Mercury Strategy for the Bay-Delta Ecosystem - CALFED Bay-Delta ...

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Wetlands, which are generally considered important sites of microbial methylation on the
landscape, can be dominant sources of methylmercury for downstream waters (Hurley et al.
1995, St. Louis et al. 1996, Waldron et al. 2000, Domagalski 2001, Sellers et al. 2001). The
restoration of wetlands, particularly in areas where the abundance of mercury in soils or
sediments has been elevated by mining or other human activities, could accelerate the production
of methylmercury and increase the contamination of aquatic biota (Naimo et al. 2000, Wiener
and Shields 2000). In addition, flooding of vegetated wetlands or uplands, or fluctuating water
levels during tidal cycles, could stimulate microbial methylation of inorganic mercury,
increasing concentrations of methylmercury in water and biota (Hecky et al. 1991, Hall et al.
1998, Paterson et al. 1998, Bodaly and Fudge 1999, Bodaly et al. 2002).
This report presents a strategy for addressing key questions concerning the biogeochemical
cycling and potential effects of mercury in the Bay-Delta ecosystem. The goal of the mercury
strategy is to provide a holistic framework for integrated investigations needed to build a
scientific foundation for ecosystem restoration, environmental planning, and the assessment and
eventual reduction of mercury-related risks in the Bay Delta ecosystem.
The Ecosystem
II. THE SAN FRANCISCO BAY-DELTA ECOSYSTEM
The modern San Francisco Bay-Delta ecosystem can be described as three physiographic areas:
the San Francisco Bay and its estuarine embayments, the Sacramento-San Joaquin River Delta,
and the Sacramento and San Joaquin River watersheds that drain into the Delta. Conditions
across this ecosystem range from the marine environment of central San Francisco Bay to highgradient
tributaries fed largely by snow melt in the Coast Ranges and the western slopes of the
Sierra Nevada. The “Delta”, once an expansive area of tidal and non-tidal wetlands, lies at the
convergence of the Sacramento and San Joaquin rivers (Figure 1).
The Sacramento and San Joaquin rivers together drain about 37 percent of California. The
Sacramento River basin is the state’s largest (nearly 70,000 square kilometers), with annual
runoff of about 27-billion cubic meters, about one-third of the total runoff in California and
about 5 to 6 times that of the San Joaquin River basin (http://waterdata.usgs.gov/nwis/). The
Sacramento River is a major source of drinking water for the state, as well as the principal source
of irrigation water for agriculture in the Sacramento and San Joaquin valleys (Central Valley).
The Sacramento River basin includes all or parts of five physiographic provinces: the
Sacramento Valley, the Sierra Nevada, the Coast Ranges, the Cascade Range, the Klamath
Mountains, and the Modoc Plateau. The northernmost area (Modoc Plateau) is a high desert
plateau with cold snowy winters, moderate rainfall (about 30 cm), and hot dry summers. Other
high-elevation portions of the basin (including the Cascade, Coast, and Sierra Nevada ranges)
receive more precipitation (~ 50-100 cm per year) with melting winter snow yielding most of the
spring and summer runoff.
The San Joaquin River basin, which drains the Central Valley from the south, is bounded by the
Sierra Nevada to the east, the Coast Ranges to the west, and the Tehachapi Mountains to the
south. The San Joaquin River basin is more arid than the Sacramento River basin, with hotter
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