Changes

Research has shown the pollutant mass removal rates of grass swales are variable, depending on influent pollutant concentrations (Bäckström et al., 2006)<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>, but generally moderate for most pollutants (Barrett et al., 1998<ref>Barrett, M.E., Walsh, P.M. Malina Jr., J.F. and Charbeneau, R.J. 1998. Performance of Vegetative Controls for Treating Highway Runoff. Journal of Environmental Engineering. November 1998. pp. 1121-1128.</ref>; Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Median pollutant mass removal rates of swales from available performance studies are 76% for total suspended solids, 55% for total phosphorus, and 50% for total nitrogen (Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Significant reductions in total zinc and copper event mean concentrations have been observed in performance studies with a median value of 60%, but results have varied widely (Barrett, 2008<ref>Barrett, M.E. 2008. Comparison of BMP performance using the international BMP database. Journal of Irrigation and Drainage Engineering, 134(5), pp.556-561.</ref>). Site specific factors such as slope, soil type, infiltration rate, swale length and vegetative cover also affect pollutant mass removal rates. In general, the dominant pollutant removal mechanism operating in grass swales is infiltration, rather than filtration, because pollutants trapped on the surface of the swale by vegetation or check dams are not permanently bound (Bäckström et al., 2006<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>). Designers should maximize the degree of infiltration achieved within a grass swale by incorporating check dams and ensuring the native soils have infiltration rates of 15 mm/hr or greater or specifying that the soils be tilled and amended with compost prior to planting. Several of the factors that can significantly increase or decrease the pollutant removal capacity of grass channels are provided in the table below:<br>

Research has shown the pollutant mass removal rates of grass swales are variable, depending on influent pollutant concentrations (Bäckström et al., 2006)<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>, but generally moderate for most pollutants (Barrett et al., 1998<ref>Barrett, M.E., Walsh, P.M. Malina Jr., J.F. and Charbeneau, R.J. 1998. Performance of Vegetative Controls for Treating Highway Runoff. Journal of Environmental Engineering. November 1998. pp. 1121-1128.</ref>; Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Median pollutant mass removal rates of swales from available performance studies are 76% for total suspended solids, 55% for total phosphorus, and 50% for total nitrogen (Deletic and Fletcher, 2006<ref>Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.</ref>). Significant reductions in total zinc and copper event mean concentrations have been observed in performance studies with a median value of 60%, but results have varied widely (Barrett, 2008<ref>Barrett, M.E. 2008. Comparison of BMP performance using the international BMP database. Journal of Irrigation and Drainage Engineering, 134(5), pp.556-561.</ref>). Site specific factors such as slope, soil type, infiltration rate, swale length and vegetative cover also affect pollutant mass removal rates. In general, the dominant pollutant removal mechanism operating in grass swales is infiltration, rather than filtration, because pollutants trapped on the surface of the swale by vegetation or check dams are not permanently bound (Bäckström et al., 2006<ref>Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm</ref>). Designers should maximize the degree of infiltration achieved within a grass swale by incorporating check dams and ensuring the native soils have infiltration rates of 15 mm/hr or greater or specifying that the soils be tilled and amended with compost prior to planting. Several of the factors that can significantly increase or decrease the pollutant removal capacity of grass channels are provided in the table below:<br>

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|+Factors that Influence the Pollutant Removal Capacity of Swales

|+Factors that Influence the Pollutant Removal Capacity of Swales