Stormwater Quality

Urban runoff contains a variety of pollutants that are washed off impervious, and to a lesser extent, the pervious surfaces of urban areas. Water-quality challenges related to pollution prevention, treatment, and impacts must consider the high variability of runoff concentrations during a storm event. More details on the dynamic nature of stormwater quality are presented in this chapter. Because the concentration of a water-quality parameter is not constant throughout a storm event, a volume-weighted average concentration is commonly used to describe a representative concentration for a particular storm event. This concentration is known as the event mean concentration or simply the EMC. Because of the heterogeneity of urban land areas, many sources contribute to the pollutant load in stormwater runoff. Stormwater characteristics are highly dynamic and can vary geographically due to climate variations and diverse activities taking place on the watershed.