A "first flush" phenomenon of pollutants in stormwater occurs when rainfall after a dry period entrains a
greater pollutant load from catchment surfaces than that of subsequent rainfall. The occurrence and
magnitude of first flush events may influence the effective management of urban runoff pollution. To
facilitate the understanding of the first flush phenomenon on a seasonal scale, the City of San Jose, California
carried out an investigation between May 1997 and April 2000 to characterize concentrations of
pollutants in local waterbodies during eight storm events.
The purpose of the investigation was twofold: to determine if concentrations of specific constituents
in stormwater runoff are elevated during storms preceded by an extended dry period; and, if so, to
identify the environmental conditions surrounding such events (first flush events). Concentration data for
total and dissolved metals, pesticides, PAHs, anions, TSS, TOC, conductivity, gasoline and diesel, and
volatile and semi-volatile organics were collected at over twenty-five sites within the Guadalupe River
and Coyote Creek watersheds (City of San Jose, CA). The numerical analysis of the monitoring data
focused on identifying combinations of circumstances yielding increased levels of pollutants during the
first substantial storms of the rainy season compared to other storm events. Factors potentially impacting
the 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 were
below detectable levels. The results of the investigation suggest that first flush phenomena did not occur
consistently 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 that
result in a first flush effect with respect to dissolved metals. Based on the results of this and related
investigations, implications for water resource and urban runoff management are discussed. Includes 14 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 510 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 16 |
| Published : | 01/11/2004 |
