Technical Support for Tomales Bay Watershed Mercury TMDL Project
Project Status: Completed
Tomales Bay appears on the federal Clean Water Act 303(d) list of impaired waters for mercury, which could threaten the health of humans and wildlife. The purpose of this study is to assist the San Francisco Bay Regional Water Quality Control Board (SFBRWQCB) to better understand the risks to wildlife of mercury in Tomales Bay. The data will also enable the SFBRWQCB to develop a Total Maximum Daily Load (TMDL) for mercury, the plan required by the EPA to attain water quality objectives.
Data collection focused on three factors from which to assess impairment to piscivorous (fish-eating) wildlife in Tomales Bay due to mercury: (1) surface sediment samples, (2) water samples, and (3) small fish sampling.
Elevated total mercury in sediment was largely confined to the Walker Creek Delta. Methylmercury, the organic form of mercury that most easily accumulates in animals) in sediment was also highest in the Walker Creek Delta, although there were elevated concentrations from the Lagunitas Delta as well. Higher methylmercury concentrations were found in tidal marsh sediment compared to mudflat sediment.
In this study, fish 5-15 cm in length had a mean mercury concentration of 0.047 ug/g wet, which is nearly equivalent to the suggested wildlife target for that size class (0.046 µg/g). Of the 46 small fish composites in this size class, half (48%) of them exceeded the estimated safe concentration. Thus, fish meet or exceed the safe concentrations. A larger dataset would provide us a more definitive interpretation of how fish meet these safe concentrations (with a more certain margin of error).