We need to better understand the processes (e.g., fluvial and biotransformation) that cause and contribute to the impairment in this watershed. We need to identify the cause of impairment (i.e., methylmercury, or organic mercury). We also need to determine under what circumstances the waterbodies can be delisted (i.e., determine they are no longer impaired).
Available information suggests that the major sources of total mercury loads in the Guadalupe Watershed are abandoned mercury mines. Mercury loading from these sources continues to occur as a result of drainage and erosion from the abandoned mine sites. Historic river processes have distributed mercury from these known sources throughout the watershed. These deposits have not been identified and act as potential sinks and sources of future mercury loadings to the watershed. In all, we do not fully understand the fate and transport of all the sources of mercury in the watershed and therefore cannot predict the effectiveness of control measures on them. We have evidence to indicate that mercury loading peaks in the Guadalupe Watershed are associated with pulse flows from first-flush events. We also have evidence that shows in San Francisco Bay a consistent, long-term pattern of mercury bioaccumulation up the food chain. Runoff and erosion from abandoned mercury mines, sinks and sources within the Guadalupe Watershed need to be confirmed as significant, ongoing sources of organic mercury discharges to South San Francisco Bay.
Some known, ongoing sources of mercury discharges to the watershed are tailings piles and uncontrolled drainage from abandoned mines. Active mining in the region ceased after 1975. Mercury primarily occurs as cinnabar (mercuric sulfide) in Franciscan assemblage and serpentine rocks. Elemental mercury may also be present. There are at least 10 open pit mines located within the Guadalupe Watershed. Surface mining also occurred at some of the mines. New Almaden is probably the largest abandoned mine and is a State Superfund site, regulated by the Department of Toxic Substances Control (lead agency).
Reservoirs, overbank deposits and point bars located downstream from these abandoned mines are also potential sources of mercury discharges to the system. The Calero Reservoir, which is reportedly not downstream from any known mercury mines, is a source of mercury because but water is transferred to it from Almaden Reservoir. Mercury-laden tailings that have been transported from the mines and deposited into downstream pools, banks and floodplains are also potential sources of mercury discharges to the system. Historic processing of mercury resulted in atmospheric release and deposition of mercury. This phenomena makes it difficult to estimate “background” levels of mercury in South Bay soils.