Tmdl schedule and workplans status july 1999


Summary of Water Quality Monitoring Results



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Summary of Water Quality Monitoring Results


Water Body

Total Mercury (ng/l)

Dissolved Mercury (ng/l)

Walker Creek Ranch

343 – 7068

4.79 – 15.9

Walker Creek Mouth

255 – 2607

5.62 – 8.96

San Francisco Bay

0.01 – 105

Non-detect – 2.7

In May 1998, Board staff conducted an extensive study to determine mercury concentrations in native and cultured shellfish in Tomales Bay. The goal of the study was to determine the human health risk associated with consumption of shellfish. Cultured oysters in Tomales Bay ranged from 0.012-0.057 ppm mercury (wet weight), cultured mussels ranged from 0.021-0.032 ppm mercury, and cultured clams range from 0.048-0.053 ppm mercury. All results for cultured shellfish are well below the FDA limit (1 ppm) and the Regional Board’s screening criteria (0.17 ppm) although, a spatial trend was observed in the data indicating increased biological uptake of Mercury by oysters farmed closer to the mouth of Walker Creek. In contrast to cultured shellfish, native clams sampled at Hamlet Point (just south of the Walker Creek estuary) contained alarmingly high levels of mercury, ranging from 0.28 - 0.67 ppm (wet weight). Sediments from this area also contained high levels of mercury (up to 23 ppm). Based on these results, a more extensive study of native clams and mercury/sediment deposition is underway.


A recent published study evaluated mercury concentrations in livers of diving ducks in Suisun and Tomales Bay. Reported liver mercury concentrations for Tomales Bay birds were 19 ppm, double the amount found in Suisun Bay ducks. While the biological effects of elevated mercury concentrations on diving ducks are not well studied, the concentrations reported are high enough that overwinter survival and reproductive success are at risk. As a result of its tendency to bioaccumulate in the food chain, mercury tissue concentrations tend to be higher in upper trophic level fish and wildlife predators. The high concentrations found in diving ducks in Tomales Bay support this hypothesis. Shark, halibut and jack smelt are upper trophic level fish found in Tomales Bay that are at risk, as are their consumers. Because mercury is closely associated with sediment, detritus feeders such as crabs, which are also consumed by humans, are at risk. A study of mercury levels in fish tissue from Tomales Bay is underway. The purpose of this study is to evaluate whether consumption of seafood from Tomales Bay poses a threat to human health.


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