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Autumn:WhenLeavesFall,SoCanWaterQuality

BySteveHubbs,P.E.

Nowthatsummerisover—andhopefullytherecord-setting2017hurricaneseason—manyofuscanturnourattentiontothecoolertemperatures,shorterdaysandthecolorfulsplendorofautumnleaves.Ofcourse,allofthosered,orange,andyellowleavesareshort-livedandfalltotheground,formingtrulymassiveamountsoforganicdebris.Butdidyouknowthatthisyearlyspectacle,andallthatleaflitter,canhaveanegativeeffectonwaterquality?

LeafLitter,NutrientsandStormwaterResearch

Priortothe1970s,most“urbanleaves”wereburnedinyardsandprovidedtheunmistakeablesmellofautumn…tothemarkeddisadvantageofairquality.Beginningintheearly1970s,manycitiesbannedtheopenburningofleavesandsomeeventuallybegancollectingleavesatcurbside.In1972,theNationalScienceFoundationfundedagroupof12students(myselfincluded)toconsideralternativestoburningleavesinmyhometownofLouisville,Kentucky;weinvestigatedtheirconversionintocarbon,fertilizerandevenbiodegradablereplacementsforpolystyrenefoamcupsusedforfishbait.

Today,amajorconcernisthatleaflitter(i.e.,organicdebris)spikesstormwatersystemsandwatershedswithnutrientsduringandafterrainfall,especiallywhenitgathersalongstreetcurbs.Excessphosphorusandnitrogenoftenleadtomajorwaterqualityproblemsformunicipalities,aprocesscalledeutrophication,1andharmfulalgalblooms(HABs),suchasthosethatplaguedtheLakeEriesourceofdrinkingwaterinToledo,Ohio,in2014.Overgrowthofblue-greenalgae(alsocalledcyanobacteria)isparticularlyproblematicbecauseitnotonlycandeprivethewaterofoxygen,2butalsocanreleasetoxinsthatarebothdifficultandexpensivetoremoveduringthedrinkingwatertreatmentprocess.Thesourcesofnitrogenandphosphorusaremanyandvaried,butleavesthatfallonstreetsandintocatch-basinsduringautumncanaddtothenutrientprobleminurbanstreamsandlakes—particularlyinsubsequentsummermonthsthatalsosupportconditionsforHABs.

1Eutrophicationistheenrichmentofanecosystemwithchemicalnutrients,typicallycompoundscontainingnitrogen,phosphorus,orboth.https://www.sciencedaily.com/terms/eutrophication.htm.2AsHABcyanobacteriacellsreproduce,othersdie,becomingfoodforbacteriathatdecomposeorganicmatter.2AsHABcyanobacteriacellsreproduce,othersdie,becomingfoodforbacteriathatdecomposeorganicmatter.Thisovergrowthblockslightandusesupthedissolvedoxygeninthewatertothepointwhereotheraquaticorganismslikefishandinsectscannotsurvive.https://www.sciencedaily.com/terms/algal_bloom.htm.

Theconnectionbetweenfallenleavesandexcessnutrientloadingmayappearintuitive,buttheimpactonurbanwaterquality,includingreleaseoforganiccompounds,isnotfullyunderstood.AU.S.GeologicalSurveyresearchernamedWilliamSelbigreportedonresearchconductedduringthemonthsofAprilthroughNovember,2013to2015,inMadison,Wisconsin.3Henotedthat“Whilethesourcesofnutrientstourbanstormwateraremany,theprimarycontributorisoftenorganicdetritus,especiallyinareaswithdenseoverheadtreecanopy…makingsourcecontrolthroughleafremovaloneofthefewtreatmentoptionsavailabletoenvironmentalmanagerswhenreducingtheamountofdissolvednutrientsinstormwaterrunoff.”Moreover,thefindingsareapplicabletoanyurbanareasthatarerequiredtoreducestormwater-associatednutrientloading,suchasthroughaTotalMaximumDailyLoadorTMDL.

ImportanceofCommunityLeafRemoval

Selbig’sresearchdemonstratesthatcommunityleafremovalcansignificantlyreducetheconcentrationofnutrients—particularlyphosphorus—thatreachstreamsandlakesthroughstreetstormwaterdrains.However,timingisimportantforoptimalnutrientmitigation.TheCityofMadisoncollectedleavesandcleanedthestreetsonceperweek,butwouldalsoendeavortodosopriortoapredictedrainevent.

SoWhatCanIDo?

Thinkregionally(areastreamsandlakes)andactlocally(yourfrontyard).Thereareseveralthingsyoucandothisfalltoreducetheeffectofleavesonurbanwaterquality—particularlyifyouknowyouliveinaregionwherenearbylakesandstreamsareimpactedbyeutrophication:

• Considermulchingin-placewithyourlawnmower,especiallyifyouryardisrelativelyflat,andthosenutrientswillsoakintoyourlawn.4Andit’sprobablyeasierthanraking!

• Gatherleavesandother“yardwaste”intoacompostpileforusenextspringtofertilizeflowerbedsandvegetablegardens.

• Ifyouknowwhenleaveswillbecollectedforyourcommunity,waittorakethemclosetothestreetuntiljustbeforecollectiontime.

• Ifyouhavecurbsideleafcollection,rakeleavesneartheedgeofthestreet(keepaboutthreefeetoflawnbetweenthecurbandthepile)butnotintothestreet.Thesoilundertheleaveswilladsorbsomeofthenutrientswhenitrains.

• Keepleavesoffdriveways,sidewalksandotherimpermeablesurfaces.• Keepthestreetsandcatch-basinsfreeofleaves…theyusuallyleadstraighttoastream!

3Evaluationofleafremovalasameanstoreducenutrientconcentrationsandloadsinurbanstormwater.https://pubs.er.usgs.gov/publication/70175058.4Skiptherakeandleavetheleavesforahealthier,greeneryard.https://www.treehugger.com/lawn-garden/skip-rake-and-leave-leaves-healthier-greener-yard.html.

SteveHubbsretiredfromwatertreatmentoperationsattheLouisvilleWaterCompanyin2004.HisfirstscientificadventurewasleadingaNationalScienceFoundationstudentprojectlookingintoalternativesforleafdisposalfollowingabanonopenleafburningin1972.

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