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1、TPO-1GroundwaterGroundwateristhewordusedtodescribewaterthatsaturatestheground,fillingalltheavailablespaces.Byfarthemostabundanttypeofgroundwaterismeteoricwater;thisisthegroundwaterthatcirculatesaspartofthewatercycle.Ordinarymeteoricwateriswaterthathassoakedintothegroundfrom
2、thesurface,fromprecipitation(rainandsnow)andfromlakesandstreams.Thereitremains,sometimesforlongperiods,beforeemergingatthesurfaceagain.Atfirstthoughtitseemsincrediblethattherecanbeenoughspaceinthe“solid”groundunderfoottoholdallthiswater.Thenecessaryspaceisthere,however,inma
3、nyforms.Thecommonestspacesarethoseamongtheparticles—sandgrainsandtinypebbles—ofloose,unconsolidatedsandandgravel.Bedsofthismaterial,outofsightbeneaththesoil,arecommon.Theyarefoundwhereverfastriverscarryingloadsofcoarsesedimentonceflowed.Forexample,asthegreaticesheetsthatcov
4、eredNorthAmericaduringthelasticeagesteadilymeltedaway,hugevolumesofwaterflowedfromthem.Thewaterwasalwaysladenwithpebbles,gravel,andsand,knownasglacialoutwash,thatwasdepositedastheflowsloweddown.Thesamethinghappenstothisday,thoughonasmallerscale,whereverasediment-ladenrivero
5、rstreamemergesfromamountainvalleyontorelativelyflatland,droppingitsloadasthecurrentslows:thewaterusuallyspreadsoutfanwise,depositingthesedimentintheformofasmooth,fan-shapedslope.Sedimentsarealsodroppedwhereariverslowsonenteringalakeorthesea,thedepositedsedimentsareonalakefl
6、oorortheseaflooratfirst,butwillbelocatedinlandatsomefuturedate,whenthesealevelfallsorthelandrises;suchbedsaresometimesthousandsofmetersthick.Inlowlandcountryalmostanyspotonthegroundmayoverliewhatwasoncethebedofariverthathassincebecomeburiedbysoil;iftheyarenowbelowthewater’s
7、uppersurface(thewatertable),thegravelsandsandsoftheformerriverbed,anditssandbars,willbesaturatedwithgroundwater.Somuchforunconsolidatedsediments.Consolidated(orcemented)sediments,too,containmillionsofminutewater-holdingpores.Thisisbecausethegapsamongtheoriginalgrainsareofte
8、nnottotallypluggedwithcementingchemicals;also,partsoftheoriginalgrainsmaybecomedis
