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1、TPO28GroundwaterMostoftheworld’spotablewater–freshwatersuitablefordrinking–isaccountedforbygroundwater,whichisstoredintheporesandfracturesinrocks.Thereismorethan50timesasmuchfreshwaterstoredundergroundthaninallthefreshwaterriversandlakesatthesurface.Nearly50percen
2、tofallgroundwaterisstoredintheupper1,000metersofEarth.AtgreaterdepthwithinEarth,thepressureoftheoverlyingrockcausesporesandcrackstoclose,reducingthespacethatporewatercanoccupy,andalmostcompleteclosureoccursatadepthofabout10kilometers.Thegreatestwaterstorage,theref
3、ore,liesnearthesurface.Aquifers,Porosity,andPermeabilityGroundwaterisstoredinavarietyofrockstypes.Agroundwaterreservoirfromwhichwatercanbeextractediscalledanaquifer.Wecaneffectivelythinkofanaquiferasadepositofwater.Extractionofwaterdependsontwopropertiesoftheaquif
4、erporosityandpermeability.Betweensedimentgrainsarespacesthatcanbefilledwithwater.Thisporespaceisknownasporosityandisexpressedasapercentageofthetotalrockvolume.Porosityisimportantforwater-storagecapacity,butforwatertoflowthroughrocks,theporespacesmustbeconnected.Th
5、eabilityofwater,orotherfluids,toflowthroughtheinterconnectedporespacesinrocksistermedpermeability.Fracturesandjointshaveveryhighpermeability.Intheintergranularspacesofrocks,however,fluidmustflowaroundandbetweengrainsinatortuouspath;thiswindingpathcausesaresistance
6、toflow.Therateatwhichtheflowingwaterovercomesthisresistanceisrelatedtothepermeabilityofrock.sediment:materials(suchassandorsmallrocks)thataredepositedbywater,wind,orglacialice.Sedimentsortingandcompactioninfluencepermeabilityandporosity.Themorepoorlysortedorthemor
7、etightlycompactedasedimentis,theloweritsporosityandpermeability.Sedimentaryrocks–themostcommonrocktypenearthesurface–arealsothemostcommonreservoirsforwaterbecausetheycontainthemostspacethatcanbefilledwithwater.Sandstonesgenerallymakegoodaquifers,whilefiner-grained
8、mudstonesaretypicallyimpermeable.Impermeablerocksarereferredtoasaquicludes.Igneousandmetamorphicrocksaremorecompact,commonlycrystalline,andrarelycontain
