Temporal evolution of marine intrusion and other salinization processes in the volcanic sedimentary aquifer of the east of Gran Canaria (Canary Islands, Spain)
Abstract
The coastal volcanic‐sedimentary aquifer of the East of Gran Canaria has suffered a progressive salinization process since the 1960’s that has been attributed to intensive exploitation. It is a sector of the insular aquifer characterized as a complex, heterogeneous and multilayer aquifer in which groundwater flows from the high parts if the island towards the coast. Discharge is mostly in diffuse form. Under natural conditions the aquifer discharged also as springs in the bottom of the main gullies, but currently this discharge has been substituted by exploitation wells (Cabrera y Custodio, 2004).
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Proceedings J,ne21-2612010 SWI M21 Azores· Portugal 21st Sa~ Water Intrusion Meeting
Temporalevolutionofmarineintrusionandothersalinization processesinthevolcanic‐sedimentaryaquiferoftheeastofGran Canaria(CanaryIslands,Spain) J.Hernández1,N.Boluda‐Botella1,M.C.Cabrera2andE.Custodio3 1DepartmentofChemicalEngineering,UniversityofAlicante,Spain 2DepartmentofPhysics,UniversityoflasPalmasdeGranCanaria,Spain 3DepartmentofEngineering,UniversityofCatalonia,Spain ABSTRACT Thecoastalvolcanic‐sedimentaryaquiferoftheEastofGranCanariahassuffereda progressivesalinizationprocesssincethe1960’sthathasbeenattributedtointensive exploitation.Itisasectoroftheinsularaquifercharacterizedasacomplex, heterogeneousandmultilayeraquiferinwhichgroundwaterflowsfromthehighpartsif theislandtowardsthecoast.Dischargeismostlyindiffuseform.Undernatural conditionstheaquiferdischargedalsoasspringsinthebottomofthemaingullies,but currentlythisdischargehasbeensubstitutedbyexploitationwells(CabrerayCustodio, 2004). ThestudyareaislocatedattheEastofGranCanariaIsland.Theaveragerainfallis345 mm,rangingfrom900mmintheupperpartofthezone(1800masl)to100mminthe coast,andtheaverageyearlytemperatureis24ºC.Thisareahasbeentraditionally dedicatedtointensive,irrigatedagriculture(mainlytomatoesinthe20thCentury)with waterfromtheupperpartoftheislandandfromthelocalaquifer,butthescarcityand progressivesalinizationofgroundwateristhecauseofagricultureabandoning. Currently,theexploitedmaterialsintheareaaremostlyMiocenebasaltsandphonolites (Cabrera,1995).Pliocenebasalts(volcanicagglomeratesandlavaflowsoftheRoque NubloGroup)andQuaternarybasalts(RecentBasalts)arelocallyexploitedinsome pointsattheupperpartofthearea.TheunsaturatedzoneconsistsofRoqueNublo, Recentbasaltsanddetriticformationsinmostofthearea.Theseweretheaquifer materialsundernaturalconditions,beforedewaterleveldrawdown. ThereisanimportanthydrogeologicaldatabasethathasbeenprovidedbytheWater authoritiesoftheisland.Itincludespiezometricandchemicaldata(fieldmeasurements andlaboratorywateranalysis)from1970to1999.Threepiezometricmapshavebeen reconstructed(1970‐76;1980‐89and1990‐1999).Extensiveareaswithadrawdownto 60mbelowsealevelhavebeenidentified,correspondingwiththemoreexploitedareas (locally,somewellswentdownto100mbslin1992).Theevolutionofthewatertable showsaprogressivedrawdownfrom1970to1999,withthedevelopmentoffoursharp drawdownconesatthecoastalplane. Thehydrogeochemicalstudyhasincludedtheconstructionofseveralhydrogeochemical mapsandthetheoretical(closedsystem)watermixingcalculations.Groundwater attainselectricalconductivitiesupto10mS/cmatthesouthdeepdrawdowncones,but notatthenorthofthearea.Theanalysisofhydrogeochemicalinformationallowsto Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010 103
identifyanactivemarineintrusioninthearea,butalsotheexistenceofother salinizationcauses:returnirrigationflowsandtheeffectofweatheringbyvolcanicCO2 contributionsintwolocalizedareas.Thecontributionofbrackishrechargewatersdueto thearidityofthezonemustbealsoconsideredasanaturalcauseofsalinityatthesouth ofthearea. Watersupplyqualityproblemshavebeensolvedbymeansofreverseosmosisfacilities inthemoreproductivewellsandtheuseofseawaterdesalinationforpopulationsupply, buttheaquifersalinizationhasnotbeenarrestedintherecentyears.Awidestudyhas beencarriedoutbytheSpanishGeologicalSurvey(IGME)andtheWaterAuthorityof theIslandinordertodeclaredeaquiferasoverexploited.ItisconsideredintheSpanish WaterLegislationthatallowscontrollingtheexploitationatthewellsincoastalaquifers tominimizeseawaterintrusion.Inthiscase,thedifficulttocharacterizethisprocess duetothecomplexgeologyandtheexistenceofdifferentsalinityoriginshasprevented thedeclaration. ThisabstractisbasedontheworkofHernández‐Martín(2009),underthesupervisionof theotherauthors. REFERENCES Cabrera,M.C.1995.Caracterizaciónyfuncionamientohidrogeológicodelacuífero costerodeTelde(GranCanaria),DoctoralThesis,UniversityofSalamanca, Spain. Cabrera,M.C.andCustodio,E.2004.Groundwaterflowinavolcanic–sedimentary coastalaquifer:Teldearea,GranCanaria,CanaryIslands,Spain.Hydrogeology Journal,12:305‐320. Hernández‐Martín,J.A.2009.Caracterizaciónhidrogeoquímicayevolucióntemporalde laintrusiónmarinayotrosprocesosdesalinizaciónenelacuíferovolcánico‐ sedimentariodelEstedeGranCanaria.EngineeringGeologyProjectfor graduation.UniversityofAlicante. ContactInformation:M.CarmenCabrera.UniversidaddeLasPalmasdeGranCanaria. [email protected] Azores, Portugal SWIM21 - 21st Salt Water Intrusion Meeting June 21 - 26, 2010 104