Watershed Modeling of Copper Runoff to San Francisco Bay from Brake Pad Wear Debris

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 Bay (SF) Region. The watershed model provides runoff loads to a Bay modeling effort to assess resulting concentrations in SF Bay. The U.S. EPA's Hydrological Simulation Program-FORTRAN (HSPF) model was set up for each of the 22 BPP modeled sub-watersheds that drain to the SF Bay. HSPF model runs were performed for each sub-watershed for the entire time period of water year 1981 through water year 2005. Model results were processed for flow, sediment, and copper loads; and annual and mean annual loads were tabulated. Uncertainty in both non-brake and brake release estimates was assessed by representing alternative scenarios of source loadings. Three cases of copper release scenarios were modeled -- with high, low, and median releases. Each of these three scenarios was modeled with and without releases from brake pads in order to determine the relative contribution of copper from brake pads in runoff to the Bay. The total anthropogenic contribution from brake pad wear debris towards total loads of copper to the Bay for the median estimate case varies from 15% to 57%. As expected, the brake pad contribution is much lower for the rural sub-watersheds than for the heavily urbanized subwatersheds, reflecting alternative human activity and traffic levels. Additional scenario runs were performed to assess the impacts of copper lost through the normal buildup/washoff attenuation algorithms and the time period for copper loads to return to background levels if all sources were eliminated. Scenario runs also considered impacts of climate changes such as wet and dry periods, and the relative impacts on loadings.