THE SAN FRANCISCO BAY WATERSHED MODEL FOR COPPER RUNOFF FROM BRAKE PAD WEAR DEBRIS: Part II - Model Application and Results

This watershed modeling effort was conducted as part of a larger study by the Brake Pad Partnership (BPP) that examines the potential impact of copper from brake pad wear debris (BPWD) released to the environment in the San Francisco (SF) Bay Region. The BPP is a multistakeholder effort of manufacturers, regulators, stormwater management agencies and environmentalists working together to better understand the impacts of this source of copper on water quality. The watershed model provides runoff loads to a Bay modeling effort to assess resulting concentrations in SF Bay. This paper describes the model application process, including model conceptualization, parameterization, calibration, and scenario analyses. The GIS processing of relevant data layers to produce watershed characterization and modeling inputs is described in a companion paper at this conference. With local data for land use, soils, topography, and meteorology, the U.S. EPA's Hydrological Simulation Program- FORTRAN (HSPF) model, as part of the BASINS system, was set up for each of the 22 BPP modeled sub-watersheds that drain to the San Francisco Bay. HSPF model runs were performed for each sub-watershed for the entire time period of water year 1981 through water year 2005, i.e. October 1980 through September 2005. Model results were processed for flow, sediment and copper loads; annual and mean annual loads were tabulated; and daily flows and concentrations (both sediment and copper, total and dissolved) were reviewed as a quality assurance confirmation. Model results were analyzed to distribute and characterize the contribution of the total copper load to the Bay from three sources -- from BPWD, anthropogenic non-BPWD, and sediment/background soil levels -- from each sub-watershed and the total from all Bay Area sub-watersheds. The total contribution from BPWD varies from 15% to 57% of the total load. As expected, the brake pad contribution is much lower for the rural sub-watersheds than for the heavily urbanized sub- watersheds, reflecting alternative human activity and traffic levels. KEY TERMS: Watershed modeling, flow, sediment, copper loadings, nonpoint sources