San Francisco Bay is an important fishery on the West Coast, serving as a food source for approximately 150,000 anglers, and providing habitat for rare and endangered species. Monitoring data indicate that mercury concentrations in popular sport fish exceed acceptable risk levels for
developmental impairment of children and expectant mothers. Based on the latest criteria guidance from USEPA and local consumption surveys, mercury concentrations in popular sport fish need to be reduced by two-fold to fully protect the majority of subsistence fishers. Strategies to reduce
mercury concentrations in aquatic ecosystems must focus both on mercury loads and mercury methylation, because methylmercury is the primary chemical form that accumulates in biota. By assessing how watershed processes affect both the total mercury inventory and the mercury methylation rate
in receiving waters, a rational basis is established for implementation actions. Since the vast majority of mercury in aquatic ecosystems is bound to particulates, mercury loads are assessed by evaluating how different sources affect the concentration of mercury in Bay sediments. The Bay
is downstream of 40 percent of the land area of California, and two to three billion kilograms of sediments are annually washed into San Francisco Bay. Prior to the European settlement of California, the concentration of mercury in Bay sediments was approximately 0.06 ppm. Today, the concentration
of mercury in sediments is approximately 0.4 ppm, a six-fold excess compared to pre-settlement conditions. Half of the contemporary excess mercury concentration in Bay sediments is accounted for by background processes, including shifts in the mineralogy of watershed source sediments and
atmospheric deposition of global air sources. The other half of the excess mercury in Bay sediments is mostly attributed to mining legacy sources, with lesser fractions attributed to wastewater discharge (1-3%) and urban runoff (3-10%). Mining legacy sources include feasibly
controlled processes, such as erosion of waste rock from inoperative mercury mines, and more intractable sources, such as remobilization of mercury from historic sediment deposits. It is projected that controlling all controllable sources will, after decades of equilibration, produce a steady-state
mercury concentration of approximately 0.2 ppm, or half of the current concentration in sediments. Water column mercury concentrations in the turbid Bay waters are directly proportional to mercury concentrations of suspended sediments. Attainment of the projected mercury concentration in
Bay sediments (0.2 ppm) would result in attainment of the existing numeric objective for mercury in the water column (25 ng/L) in most regions of the Bay, so existing standards are sufficient to require watershed restorations to prevent discharge of mercury polluted sediments. Unfortunately,
it is unlikely that fish tissue targets can be attained over time through load reductions alone, because mercury bioaccumulation is mainly driven by the methylmercury concentrations in aquatic ecosystems, rather than total mercury concentrations. To effectively reduce mercury concentrations
in fish, controllable water quality factors that promote mercury methylation in the aquatic ecosystem must be considered in conjunction with mercury load reductions. Some of these water quality factors (i.e., dissolved oxygen) are already subject to regulation. Recent monitoring data demonstrates
that mercury methylation efficiency in the Bay increases four-fold when dissolved oxygen drops below 6 mg/L. Thus, while load reductions can reduce total mercury inventories by a half over long (decadal) timescales in large areas, watershed management to reduce eutrophication and anoxia
could result in rapid reductions in tissue concentrations in localized areas. Consequently, the TMDL implementation plan will call for load reductions through restoration of inoperative mine sites in rural watersheds, pollution prevention measures in the urban environment, and adaptive management
strategies to identify and control factors (e.g., nutrient loading, dissolved oxygen) that create conditions favorable to mercury methylation.
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