1492Ch03

Manahan,StanleyE.FUNDAMENTALSOFAQUATICCHEMISTRYEnvironmentalChemistryBocaRaton:CRCPressLLC,20003FUNDAMENTALSOFAQUATICCHEMISTRY____________________________________________________3.1.WATERQUALITYANDQUANTITYThroughouthistory,thequalityandquantityofwateravailabletohumanshavebeenvitalfactorsindeterminingtheirwell-being.Wholecivilizationshavedisappearedbecauseofwatershortagesresultingfromchangesinclimate.Evenintemperateclimates,fluctuationsinprecipitationcauseproblems.DevastatingdroughtsinAfricaduringthe1980sresultedincatastrophiccropfailuresandstarvation.In1997anunprecedentedfloodstruckhugesectionsofNorthDakotainundatingmostoftheCityofGrandForks,andin1998floodsproducedbytorrentialrainsfromHurricaneMitchkilledthousandsinCentralAmerica.In1999heatkilledanumberofpeopleinthecentralandeasternUnitedStatesandadroughtdevastatedcropsandwatersupplies.InSeptember1999HurricaneFloydproduceda500-yearfloodineasternNorthCarolina.Waterbornediseasessuchascholeraandtyphoidkilledmillionsofpeopleinthepastandstillcausegreatmiseryinlessdevelopedcountries.Ambitiousprogramsofdamanddikeconstructionhavereducedflooddamage,buttheyhavehadanumberofundesirablesideeffectsinsomeareas,suchasinundationoffarmlandbyreservoirsandfailureofunsafedams.Globally,problemswithquantityandqualityofwatersupplyremainandinsomerespectsarebecomingmoreserious.Theseproblemsincludeincreasedwateruseduetopopulationgrowth,contaminationofdrinkingwaterbyimproperlydiscardedhazardouswastes(seeChapter19),anddestructionofwildlifebywaterpollution.Aquaticchemistry,thesubjectofthischapter,mustconsiderwaterinrivers,lakes,estuaries,oceans,andunderground,aswellasthephenomenathatdeterminethedistributionandcirculationofchemicalspeciesinnaturalwaters.Itsstudyrequiressomeunderstandingofthesources,transport,characteristics,andcompositionofwater.Thechemicalreactionsthatoccurinwaterandthechemicalspeciesfoundinitarestronglyinfluencedbytheenvironmentinwhichthewaterisfound.Thechem-istryofwaterexposedtotheatmosphereisquitedifferentfromthatofwateratthe©2000CRCPressLLCbottomofalake.Microorganismsplayanessentialroleindeterminingthechemicalcompositionofwater.Thus,indiscussingwaterchemistry,itisnecessarytoconsiderthemanygeneralfactorsthatinfluencethischemistry.Thestudyofwaterisknownashydrologyandisdividedintoanumberofsubcategories.Limnologyisthebranchofthesciencedealingwiththecharacteristicsoffreshwaterincludingbiologicalproperties,aswellaschemicalandphysicalproper-ties.Oceanographyisthescienceoftheoceananditsphysicalandchemicalcharac-teristics.ThechemistryandbiologyoftheEarth’svastoceansareuniquebecauseoftheocean’shighsaltcontent,greatdepth,andotherfactors.SourcesandUsesofWater:TheHydrologicCycleTheworld’swatersupplyisfoundinthefivepartsofthehydrologiccycle(Figure3.1).About97%ofEarth’swaterisfoundintheoceans.Anotherfractionispresentaswatervaporintheatmosphere(clouds).Somewateriscontainedinthesolidstateasiceandsnowinsnowpacks,glaciers,andthepolaricecaps.Surfacewaterisfoundinlakes,streams,andreservoirs.Groundwaterislocatedinaquifersunderground.Lake,reservoirRunoff110Evaporationandtranspiration,195Precipitation,304Evaporation,1164CondensationSnowpack,iceInfiltrationtogroundwaterPrecipitation,1055GroundwaterOceanMovementofwatervaportoland110Figure3.1.Thehydrologiccycle,quantitiesofwaterintrillionsoflitersperday.Thereisastrongconnectionbetweenthehydrosphere,wherewaterisfound,andthelithosphere,whichisthatpartofthegeosphereaccessibletowater.Humanactivitiesaffectboth.Forexample,disturbanceoflandbyconversionofgrasslandsorforeststoagriculturallandorintensificationofagriculturalproductionmayreduce©2000CRCPressLLCvegetationcover,decreasingtranspiration(lossofwatervaporbyplants)andaffectingthemicroclimate.Theresultisincreasedrainrunoff,erosion,andaccumulationofsiltinbodiesofwater.Thenutrientcyclesmaybeaccelerated,leadingtonutrientenrichmentofsurfacewaters.This,inturn,canprofoundlyaffectthechemicalandbiologicalcharacteristicsofbodiesofwater.Thewaterthathumansuseisprimarilyfreshsurfacewaterandgroundwater,thesourcesofwhichmaydifferfromeachothersignificantly.Inaridregions,asmallfractionofthewatersupplycomesfromtheocean,asourcethatislikelytobecomemoreimportantastheworld’ssupplyoffreshwaterdwindlesrelativetodemand.Salineorbrackishgroundwatersmayalsobeutilizedinsomeareas.InthecontinentalUnitedStates,anaverageofapproximately1.48×1013litersofwaterfallasprecipitationeachday,whichtranslatesto76cmperyear.Ofthatamount,approximately1.02×1013litersperday,or53cmperyear,arelostbyevaporationandtranspiration.Thus,thewatertheoreticallyavailableforuseisapproximately4.6×1012litersperday,oronly23centimetersperyear.Atpresent,theU.S.uses1.6×1012litersperday,or8centimetersoftheaverageannualprecipitation.Thisamountstoanalmosttenfoldincreasefromausageof1.66×1011litersperdayattheturnofthecentury.Evenmorestrikingisthepercapitaincreasefromabout40litersperdayin1900toaround600litersperdaynow.Muchofthisincreaseisaccountedforbyhighagriculturalandindustrialuse,whicheachaccountforapproximately46%oftotalconsumption.Municipaluseconsumestheremainin