The Guadalupe Watershed is located in Santa Clara County, in the South San Francisco Bay Area. The watershed generally consists of some segmented tributaries (i.e., tributaries with reservoirs which do not generally release water downstream), some partially controlled tributaries (i.e., some diversion and recreational dams, but hydrologic connection to downstream), and some uncontrolled tributaries that all flow to the Guadalupe River, which then flows (partially controlled) through the City of San Jose to South San Francisco Bay. There are numerous historic mercury mines in the upper parts of the eastern side of this watershed. We believe that organic mercury from historic mining activities has been distributed throughout the watershed due to river processes, and in some areas has accumulated in areas where conditions have caused organic mercury to transform to methylmercury. This form of mercury is toxic to humans and bioaccumulates in the food web to cause elevated levels of methylmercury in the tissue of exposed fish.
Mercury concentrations in fish tissue from Calero and Guadalupe Reservoirs generally exceed 1 ppm, which is the FDA-recommended criteria for edible fish. Due to this, there is an interim health advisory for consumption of fish caught in Calero and Guadalupe Reservoirs. Fish tissue samples taken from other parts of the watershed also contain mercury at concentrations that exceed the FDA criteria for mercury. These areas are also posted to warn of unsafe levels of mercury present in fish caught there. The interim advisory and warnings constitute a violation of the narrative standards protecting REC-1 beneficial uses.
This problem is complex because it is primarily a result of methylmercury, but depends on the abundance of all types of mercury in the system, and the ability of the system to biotransform mercury to methylmercury. There are two major types of processes that the TMDL needs to consider and better understand: river processes and in-stream biotransformation processes. The river processes act as a transport vehicle for mercury through the system, and are related to the abundance and distribution of the mercury problem. Once the mercury is mobilized, it may be deposited in areas where it becomes more bioavailable. The biotransformation processes could act as methylmercury pumps by converting local, organic mercury to methylmercury and then discharging it to the system, including South San Francisco Bay. South San Francisco Bay is also listed as impaired due to mercury.