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The San Francisco Bay Watershed Model for Copper Runoff from Brake Pad Wear Debris

Client: Association of Bay Area Governments (ABAG), Oakland, CA

Description

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 multi stakeholder effort of manufacturers, regulators, stormwater management agencies and environmentalists working together to better understand the impacts of this source of copper on water quality. Phase I of the watershed model provided runoff loads to a Bay modeling effort to assess resulting concentrations in SF Bay. Phase II adjusted the model to better represent copper deposition on roadways and determined the time required for copper to return to baseline, or near baseline concentrations if the source is eliminated.



Figure 1: San Francisco bay area with delimited watershed borders shown in red

Model Development

Phase I of this project included model conceptualization, parameterization, calibration, and scenario analyses. 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. Significant meteorological data were needed to model in an area such as the San Francisco Bay Area where micro-climates provide great spatial variability in meteorological conditions. In Phase II, changes discussed were applied to 5 test watersheds, which were selected to provide a range of conditions. These test basins are shown in dark green in Figure 1.

Model Application and Results

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.

Among the Brake Pad Partnership modeled watersheds, the total contribution from brake pad wear debris towards total anthropogenic loads of copper to the Bay for the median estimate case varies from 15% (for the Sonoma sub-watershed) to 57% (for the Upper Colma sub-watershed - See Figure 2). For the rural sub-watersheds, the brake pad contribution was much lower than for the heavily urbanized sub-watersheds, such as Peninsula Central, Upper Colma, and Santa Clara Valley West.


Figure 2: Suspended solid results for studied subbasins in the San Francisco Bay

To better understand the potential importance of brake pad copper relative to other sources that introduce copper into the Bay, three cases of copper release (flux) scenarios were modeled: brakes-high, brakes-low, and median estimate. The median estimates case is based on the point value presented in the copper release inventories for both brake sources and non-brake sources. The brakes-low case explores the source term estimates from the perspective that the point values in the release inventory overestimate brake contributions relative to non-brake sources. The brakes-high case explores the source terms from the perspective that the point values in the release inventory underestimate brake contributions relative to non-brake sources of copper. Each of these 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 modeling approach that was used for this study featured a new method to account for "lost" copper. This was accomplished by moving the copper from the roadway surface to the roadway buffer where it is subject to washoff from surface waters. The results showed an increase of approximately 15% to 30% copper contribution from brake pads when compared to the "Mid Brakes" scenario for an impervious buffer, but a relatively minor impact if the buffer is pervious.


Table 1: Effects of Roadway Buffers on Copper Loads
  Mid Brakes
Load to Bay
(kg/yr)
Pervious Buffer Impervious Buffer
Cu Load to Bay
(kg/yr)
% Chg from
Mid Brakes
Cu Load to Bay
(kg/yr)
% Chg from
Mid Brakes
East Bay North 1,781 1,845 3.6% 2,045 15%
Peninsula Central 2,682 2,723 1.5% 3,452 29%
Petaluma 4,742 4,772 0.6% 5,689 20%
Santa Clara Valley Central 2,645 2,704 2.2% 3,478 31%
Upper Colma 521 539 3.3% 648 24%

Sensitivity analysis was performed for the accumulation maximum (SQOLIM in HSPF), and other model parameters to evaluate the uncertainty in some key model parameters. The testing was primarily evaluated by comparing the predicted copper loads to the Bay with the loads from the Phase 1 effort. Loads changed significantly when the accumulation limit was modified, as expected.

Analyses were performed under both wet and dry meteorological conditions to assess the impacts of climate variability. Based on rainfall time series from areas around the Bay, the entire 26-year period was run. This provided a repeat of the historic time period and its associated climate conditions a Dry meteorological run using the 11-year period from 1984 to 1994, repeated for two or three cycles, and a Wet meteorological run using the 11-year period from 1995 to 2005, repeated for two or three cycles.

Table 2:Time to Reach 5% of Brake Copper Loads (Years)
  Time Lag to Reach 5% of Brake Copper Loads (years),

95% Reduction in Brake Loads

  Historical Met
Data, WY84-WY05
Dry Scenario,
WY84-WY94
Wet Scenario,
WY95-WY05
East Bay North 6-10 6-10 6-10
Peninsula Central 6-10 10-15 6-10
Petaluma 6-10 6-10 0-5
Santa Clara Valley Central >20 6-10 6-10
Upper Colma 11-15 11-15 11-15

The results of these runs were summarized by compiling the time it takes to reach 5%, of the original BPWD loads; this percentage also corresponds to 95% reduction in the BPWD loads. The table below shows the times required to reach this level based on 5-year increments. To reach a 95% reduction level, two decades or more may be required.

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