Non-renewable groundwater resources: a ... - unesdoc - Unesco

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CHAPTER 3 ■ AQUIFER CHARACTERISATION TECHNIQUESTable 9. Summary of hydrogeological tools to guide groundwater development and managementEXPLORATION DEVELOPMENT MANAGEMENTTOOLSGroundwater useonly for ruralsubsistenceIntroductionof improved drillingand pumping technologyGroundwater level declineand possiblequality deteriorationField mappingand remotesensingDefinition ofaquifer extension/structure andpotentialrecharge areasIdentification of areas forintensive resource developmentand possible impacts of reducedaquifer natural dischargeChecking groundwaterproduction from agriculturalcrop-use and status of naturaldischarge areasRegionalgeophysicalsurveysComplementsmapping ofsurface geologyImproves identification of moreproductive aquifer zones (includeuse of aeromagnetic techniques)Further refinement possiblebut not widely usedWaterwelldrillingExploratory forreconnaissancelevel data onhydrostratigraphyProduction wellfield constructionwith associated infrastructure(usually either for urban watersupplyor irrigated agriculture)Refining production system toprovide maximum energyefficiency; in some caseswaterwell rehabilitation andpump replacementDownholegeophysicalloggingImproveshydrostratigraphicanalysisImproves correlation betweenwaterwells and refines aquiferconceptual modelAids diagnosis of waterwell yieldand quality problems, and thusfacilitates rehabilitationGeographicalinformationsystemsAt best simplemanual databasesof geo-referencedhydrogeologicaldataGIS-based informationsystems for well recordsand aquifer data – outputwell-suited for input tonumerical aquifer modelsGIS transformed into MISfor data on aquifer productionperformance andsocio-economic indicatorsGroundwaterchemical andisotopicanalysisReconnaisanceleveldeterminationof groundwatergenesis andchemical variationsMore detailed evaluation ofgroundwater genesis andevidence of contemporaryrechargeEvaluation of rates ofcontemporary recharge andprediction of groundwater qualitychanges threatening wellfieldsAnalyticalradialgroundwaterflow modelsAnalysis ofpumping test datafor estimation ofaquifer propertiesPrediction of drawdowns forwaterwell designAnalysis of causesof well deterioration andrehabilitation planningLumpedparameteraquifer models–------------------- Basic aquifer water balanceto confirm conceptual modelMass-balance type checks onissues such as ‘life of resource’or ‘bulk impact of production’Numericaldistributedparameteraquifer models–------------------- Testing conceptual modelvalidity, optimising wellfielddesign/production anddetermining potential long-termimpactsImproving operational efficiencyand evaluating long-termsustainability of wellfieldsGroundwaterdevelopmentrisk assessment–------------------- Assessment of risks of prematurefailure/quality deterioration ofwellfieldsAssessment of risks togroundwater-based economicdevelopment related toexternalities, such as changesin energy costs and crop prices37
NON-RENEWABLE GROUNDWATER RESOURCESPhases of aquifer modelling followed by exploration drilling have been carried out from themid-1970s in southern Jordan. In the final phase of exploration (before the implementation of amajor, and very costly, groundwater transfer scheme) an extensive exploration drilling programmewas employed to allow a critical re-evaluation of the Rum-Saq aquifer, shared by Jordanand Saudi Arabia (Puri et al., 1999). The programme involved some 18,000 m of drilling and wascomplemented by a host of other measurements and analyses. These included detailed downholegeophysical logging, laboratory testing of aquifer rock-cores, long-term pumping tests andgroundwater quality analyses – which were synthesized into a regional numerical model of theaquifer over 70,000 km 2 .Geographical information systemsMany thousands of data-points can be collected in the evaluation of a large regional aquifersystem – from waterwell drilling, geophysical logging, pump testing, groundwater quality analyses,etc. Quality assurance of these data can become unmanageable unless a well-suited geographicalinformation system (GIS) is used. There are various commercially-available suites ofcomputer programmes marketed as packages for this purpose. Numerical models of aquiferbehaviour have to be based on data collated within reliable and interactive information systems.The nature and type of data stored in a GIS has to evolve with time and the level of aquiferexploitation.Approaches to resource evaluationBy definition the assessment of the non-renewable groundwater resource potential of a specificaquifer system requires the evaluation of its storage reserves – or more specifically its extractablestorage reserves – and two complementary approaches will need to be pursued.Bulk storage estimation from hydrogeological structureAs a first approximation, the calculation of aquifer storage reserves can be made using structuralhydrogeological data (geometry/volume of the aquifer reservoir and aquifer specific yield or unitdrainable storage), providing that the possible presence of groundwater of inferior qualityis also considered (Margat, 1991). The total storage reserve thus calculated (see Table 10 forexamples) remains theoretical, however, since any estimate of extractable groundwater reservesmust also reflect the technical and economic limits on aquifer exploitation. In the case ofa confined aquifer system, the calculation of exploitable storage reserves must be basedexclusively upon the confined storage coefficient (and not on aquifer specific yield) and thus willbe relatively small in volume – although complex multi-layer aquifer systems with interbeddedaquitards can also contain large exploitable storage reserves.Dynamic storage computation by aquifer modelling38The use of numerical hydrodynamic simulation of the aquifer system under consideration is apowerful tool to estimate the availability of aquifer storage reserves. This approach normallyuses a computerised model of the aquifer system to calculate and compare the feasibility of
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