Baykeeper Update

Maps Highlighting Local Mercury and PCB Sources

We recently developed a few maps highlighting local sources of mercury and polychlorinated biphenyls (PCBs). These maps indicate that although these pollutants are generally thought to be a thing of the past the Bay Area continues to discharge them in considerable quantities into San Francisco Bay. This data was made available through development of Total Maximum Daily Loads (TMDLs) for mercury and PCBs in 2006 and 2008, respectively. TMDLs are calculations of the maximum amount of a pollutant a water body can receive and still meet water quality standards. These standards are supposed to be developed, according to the Clean Water Act, whenever a water body is deemed ‘impaired’ due to discharges of a particular pollutant.

Click here to view the mercury map and here for the map of local PCB sources.

Local Mercury Sources

Local sources of mercury and PCBs identified by local regulators largely include municipal wastewater (sewerage) agencies, industrial sources and municipal stormwater agencies. However, urban stormwater represents by far the largest local contribution of both mercury and PCBs. When the mercury TMDL was completed in 2006 local scientists believed the vast majority of mercury reaching the Bay was delivered from the Central Valley watershed and through bed erosion, a process by which mercury buried in Bay sediment becomes available for biological uptake following erosion of overlying sediment. Based on 2003 data, urban stormwater was believed to represent only about 20% of the annual contribution, or load, of mercury reaching the Bay, as shown below.

2003 Annual Mercury Loads to San Francisco Bay by Source Category (kilograms/year)

Following recent monitoring of small urban watersheds in the Bay Area, scientists at the San Francisco Estuary Institute (SFEI) now believe urban stormwater may contribute up to 40% of the mercury load. This is likely from runoff of contaminated sediments in current and former industrial areas and from the settling of airborne mercury from local industrial sources onto roads and other impermeable surfaces. One significant source of airborne mercury are the refineries along the Carquinez Strait, which contribute much more mercury to the air through their stacks than is represented in the TMDL, which is restricted to water-related discharges.

In 2009, the Western States Petroleum Association released a study documenting airborne discharges of mercury from Bay Area refineries. This study was commissioned at the request of the Regional Water Quality Control Board following a Baykeeper inquiry. It indicated that Bay Area petroleum refineries emit approximately 19 kilograms of mercury per year to the air. This amount differs significantly from Toxic Release Inventory (TRI) data reported by the refineries themselves, which indicates that petroleum refineries in Contra Costa and Solano counties release approximately 54 kg/yr. To date, the San Francisco Regional Water Quality Control Board has not pursued this issue.

Of even greater significance is the Lehigh Southwest Cement plant in Cupertino, which emits approximately 450 pounds of mercury to the air each year. Like other discharges of mercury to air, this facility is not represented in the TMDL and air emissions from the plant are not regulated under the Clean Water Act. However, in 2010 the U.S. EPA set rules to reduce mercury emissions from cement plants in an effort to address this nation-wide issue. Unfortunately, this rule is now under Republican attack in Washington, which could undermine EPA’s efforts to reduce this source of water and air pollution that poses a significant public health risk in many parts of the nation.

Local PCB Sources

Although banned from production since 1977 PCBs remain a persistent toxic contaminant in the Bay and, along with mercury, is a principal reason why you should avoid eating fish caught from the estuary on a regular basis. Based on the 2008 TMDL for PCBs in San Francisco Bay, stormwater runoff from within the Bay Area represents the largest annual contribution of PCBs. The relative stormwater contributions from the 9 Bay Area counties, along with wastewater contributions, are indicated on the PCB sources map. As indicated below, wastewater contributions represent a relatively small portion of the total load, yet are subject to significant reductions under the TMDL.

Annual PCB Loads by Source Category (kilograms/year)


As noted on the map, although municipal and industrial wastewater dischargers are believed to represent a small portion of the total load, under existing Clean Water Act permits, these facilities and agencies are permitted to discharge much greater amounts of PCBs than indicated in current estimates of total discharges as well as their future TMDL load allocations. Dischargers have 20 years to comply with the allocations provided in the TMDL. However, mechanisms for monitoring and implementation are poorly defined. Baykeeper is active in the development of stringent permit requirements for wastewater and stormwater-related discharges of PCBs and is hopeful that additional monitoring activities intent on identifying PCB hotspots will result in remediation of contaminated sites and efforts to contain or treat stormwater carrying PCBs and other harmful chemicals.