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Mercury Fall Out

Atmospheric deposition contributes almost seven times as much mercury to the Bay as wastewater discharges, but is nonetheless not the primary source of the pollutant, according to a pilot study released last summer by the San Francisco Estuary Institute (see Now in Print).

The study analyzed mercury in the ambient air and in precipitation to estimate dry and wet deposition from both direct and indirect sources. (Indirect loading derives from mercury being deposited to the watershed and then transported to the Estuary through runoff and tributaries.) The study did not include estimates of indirect loading from remote watersheds, such as the Sacramento and San Joaquin River drainage areas.

Among the study’s findings were that the ambient air concentration of mercury in the Bay Area is approximately twice that considered the global background level, although the Institute’s Don Yee says that level is similar to that of many other U.S. urban areas.

Annually, direct atmospheric deposition contributes approximately 27 kg to the Estuary, and indirect deposition contributes approximately 55 kg. Nevertheless, the most significant source of mercury continues to be the erosion of mercury-laden soils and runoff from abandoned mines.

The study concludes that strategies for minimizing mercury loading to the Estuary should include an investigation of measures to mitigate sources that contribute to mercury in the atmosphere, such as electric utility boilers and municipal waste combustion. However, Yee says that will come down the road. "The dominant sources of mercury are still the tributaries and mercury already in the Bay, and those will have to be dealt with first," he says.

Contact: Don Yee (510)746-7369

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