A "first flush" phenomenon of pollutants in stormwater occurs when rainfall after a dry period entrains agreater pollutant load from catchment surfaces than that of subsequent rainfall. The occurrence andmagnitude of first flush events may influence the effective management of urban runoff pollution. Tofacilitate the understanding of the first flush phenomenon on a seasonal scale, the City of San Jose, Californiacarried out an investigation between May 1997 and April 2000 to characterize concentrations ofpollutants in local waterbodies during eight storm events.The purpose of the investigation was twofold: to determine if concentrations of specific constituentsin stormwater runoff are elevated during storms preceded by an extended dry period; and, if so, toidentify the environmental conditions surrounding such events (first flush events). Concentration data fortotal and dissolved metals, pesticides, PAHs, anions, TSS, TOC, conductivity, gasoline and diesel, andvolatile and semi-volatile organics were collected at over twenty-five sites within the Guadalupe Riverand Coyote Creek watersheds (City of San Jose, CA). The numerical analysis of the monitoring datafocused on identifying combinations of circumstances yielding increased levels of pollutants during thefirst substantial storms of the rainy season compared to other storm events. Factors potentially impactingthe occurrence of a first flush event were grouped according to whether they pertained to pollutant, site,and/or storm characteristics.Quantitative analysis focused on metals and anions because most observations for other constituents werebelow detectable levels. The results of the investigation suggest that first flush phenomena did not occurconsistently throughout most of the stations investigated for either (total or dissolved) metals or anions.The results do however suggest that there are specific combinations of site and storm circumstances thatresult in a first flush effect with respect to dissolved metals. Based on the results of this and relatedinvestigations, implications for water resource and urban runoff management are discussed. Includes 14 references, tables, figures.
