1492Ch15

Manahan,StanleyE.THEGEOSPHEREANDGEOCHEMISTRYEnvironmentalChemistryBocaRaton:CRCPressLLC,200015THEGEOSPHEREANDGEOCHEMISTRY____________________________________________________15.1.INTRODUCTIONThegeosphere,orsolidearth,isthatpartoftheearthuponwhichhumansliveandfromwhichtheyextractmostoftheirfood,minerals,andfuels.Oncethoughttohaveanalmostunlimitedbufferingcapacityagainsttheperturbationsofhumankind,thegeosphereisnowknowntoberatherfragileandsubjecttoharmbyhumanactivities.Forexample,somebillionsoftonsofearthmaterialareminedorother-wisedisturbedeachyearintheextractionofmineralsandcoal.Twoatmosphericpollutantphenomena—excesscarbondioxideandacidrain(seeChapter14)—havethepotentialtocausemajorchangesinthegeosphere.Toomuchcarbondioxideintheatmospheremaycauseglobalheating(“greenhouseeffect”),whichcouldsignificantlyalterrainfallpatternsandturncurrentlyproductiveareasoftheearthintodesertregions.ThelowpHcharacteristicofacidraincanbringaboutdrasticchangesinthesolubilitiesandoxidation-reductionratesofminerals.Erosioncausedbyintensivecultivationoflandiswashingawayvastquantitiesoftopsoilfromfer-tilefarmlandseachyear.Insomeareasofindustrializedcountries,thegeospherehasbeenthedumpinggroundfortoxicchemicals.Ultimately,thegeospheremustprovidedisposalsitesforthenuclearwastesofthemorethan400nuclearreactorsthathaveoperatedworldwide.Itmaybereadilyseenthatthepreservationofthegeosphereinaformsuitableforhumanhabitationisoneofthegreatestchallengesfacinghumankind.Theinterfacebetweenthegeosphereandtheatmosphereatearth’ssurfaceisveryimportanttotheenvironment.Humanactivitiesontheearth’ssurfacemayaffectclimate,mostdirectlythroughthechangeofsurfacealbedo,definedasthepercentageofincidentsolarradiationreflectedbyalandorwatersurface.Forexample,ifthesunradiates100unitsofenergyperminutetotheouterlimitsoftheatmosphere,andtheearth’ssurfacereceives60unitsperminuteofthetotal,thenreflects30unitsupward,thealbedois50percent.Sometypicalalbedovaluesfordifferentareasontheearth’ssurfaceare:evergreenforests,7-15%;dry,plowed©2000CRCPressLLCfields,10-15%;deserts,25-35%;freshsnow,85-90%;asphalt,8%.Insomeheavilydevelopedareas,anthropogenic(human-produced)heatreleaseiscomparabletothesolarinput.Theanthropogenicenergyreleaseoverthe60squarekilometersofManhattanIslandaveragesabout4timesthesolarenergyfallingonthearea;overthe3500km2ofLosAngelestheanthropogenicenergyreleaseisabout13%ofthesolarflux.Oneofthegreaterimpactsofhumansuponthegeosphereisthecreationofdesertareasthroughabuseoflandwithmarginalamountsofrainfall.Thisprocess,calleddesertification,ismanifestedbydeclininggroundwatertables,salinizationoftopsoilandwater,reductionofsurfacewaters,unnaturallyhighsoilerosion,anddesolationofnativevegetation.Theproblemissevereinsomepartsoftheworld,particularlyAfrica'sSahel(southernrimoftheSahara),wheretheSaharaadvancedsouthwardataparticularlyrapidrateduringtheperiod1968-73,contributingtowidespreadstarvationinAfricaduringthe1980s.Large,aridareasofthewesternU.S.areexperiencingatleastsomedesertificationastheresultofhumanactivitiesandaseveredroughtduringthelatter1980sandearly1990s.Asthepopulationsofthewesternstatesincrease,oneofthegreatestchallengesfacingtheresidentsistopreventadditionalconversionoflandtodesert.Themostimportantpartofthegeosphereforlifeonearthissoil.Itisthemediumuponwhichplantsgrow,andvirtuallyallterrestrialorganismsdependuponitfortheirexistence.Theproductivityofsoilisstronglyaffectedbyenvironmentalconditionsandpollutants.Becauseoftheimportanceofsoil,allofChapter16isdevotedtoitsenvironmentalchemistry.Withincreasingpopulationandindustrialization,oneofthemoreimportantaspectsofhumanuseofthegeospherehastodowiththeprotectionofwatersources.Mining,agricultural,chemical,andradioactivewastesallhavethepotentialforcontaminatingbothsurfacewaterandgroundwater.Sewagesludgespreadonlandmaycontaminatewaterbyreleaseofnitrateandheavymetals.Landfillsmaylikewisebesourcesofcontamination.Leachatesfromunlinedpitsandlagoonscontaininghazardousliquidsorsludgesmaypollutedrinkingwater.Itshouldbenoted,however,thatmanysoilshavetheabilitytoassimilateandneutralizepollutants.Variouschemicalandbiochemicalphenomenainsoilsoperatetoreducetheharmfulnatureofpollutants.Thesephenomenaincludeoxidation-reductionprocesses,hydrolysis,acid-basereactions,precipitation,sorption,andbiochemicaldegradation.Somehazardousorganicchemicalsmaybedegradedtoharmlessproductsonsoil,andheavymetalsmaybesorbedbyit.Ingeneral,however,extremecareshouldbeexercisedindisposingofchemicals,sludges,andotherpotentiallyhazardousmaterialsonsoil,particularlywherethepossibilityofwatercontaminationexists.15.2.THENATUREOFSOLIDSINTHEGEOSPHERETheearthisdividedintolayers,includingthesolidiron-richinnercore,moltenoutercore,mantle,andcrust.Environmentalchemistryismostconcernedwiththelithosphere,whichconsistsoftheoutermantleandthecrust.Thelatteristheearth’souterskinthatisaccessibletohumans.Itisextremelythincomparedtothediameteroftheearth,rangingfrom5to40kmthick.©2000CRCPressLLCMostofthesolidearthcrustconsistsofrocks.Rocksarecomposedofminerals,whereaminer