Mercury, Restoration and Management in Tidal Wetlands of the San Francisco Estuary: an Ecosystem Approach to TMDL Implementation

A TMDL for mercury in the San Francisco Estuary has been adopted by the San Francisco Bay Regional Water Quality Control Board. This "legacy" contaminant is widely distributed in the estuary and a large reservoir resides in sediment profiles. If all loads currently entering the estuary are eliminated, it is uncertain that inherent assimilative capacity would eliminate impairment in this century. The most toxic and bioavailable form, methylmercury, is generated by complex biogeochemical processes that are in turn influenced by certain environmental conditions. Given the uncertainty in the ability of load reduction alone to achieve impairment elimination, mercury risk is being addressed by identifying means to modify estuarine conditions to reduce methylation of mercury. Because tidal wetlands may be the primary location in the estuary where methylation occurs and because they are the most feasibly engineered and managed of estuarine habitats, this project is focused on restoration design and management of tidal wetlands. This paper describes the conceptual framework that is being employed to identify and evaluate potential control options. Habitat methylation potential, habitat spatial extent, environmental parameters, and temporal aspects are being evaluated on an ecosystem scale. Initial results include identification of major wetland habitats and potential control options. Examples of how design and management options are being identified and evaluated will be presented and discussed.