PROBLEM DEFINITION AND BACKGROUND. The Santa Xxxxx Valley Water District has been designated as the lead agency on behalf of the Santa Xxxxx Basin Watershed Management Initiative (WMI) for the development of a Total Maximum Daily Load (TMDL) for mercury in the entire Xxxxxxxxx River Watershed. Former mercury mines are located in the upper Xxxxxxxxx River Watershed, which have contributed mercury to downstream surface waters and San Francisco Bay. The New Almaden Mining District in the Guadalupe Creek and Alamitos subwatershed was the largest producer of mercury in North America. Mercury in the mining district is primarily present as the mineral cinnabar. Mercury in downgradient water and sediment can be present in dissolved or particulate forms. Under appropriate conditions, bacteria can convert inorganic mercury to the organic form, methylmercury, which is the form of primary concern from a human health and ecological perspective due to its greater toxicity and ability to bioaccumulate. Other sources of mercury to the watershed include atmospheric deposition from global and local sources, stormwater runoff, and seepage from contaminated sites and landfills.
