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1、GroundwaterMostoftheworld’spotablewater-freshwatersuitablefordrinking-isaccountedforbygroundwater,whichisstoredintheporedandfracturesinrocks.Thereismorethan50timesasmuchfreshwaterstoredundergroundthaninallthefreshwaterriversandlakesatthesurface.Nearly50percentofallgroundwaterisstoredinth
2、eupper1,000metersofEarth.AtgreaterdepthswithinEarth,thepressureoftheoverlyingrockcausesporesandcrackstoclose,reducingthespacethatporewatercanoccupy,andalmostcompleteclosureoccursatadepthofabout10kilometers.Thegreatestwaterstorage,therefore,liesnearthesurface.Aquifer,Porosity,andPermeabil
3、ityGroundwaterisstoredinavarietyofrocktypes.Agroundwaterreservoirfromwhichwatercanbeextractediscalledanaquifer.Wecaneffectivelythinkofanaquiferasadepositofwater.Extractionofwaterdependsontwopropertiesoftheaquifer,porosityandpermeability.Betweensedimentgrainsarespacesthatcanbefilledwithwa
4、ter.Thisporespaceisknownasporosityandisexpressedapercentageofthetotalrockvolume.Porosityisimportantforwater-storagecapacity,butforwatertoflowthroughrocks,theporespacesmustbeconnected.Theabilityofwater,orotherfluids,toflowthroughtheinterconnectedporespacesinrocksistermedpermeability.Fract
5、uresandjointshaveveryhighpermeability.Intheintergranularspacesofrocks,however,fluidmustflowaroundandbetweengrainsinatortuouspath;thiswindingpathcausesaresistancetoflow.Therateatwhichtheflowingwaterovercomesthisresistanceisrelatedtothepermeabilityofrock.Sedimentsortingandcompactioninfluen
6、cepermeabilityandporosity.Themorepoorlysortedorthemoretightlycompactedasedimentis,theloweritsporosityandpermeability.Sedimentaryrocks-themostcommonrocktypenearthesurface-arealsothemostcommonreservoirsforwaterbecausetheycontainthemostspacethatcanbefilledwithwater.Sandstonesgenerallymaegoo
7、daquifers,whilefiner-grainedmudstonesaretypicallyimpermeable.Impermeablerocksarereferredtoasaquicludes.Igneousandmetamorphicrocksaremorecompact,commonlycrystalline,andrarelycontainspacesbetweengrains.However,evenigneousandmetamorphicrocksmayactasundergroundreservoirifexte
