A regional mass balance of methylmercury in San Francisco Bay, California, USA

Abstract

The San Francisco Bay (California, USA) is a water body listed as impaired because of Hg contamination in sport fish for human consumption, as well as possible effects on resident wildlife. A legacy of Hg mining in local watersheds and Hg used in Au mining in the Sierra Nevada (USA) has contributed to contamination seen in the bay, with additional more recent and ongoing inputs from various sources. Methylmercury is the species of Hg most directly responsible for contamination in biota, so better understanding of its sources, loads, and processes was sought to identify the best means to reduce impacts. A regional scale model of San Francisco Bay was developed to characterize major methylmercury inputs and processes. The model was used to evaluate the potential impact of uncertainties in estimates for methylmercury loading pathways and environmental processes, identify major data gaps, and explore management prospects for reducing methylmercury contamination. External loading pathways considered in the mass balance include methylmercury loads entering via atmospheric deposition to the bay surface, and discharges from the Sacramento/San Joaquin Delta, local watersheds, municipal wastewater, and fringing wetlands. Internal processes considered include exchange between bed and suspended sediments and the water column, in situ production and demethylation, biological uptake, and losses via hydrologic transport to the ocean through the Golden Gate. In situ sediment methylation and demethylation were dominant sources and losses determining ambient steady-state concentrations in the model, with changes in external loads and export causing smaller changes. Better information on methylation and demethylation is thus most critical to improving understanding of methylmercury balances and management.

Mercury offloaded in Northern elephant seal hair affects coastal seawater surrounding rookery

“…The distinct increase in average percentage MeHg to HgT (wt/wt) in seawater during the molting season (24%) compared with the breeding season (3.9%) and with the comparison coastal sites (2.0%) further indicates that the molted pelage, which likely contained a high concentration of MeHg, was the primary source of that Hg enrichment (Table 1). This assertion is supported by other studies, which report an elevated MeHg percentage in sediments with biological sources of Hg (21), whereas a lower percentage of MeHg is typically associated with industrial and anthropogenic sources of Hg contamination (1,5,25). For example, storm water inputs containing 0.5-1.6% (wt/wt) MeHg were a main source of Hg to the local watersheds of the San Francisco Bay estuary (25).…”

“…This assertion is supported by other studies, which report an elevated MeHg percentage in sediments with biological sources of Hg (21), whereas a lower percentage of MeHg is typically associated with industrial and anthropogenic sources of Hg contamination (1,5,25). For example, storm water inputs containing 0.5-1.6% (wt/wt) MeHg were a main source of Hg to the local watersheds of the San Francisco Bay estuary (25).…”

“…On the basis of this estimate, we calculate that ∼0.2 kg organic Hg entered the nearshore environment of Año Nuevo during that molting season. To put that amount in perspective, the San Francisco Bay covers ∼1,100 km 2 , drains 40% of the state of California (26,27), and receives an estimated annual load of 8.0 kg MeHg from external sources such as rivers and point source discharges (25). Conversely, the Año Nuevo State Marine Conservation Area covers only 26.4 km 2 (28), indicating that the magnitude of annual inputs from pinniped colonization constitutes a relevant, previously unaccounted for, source of MeHg to that marine reserve.…”

Seasonal and annual trends in forage fish mercury concentrations, San Francisco Bay

“…In San Francisco Bay, PCBs and legacy pesticides have declined but Hg concentrations have exhibited no consistent upward or downward trends in Bay biota (Greenfield et al, 2005;Gunther et al, 1999;Stephenson et al, 1995). This is likely due to continued atmospheric and fluvial inputs in addition to disturbance of legacy deposits, resulting in stable concentrations of total Hg in sediment and projected rates of change on the order of decades to centuries (Davis et al, 2012;Yee et al, 2011).…”

Coupled impacts of sea-level rise and tidal marsh restoration on endangered California clapper rail

“…Due to the large inventory of mercury from regional historic ore mining, the concentration of inorganic mercury in estuarine sediment is expected to remain at high levels for several decades despite existing and planned source control actions (Davis et al, 2012;Marvin-DiPasquale and Agee, 2003;Miles and Ricca, 2010;Yee et al, 2011). Based on theories of chemical equilibrium and solute diffusion, the abundance of sediment mercury would control the production and release of biotoxic mercury species (methylmercury) and maintain its presence in the estuarine environment.…”