A washoff model for stormwater pollutants

A washoff rate equation for stormwater pollutants is presented based on linear buildup of pollutant mass in the watershed. The expression was applied to residential and open land areas that have not previously been considered even though these areas are major sources of metals, nutrients, and bacteria in stormwater runoff. The transport coefficient c was determined from the nearly constant portion of the hydrograph at the peak of each runoff event. Values of c (cm− 1) for seven metals, i.e., Zn, Cu, Cd, Ni, Pb, Hg, and Ag (3.03 ± 1.34–6.10 ± 2.55 cm− 1), TSS, BOD5, total phosphorus (TP), E. coli, and fecal coliform (2.12 ± 0.907–7.16 ± 2.72 cm− 1) were estimated using this equation. The washoff rate was developed to reflect mixed land uses based on mass deposition and effective area. The order of transport coefficients for metals is Pb > Ag > Zn > Cu > Ni > Hg > Cd reflecting decreasing particle association. For all parameters this order is TSS > fecal coliform > E. coli > Pb > BOD5 BOD5 > Ag > Zn > Cu > Ni > Hg > Cd > TP. The washoff rate and a related pollutant concentration model are expected to be useful for the development of best management practices for stormwater.