Research Areas > Stormwater > Time Variable Stormwater Pollutants

Project: Time Variable Runoff and Pollutant Loading from Watersheds and Land Uses

Background and Objectives

Pollutant-bearing storm water runoff from urban areas is currently one of the leading sources of water quality degradation in the US. Runoff from pervious and impervious areas (i.e., streets, parking lots, lawns, golf courses, and agricultural land) tends to carry accumulated contaminants (i.e., atmospheric dust, trace metals, street dirt, hydrocarbons, fertilizers, and pesticides) directly into receiving waters. Because of the environmental effects of these contaminants, effective storm water monitoring and management requires identification and characterization of the sources, patterns, and mechanisms that influence pollutant concentrations and loads.

The objectives of this project were 1) to examine constituent event mean concentrations (EMC), fluxes, and mass loadings associated with storm water runoff from representative land uses, 2) to investigate within-storm and within-season factors that affect constituent concentrations and fluxes, 3) to evaluate how constituent loadings compare to loadings from point sources, and 4) to assess how the concentrations of constituents in runoff compare to published data and water-quality criteria.

Status

This project was conducted from 2000 to 2006. Periodic monitoring is ongoing.

Methods

To understand complex spatial and temporal patterns, a variety of land uses and mass emission sites were sampled over a range of different storm sizes and antecedent conditions. Between 10 and 15 discrete grab samples per event were collected for each site, and the samples were analyzed individually to provide time vs. concentration plots (i.e., pollutographs) for each site-event. Samples were analyzed for a broad range of constituents including trace metals, organic compounds, and bacteria. Storms were targeted to capture early vs. late season conditions and large vs. small rainfall events.

Findings

Major findings from this study were:

• Storm water runoff from watershed and land use sources was a significant contributor of pollutant loading, and the runoff often exceeded water quality standards. For example, at storm water mass emission sites, trace metal levels exceeded water quality criteria based on the California Toxics Rule for more than 80% of the wet weather samples.

• All constituents were strongly correlated with total suspended solids (TSS), suggesting that reducing TSS may lead to a reduction in other particle-bound constituents.

• No single land use type was responsible for contributing the highest loading for all constituents measured. For example, industrial land use sites contributed higher storm EMCs and fluxes of all trace metals than other land use types. Recreational (horse) land use sites contributed significantly higher storm fluxes for E. coli, while agricultural land use sites contributed the highest TSS fluxes.

• Storm water runoff contributed a similar range of constituent loading as regional point source discharges. When combined with dry estimates of pollutant loading from other SCCWRP studies, the total non-point source contribution from all watersheds in the greater Los Angeles area far exceeded that of the point sources.

• The Los Angeles region had a similar range of storm water runoff pollutant loads as that of other regions of the United States.