First Flush Stormwater Runoff from Highways

Abstract

Stormwater is now receiving attention from regulatory agencies and has become an important component in watershed planning. In many cases, pollutant mass emissions from stormwater exceed those from wastewater treatment plants. Land use has been identified as an important parameter in predicting stormwater quality. Land uses associated with vehicular activity, such as parking lots, are thought to be high contributors of stormwater pollutants. Other factors, such as greater pollutant concentrations or mass emissions at the onset of rainfall, usually called a “first flush,” or higher emissions from the first storm of the season, usually called a “seasonal first flush,” have been identified. In order to determine the magnitude of the first flush from freeway runoff, three sites in the west Los Angeles area were sampled for 14 storms during the 1999-2000 rainy season. Samples were collected very early in the storm in order to compare water quality from the first runoff to water quality from the middle of the storm. A large range of water quality parameters and metals were analyzed. The data show large first flushes in concentration profiles and moderate first flushes in mass emission rates.