oration line (Fig. 14). It should be considered as an outlier
Elev. 216198891568933233293224 sincetherestofthesamplesareallwellalignedontheloga-
rithmicfitderivedfromthemixinglineofFig.13.
ng. 919979042799042837837381521 The phreatic waters that are close to cluster I (Fig. 13)
Lo m) 211817161732322323 correspond to groundwaters occurring in the edges of the
( 777777777 basin (Hassi Miloud, piezometer P433) (Fig. 14). They are
162220036 lowmineralizedandacquiretheirsalinityviatwoprocesses,
559297745
at. 046211881154544816 namely dissolution of evaporites along their underground
L 455554434
353535353535353535 transit up to Sebkhet Safioune and dilution through upward
leakage by the less-mineralized waters of the CI and CT
uifer MSMMMMMMS aquifers (for example, Hedeb I for CI and D7F4 for CT)
Aq (Fig.14)(Guendouz,1985;GuendouzandMoulla,1996).
The rates of the mixing that are due to upward leakage
Site D6F69D6F51D1F138D6F49D1F138D3F8D3F8D3F26D5F80 onianaquifer. fbinryogmmtheCeaInδstvooaClfuTaesmtooawfseasarbdcashlatfhnraecceptiheorqneuatahttiaicotaniqs.uIintifvoeonrllvcyaerndeibqneutichraeelscmkuinlxaoitwnedg-
n
hott =Se process.
c S Theδvalueofthemixtureisgivenby
able2.Continued. Locality OumerRanebN’GoussaElHouH.MiloudBenyazaAïnLarbaâH.MiloudBenyazaRouissatRouissatAïnElArchStationdePompage =Mio-Plioceneaquifer; owδmfhitexhre=eCffTi×sanδthd1e+δ1fr,(a1δc2−tiaofrne)ot×hfetδhr2ee,sCpeIcatiqvueifiesro,to1p−efconthteenftrsa.cti(o2n)
T M
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1678R.Slimanietal.:IdentificationofdominanthydrogeochemicalprocessesforgroundwatersintheAlgerianSahara
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RRBPHDHSRRDERRGRHPMRRHHRFSBGFMOHEM HAHBB L a
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6688SN060670277564855784220851109613036933 ep
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4443338333334333733344737343736463 8 499 L a
9946871821788172777387775847277747 4 777 a q
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71717171727173727171717271717171717271717171717271727171717273717172 71 717171 (m L
7799090133907383418476448198900890 7 899 ) o
022022930558442746922404715298746170353298744715428119419060353520428063875404930744721114058358421114 604 358626626 ng.
1111111111111111111111111111111111 1 111 E
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2122222333122121131112212111222111121212 D
6Jan20139927Jan20131Jan20137Jan20138Jan20134Jan20131Jan2013Feb2013Feb20139921Jan20136Jan20139921Jan20139929921/01/20139929929927Jan20130Jan20139920Jan20139929929920Jan20138Jan20134Jan20139929929929920Jan20139927/01/20139920Jan2013 ate
(m
S
c
m
676665555565544544 253444522334232424222 − E
.0.6.2.1.1.5.5.3.8.5.1.7.7.7.7.6.5.7 .6.3.7.1.7.1.5.4.5.4.7.5.8.0.5.1.9.0.1.0.1 )1 C
24.623.623.124.525.424.623.725.122.525.023.722.926.223.723.323.723.722.223.923.723.424.625.223.624.223.823.523.527.522.227.523.523.423.823.723.023.823.922.122.7 (C)◦ T
7788788787787787777778778777788778787889 p
.6.9.1.1.8.4.8.8.1.7.7.2.6.7.2.6.6.9.8.6.7.1.6.2.4.4.8.7.5.2.3.8.9.1.1.1.5.2.9.2 H
2011120413122111111213324323243211521111 A
.2.6.8.8.6.4.3.1.7.3.3.0.5.5.8.4.5.8.7.8.3.0.0.0.4.0.4.0.2.2.3.3.3.5.3.0.9.9.5.6 lk
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4443333333333333333222222222222211111111 C
2.52.12.09.89.78.88.68.46.35.45.03.63.52.82.41.91.51.20.98.88.27.76.25.75.34.34.23.53.32.62.10.89.08.98.27.74.23.02.02.0 l−
17.910.719.111.811.716.918.014.611.613.813.512.111.912.814.612.810.115.416.714.511.510.69.810.49.521.213.214.013.48.612.49.47.77.810.09.417.97.36.97.3 4SO2−
3123332223 22322222511212245222312211111 N
2.18.97.50.66.06.92.38.58.57.18.69.27.70.27.82.20.11.34.98.77.69.04.04.83.74.31.90.61.88.41.84.22.46.14.36.65.92.61.63.0 (mm a+
o
8.01.913.25.28.41.90.94.53.23.01.93.35.91.00.80.85.93.91.00.12.02.32.30.21.80.96.12.81.92.22.64.32.70.60.40.90.61.30.91.0 lL1− K+
)
M
111 1 1 1 11 1 2 1 g
2237594168966960715019594021848152150444 2
.5.6.4.1.1.1.8.6.8.4.4.4.0.2.8.6.5.2.7.8.5.1.0.3.2.2.3.0.3.0.6.4.3.1.4.8.6.4.4.3 +
C
898869218857875108684056777200635053857322 a2
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0.030.03 0.02 0.03 0.03 0.02 Br−
33 5 2 1 4
Hydrol.EarthSyst.Sci.,21,1669–1691,2017 www.hydrol-earth-syst-sci.net/21/1669/2017/