Nutrient contribution of leaf litter in urban stormwater

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<ul><li><p>Journal of Environmental Management (1998) 54, 269272</p><p>Article No. ev980334</p><p>Nutrient contribution of leaf litter inurban stormwater</p><p>R. A. Allison, F. H. S. Chiew and T. A. McMahon</p><p>This paper investigates the nutrient contribution from leaf litter in urban waterways, using data from a grosspollutant monitoring programme in a 50 ha catchment in an inner-city suburb of Melbourne, Australia. Thedata indicate that the potential nutrient contribution of stormwater leaf litter (greater than 5 mm) is abouttwo orders of magnitude smaller than the typical nutrient loads in urban stormwater. The results suggestthat removing leaf litter from urban waterways will do little to reduce the total stormwater nutrient load.</p><p> 1998 Academic Press</p><p>Keywords: leaf litter, nutrient, stormwater, urban, organic.</p><p>Nutrients in leaf litterIntroduction</p><p>The analyses of Prasad et al. (1980) of autumnTrees in urban areas can shed large quant-leaf litter from five deciduous tree species inities of leaves. The fallen leaves are potentialmetropolitan Toronto, Canada, indicate thatcontributors of nutrients as they break down</p><p>in catchments and in urban waterways. the total phosphorus (TP) and total nitrogen(TN) compositions range from 007026%This paper focuses only on organic matter</p><p>(mainly leaf litter) in urban stormwater and 0712% respectively, (expressed as per-centage of dry weight of leaves). Dorneythat can be removed by structural devices.</p><p>Specifically, the paper investigates whether (1986) reported TP composition of leaf litterranging from 006044% for residentialremoving leaf litter from stormwater can</p><p>significantly reduce nutrient loads in urban street trees in Milwaukee and Shorewood,USA. Dorney also acknowledged that TP inwaterways.</p><p>There is some information in the lit- the urban street leaves are similar to TP oftrees of similar species in natural ecosystems.erature on the nutrient composition of leaf</p><p>litter and the proportion of this that can Attivil and Leeper (1990) report TP and TNcompositions of up to 01 and 12%, re-leach into water. However, the nutrient</p><p>impact also depends on the amount of spectively in leaf litter in Australian forests.The experiments of Prasad et al. (1980) in-leaves in the stormwater, and there is</p><p>practically no data on this. In this paper, dicatethat0006007%and005024%ofdryleaf weight of TP and TN, respectively, canthe amount of nutrients that can leach</p><p>from stormwater leaf litter is estimated leachintode-ionisedwater.TheirexperimentsCooperative Researchusing data from a 3 month monitoring also suggest that 48 h is adequate to leach outCentre for Catchmentprogramme in Melbourne, Australia, where most of the soluble substances. Dorney (1986),Hydrology, Department of</p><p>a device is used to trap all gross pollutants soaking leaves in distilled water for 2 h, re- Civil and EnvironmentalEngineering, University of(greater than 5 mm) from a stormwater ported leaching of TP of 0004 and 0026% ofMelbourne, Parkville VICdrain. This estimate is then compared with dry leaf weight. Cowan and Lee (1973), soak-3052, Australia</p><p>typical nutrient loads in urban stormwater, ing oak and poplar tree leaves from Madison,*Corresponding authorand some implications on the management USA, for 15 h in distilled water, reported</p><p>of leaf litter in urban waterways are dis- leaching of TP of 00050023%. Riley and Received 6 January 1997;accepted 19 July 1998cussed. Abood (1995) applied different treatments to</p><p>03014797/98/040269+04 $30.00/0 1998 Academic Press</p></li><li><p>270 R. A. Allison et al.</p><p>a rubbish sample from a shopping centre inSydney, Australia (soaking the sample in aer-ated and non-aerated stormwater and de-ion-ised water) and recorded concentrations ofseveral water quality parameters over 262days. Assuming that all recorded TP and TNcame from leaf litter, their data indicate thatapproximately 0003 and 005% of dry leafweightofTPandTN,respectively, leachedintode-ionised water.</p><p>Diversion weir</p><p>Flow</p><p>Main drain</p><p>ScreenContainmentchamber</p><p>. .. .. .. . .....</p><p>. ...</p><p>.. .. .</p><p>. .</p><p>In summary, there is general agreement inFigure 1. Schematic of the continuous deflective</p><p>the values reported by the various studies. separation (CDS) device.The samples in the North American studiesconsist mainly of leaf litter from deciduoustrees, while urban areas in Australia gen-erally have a mixture of evergreen and de- The CDS device was cleaned 10 times be-</p><p>tween May and August 1996. During eachciduous trees. The TP in leaf litter is about005045% of dry leaf weight, while the TN clean, the water in the chamber was pumped</p><p>out and the trapped pollutants removed man-is about 0712% of dry leaf weight. About520% of the nutrient in leaf litter can leach ually. The collected pollutants were then</p><p>taken to a laboratory and sorted into variousinto stormwater.classifications (for example, plastic, paper,metal and organic material). The bulk of thematerial was organic matter, and ob-Monitoring and resultsservations during the sorting suggest thatmore than 90% of this was leaf litter (leaves</p><p>The monitoring was carried out in Coburg, and twigs). Samples of the organic materialwere tested for TP and TN contents at aabout 8 km north of central Melbourne. The</p><p>location is typical of inner-city suburbs in NATA (National Assessment and Testing As-sociation) registered laboratory. The testingsouth-east Australia. The monitoring was</p><p>part of a larger programme established to was carried out using standard proceduresset out by American Public Health As-study the characteristics of human-derived</p><p>litter and litter trapping devices (see Allison sociation (1995).Table 1 shows the dry mass of an-and Chiew, 1995; and Allison et al., 1997,</p><p>1998). thropogenic litter and organic material, theTP and TN compositions of the organic ma-As part of the monitoring, a continuous</p><p>deflective separation (CDS) trapping device terial, and the runoff for the periods betweencleans. The TP and TN compositions in thewas installed at the outlet of a 50 ha catch-</p><p>ment (65% residential, 30% commercial and organic material are similar to the nutrientcompositions of leaf litter reported in the5% light industrial). The CDS unit diverts</p><p>incoming flow and associated pollutants into literature (see the previous section). The totalTP and TN in the organic material in thea screen separation chamber, which is de-</p><p>signed to maintain a self-cleaning screen. The stormwater, calculated as the nutrient com-position multiplied by the total organic ma-trapped litter then settles into a containment</p><p>chamber (see Figure 1). A detailed description terial, are shown in the last two columns ofTable 1.of the CDS mechanism can be found in Wong</p><p>and Wootton (1995) and Allison et al. (1996, The table indicates that almost 80% of thestormwater gross pollutant consists of or-1998). Depth sensors placed on the weir that</p><p>bypasses the high flows indicated that less ganic material (which is mainly leaf litter).It also indicates that the total TP and TN inthan 1% of flow bypassed the weir during the</p><p>monitoring period. As such, practically all the organic material over the 3 month periodwere 04 and 40 kg, respectively. With thegross pollutants larger than the screen per-</p><p>forations of the containment chamber (18 total runoff volume of 385 Ml, the TP andTN in the stormwater organic material canby 47 mm diamond) were captured by the</p><p>device. also be expressed as an average of 001 mg l-1</p></li><li><p>Leaf litter in urban stormwater 271</p><p>Table 1. Variables from the 10 cleanouts of the continuous deflective separation (CDS) device</p><p>Period Runoff Runoff Organic Anthropo- TP in TN in Total TP in Total TN inevents (ml) material genic organic organic stormwater stormwater</p><p>(kg dry) litter material material organic organic(kg dry) (% of dry (% of dry material material</p><p>weight) weight) (gram) (gram)</p><p>7 May13 May 1 365 400 82 008 07 320 28013 May22 May 1 085 134 69 009 14 121 18822 May24 May 1 040 24 07 009 13 22 3124 May21 June 1 170 193 109 028 19 540 36721 June27 June 1 1430 647 162 020 22 1294 142327 June4 July 3 395 613 92 011 10 674 6134 July10 July 1 110 112 37 010 10 112 11210 July18 July 1 185 171 51 010 12 171 20518 July24 July 1 165 94 46 011 13 103 12224 July2 August 2 905 440 117 014 16 616 704Total 13 385 2828 772 3973 4045</p><p>TP, total phosphorus; TN, total nitrogen.</p><p>and 011 mg l-1, respectively. Of these, Management implicationsapproximately 520% can potentially leachinto the stormwater (see the previous sec-</p><p>Leaf litter and other vegetation are a po-tion).tential source of nutrients in urban wa-Water quality testing of stormwaterterways. A considerable amount of leaf littersamples taken from the same catchment in-is removed before it enters the stormwaterdicated that TP concentrations of stormwatersystem through practices such as domesticwere generally between 03 and 06 mg l-1sweeping and council street cleaning. How-(although concentrations as high as 3 mg l-1ever, this study shows that a large amountwere found) and TN concentrations were be-of organic material is still transported by thetween 15 and 4 mg l-1 (concentrations asdrainage network. Nevertheless, despite thehigh as 6 mg l-1 were recorded) (see Allisonlarge amounts, organic material contributesand Chiew, 1995). These concentrations arelittle to the total stormwater nutrient load.within the large range of values reported inAs such, removing leaf litter from waterwaysthe literature for fully developed urban areaswould not significantly reduce nutrient loadsin Australia and other parts of the worldreaching receiving waters. However, because(Athayde et al., 1983; Chiew et al., 1997; andof their large volume, leaf litter and plantDuncan, 1997).matter may need to be considered when de-The above values indicate that the po-signing litter trapping devices, and wheretential nutrient contribution of stormwaterthey could cause blockages or smotherleaf litter in this catchment is about twoaquatic habitat.orders of magnitude smaller than the nu-</p><p>Another implication relates to the moni-trient loads measured from water samples intoring of stormwater nutrient loads. Althoughthe stormwater. The catchment has a mixtureconventional methods use sampling bottlesof evergreen and deciduous trees, and is typ-that do not admit large material, they provideical of many inner-city suburbs in south-eastan accurate estimate of nutrient con-Australia. The monitoring was carried out incentrations because stormwater gross pol-late autumn and early winter, and it is likelylutants are not a significant source ofthat the gross pollutant characteristics wouldnutrients.be different at other times of the year. Never-</p><p>theless, even allowing for the seasonal dif-ferences, and the variability in tree species Conclusionsand gross pollutant characteristics betweencatchments, the values reported here indicate</p><p>The nutrient impact of stormwater leaf litterthat leaf litter in stormwater contributeslittle to the total stormwater nutrient load. depends on the nutrient composition in leaf</p></li><li><p>272 R. A. Allison et al.</p><p>for Catchment Hydrology, Report 98/3, 102 pp.litter, the amount that can leach into theAllison, R. A., Wong, T. H. F. and McMahon, T. A.stormwater and the quantities of leaf litter</p><p>(1996). The Pollutec stormwater pollution trap:in the stormwater. There is some information field trials. Water Australian Water and Waste-in the literature on leaf composition and nu- water Association Journal 23, 2933.trient leaching rates, but practically no data American Public Health Association (1995).</p><p>Standard methods for the examination of wateron leaf litter quantities in stormwater. The 3and wastewater. pp. 1108. Maryland: Unitedmonth monitoring programme in Melbourne,Book Press.Australia, indicated that the potential nu- Athayde, D. N., Shelley, P. E., Driscoll, E. D.,</p><p>trient contribution from stormwater leaf lit- Gaboury, D. and Boyd, G. (1983). Results of theter (greater than 5 mm) is about two orders Nationwide Urban Runoff Program. Wash-</p><p>ington, DC: Environmental Protection Agency.of magnitude smaller than the nutrient loadsPB84-185537.measured from water samples in the storm-</p><p>Attivil, P. M. and Leeper, G. W. (1990). Forestwater. The results thus suggest that re- Soils and Nutrient Cycles. 202 pp. Melbourne:moving leaf litter from urban waterways will University Press.do little to reduce the total stormwater nu- Chiew, F. H. S., Mudgway, L. B., Duncan, H. P.</p><p>and McMahon, T. A. (1997). Urban Stormwatertrient load.Pollution Industry Report. Cooperative Re-search Centre for Catchment Hydrology, Mel-bourne, Australia, Report 97/5, 18 pp.</p><p>Acknowledgements Cowan, W. F. and Lee, G. F. (1973). Leaves as asource of phosphorus. Environmental Scienceand Technology 7, 853854.</p><p>The authors would like to acknowledge Melbourne Dorney, J. R. (1986). Leachable and total phos-Water and Moreland City Council for their in- phorus in urban street tree leaves. Water Airvolvement in the gross pollutant project. The au- Soil Pollution 28, 439443.thors would also like to thank Sharyn Ross, Ian Duncan, H. P. (1997). An overview of urban storm-Findlay and Brendan Salmon for their help in the water quality. Proceedings of the 24th Inter-monitoring and sorting of gross pollutants. national Hydrology and Water Resources</p><p>Symposium. Auckland, November 1997, NewZealand Hydrological Society, pp. 143148.</p><p>Prasad, D., Henry, J. G. and Kovacko, R. (1980).ReferencesPollution potential of autumn leaves in urbanrunoff. Proceedings of the International Sym-posium on Urban Storm Runoff. Kentucky, JulyAllison, R. A. and Chiew, F. H. S. (1995). Moni-1980, (M. E. Meadow and R. W. De Vore, eds),toring of stormwater pollution from various landpp. 197202.uses in an urban catchment. Proceedings of</p><p>Riley, S. J. and Abood, M. (1995). Impact of waterthe Second International Symposium on Urbanquality of gross pollutants. Proceedings of theStormwater Management, Melbourne, JulyThird Annual Conference on Planning for Cre-1995. Institution of Engineers, Australia, Na-ative Stormwater Management. Sydney, Sep-tional Conference Publication 95/3(2), 511516.tember 1995, International Erosion ControlAllison, R. A., Chiew, F. H. S. and McMahon, T. A.Association, pp. 357370.(1997). Stormwater Gross Pollutants Industry</p><p>Wong, T. H. F. and Wootton, R. M. (1995). AnReport. Cooperative Research Centre for Catch-innovative gross pollutant trap for stormwaterment Hydrology, Melbourne, Australia, Reporttreatment. Proceedings of the Second Inter-97/11, 17 pp.national Symposium on Urban StormwaterAllison, R. A., Walker, T. A., Chiew, F. H. S.,Management. Melbourne, July 1995. InstitutionONeill, I. C. and McMahon, T. A. (1998). Fromof Engineers, Australia, National ConferenceRoads to Rivers Gross Pollutant Removal from</p><p>Urban Waterways. Cooperative Research Centre Publication 95/3(2), 407412.</p></li></ul>

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