Quantifyingthehydraulicperformanceoftreatmentwetla

EcologicalEngineering36(2010)1691–1699ContentslistsavailableatScienceDirectEcologicalEngineeringjournalhomepage:∗,LarryC.Browna,AlfredO.Soboyejoa,JayMartina,BinDongbaOhioStateUniversity,Dept.ofFood,Agricultural,&BiologicalEngineering,UnitedStatesbWuhanUniversity,CollegeofWaterResources&Hydropower,ChinaarticleinfoArticlehistory:Received13January2010Receivedinrevisedform12July2010Accepted19July2010Keywords:ConstructedwetlandHydraulicefficiencyHydraulicindexResidencetimedistribution(RTD)RetentiontimeMomentindexNominaldivideTreatmentwetlandabstractAnewhydraulicindexwasderivedaccordingtoresidencetimedistributiontheory.Theapproachquan-tifieshydraulicinefficienciesaccordingtothejuxtapositionoftheholdbackparameterrelativetotheresidencetimedistribution.Theindexwasevaluatedforitsabilitytodetectvariation,forconformitywithqualitativeassessments,andforcorrelationtoeffluentpollutantfractionsinordertoassessitssuitabilityasapredictoroftreatment.Themomentindexovercomesmanyoftheweaknessesinherentinexistingindices.Theindexcanbecomputedfromadatasetconsideringjustonevolumeexchangesoarbitrarytruncationofdataduetothefinitenatureofdatacollectionhasnoimpactonthemomentindex.Themomentindexappearstobemoresensitivethanexistingindicesindetectingattenuationofaresidencetimedistributionaswell.Thenewindexdemonstratedexcellentcorrelationtotheeffluentpollutantfractionpredictedbyafirst-orderreductionimplyingtheindexcouldbethegoodpredictoroftreatment.Inadditiontocorrelationwithtreatment,themomentindexmatchedqualitativeassessmentpreciselyforeightspecificcasesconsidered.Themomentindexcouldsubstantiallyaidinthedesignandmanagementoftreatmentwetlandsforbalancingcostandefficacybyresolvingsomeoftheuncertaintyassociatedwithresidencetime.Theindexcouldbeusedtohelpidentifytheoptimalwetlandconfigurationformaximizingresidencetime.Notonlywoulditbeusefulinquantifyingtheeffectsofvegetation,bathymetry,andwetlandshapeonresidencetime;itcouldhaveutilityinsupplyingtheboundsforpollutantreduction.©2010ElsevierB.V.Allrightsreserved.1.Introduction1.1.ProblemdefinitionAgriculturalpracticesimpactwaterqualityasproductioninten-sifiesthroughtheuseofchemicalfertilizersandpesticides,incombinationwithintensivedrainagepractices(ZuckerandBrown,1998).Drainageincreasestheamountofarablelandandimprovesfieldtrafficability.MuchofOhioandmanypartsoftheMidwest-ernUnitedStatesareintensivelysubsurfacedrained(Fauseyetal.,1995).Intensivelydrainedcroplandhasreduceddetentionstoragecomparedwithundrainedcropland.Asaresult,morenutrients∗Correspondingauthorat:TheOhioStateUniversity,Dept.ofFood,Agricultural,&BiologicalEngineering,AgriculturalEngineeringBuildingRoom250,590WoodyHayesDrive,Columbus,OH43210-1057,UnitedStates.Tel.:[phone];fax:[phone].E-mailaddresses:[e-mail],[e-mail](M.D.Wahl).areflushedfromthesoilandthenaccumulateinsurfacewater(Hubbardetal.,2004).Nitrogencontaminationindrinkingwaterintheformofnitratecancausemethemoglobinemia,apotentiallyfataldiseaseininfants(Vigiletal.,1965).Coastaleutrophicationdrivenprimarilybynitrogen,andsometimesphosphorus,resultsinharmfulalgalbloomsandwidespreadhypoxiaoranoxia(Howarth,2008).ThisisagrowingconcernintheGreatLakesregionoftheUnitedStates,aswellasintheGulfofMexico,wherehypoxiathreatenstoupsetdelicatefoodchainswithpotentialimpactsoncommercialandrecreationalfishingindustries.Onestrategytoprotectpublichealth,promoteeconomicvital-ity,andimproveecologicalhealthistoreducetheamountofexcessnutrientsenteringsurfacewaterssothatcumulativedownstreamconcentrationsarenotexcessive(MitschandGosselink,2007).Conventionalwatertreatmentprocessesusedinpointsourceapplicationsaregenerallynotpracticalinagriculturalsettingswherealargepollutioncomponentisfromnon-pointsources.Con-structedwetlandsareusefulasalow-techwatertreatmentoptioninruralsettings;particularlyinanagriculturallandscapewhereintensivefarmingpracticescontributetohighnutrientandsedi-0925-8574/$–seefrontmatter©2010ElsevierB.V.Allrightsreserved.doi:10.1016/j.ecoleng.2010.07.0141692M.D.Wahletal./EcologicalEngineering36(2010)1691–1699mentloadsindrainagewater(Dongetal.,2009;Maoetal.,2009).Withtheirlowcapitalcostandminimaloperationalexpenserel-ativetoconventionalwatertreatment,constructedwetlandsareanappropriatewatertreatmenttechnologyinmanypartsoftheworld(Brix,1994).Wetlandsprovideconsiderablebenefitandpromotebiodi-versitybyofferinghabitatforwaterlovingplantsandinsects(MitschandGosselink,2007).Wetlandsprovidehydrologicbene-fitsthroughstormwatercaptureanddetentionwhichcandelaytheonsetandlimittheseverityofdownstreamflooding.Additionally,wetlandsremoveimpuritiesthroughphysical,chemical,andbio-logicalmechanisms(MitschandGosselink,2007).Consequently,constructedwetlandsareanincreasinglycommonbestpracticeforreducingnutrientloadsandotherpollutants(Brix,1994;KadlecandKnight,1996).Wide-scaleimplementationofastrategywhichincorporatesconstructedwetlandsforthetreatmentofagriculturalrunoffischallenging.C