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

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The modern San Francisco Bay can be characterized as an ecologically young, but extensively
modified, estuarine ecosystem. The estuary was formed 15,000 to 18,000 years ago, when rising
sea waters from glacial melting entered the Golden Gate, inundating what are now the major
embayments of the San Francisco Bay (San Pablo Bay, Carquinez Strait, Suisan Bay, Grizzly
Bay, Honker Bay), transforming a riverine system into an extensive and complex estuary
(Atwater 1979). Together, the Sacramento-San Joaquin Delta and the embayments of San
Francisco Bay form the largest estuary on the West Coast of the United States, with a combined
area of about 3000 square kilometers. The Delta is estuarine through its lower end, but is almost
completely influenced by tidal cycles. About 72 percent of the Delta land area is in agricultural
production, which was engineered via a complex system of dikes, drainage ditches, irrigation
diversions, pumps, and floodgates. This complex drainage pattern combined with a strong tidal
currents create large tidal excursions, where distinct water parcels, with distinct chemical
characteristics, can travel many miles on a given ebb or flood tide in patterns that are difficult to
predict or anticipate. Freshwater inflows (excluding precipitation) to the Delta are mainly from
the Sacramento River (about 75-80 percent), with most of this inflow during January to April.
Mining and Mercury
The mountain ranges that surround California’s Central Valley and drain into the Sacramento
and San Joaquin watersheds contain extensive mineral deposits. Discovery of gold deposits in
the Klamath Mountains and Sierra Nevada stimulated the California Gold Rush in 1848, and an
abundance of mercury – mined from deposits in the Coast Ranges – facilitated the rapid historic
proliferation of gold-mining operations (Figure 2) that used the mercury-amalgamation process
to extract gold (Alpers and Hunerlach 2000). Hundreds of hydraulic gold-placer mines operated
on the east side of the Central Valley, where tens of millions of cubic meters of rock and earth
were excavated annually by hydraulic mining. The resulting mining debris choked streams and
rivers downstream of mining sites, and in some cases valleys were nearly filled with debris.
About 100,000 metric tons of mercury were produced by mercury-mining operations in the Coast
Ranges, and about 12,000 metric tons of this were used in gold mining in California, with annual
losses at mine sites ranging from about 10 to 30 percent of the mercury used (Alpers and
Hunerlach 2000). The effects of these mining activities are evident in the Bay-Delta estuary far
downstream (Conomos et al. 1985). Consequently, mercury from a mineral belt associated with
Cenozoic hydrothermal deposits in the Coast Ranges (Rytuba 1996) now contaminates
environments extending from San Francisco Bay (Hornberger et al. 1999) to the Sierra Nevada
and far beyond (Schuster et al. 2002).
The accumulation of contaminated debris from gold mining caused a notable loss of depth in
parts of the San Francisco Bay (Nichols et al. 1986, Cappiella et al. 1999). In the past 50 years,
however, the amount of additional sedimentation attributable to the Gold Rush has declined
substantially, and further declines are predicted (Jaffee et al. 1998). All of the major rivers in the
Sacramento River basin (Sacramento, Feather, American, Yuba) are impounded. The
impoundments have decreased sediment export from the basin (Goals Project 1999), and the
suspended sediment load of the Sacramento River has declined since 1960 (Krone 1996). Given
that about 90 percent of the total mercury load to the Bay-Delta ecosystem from the Sacramento
River is sediment borne (Foe 2002), it can be reasonably inferred that mercury loads have
correspondingly declined and that future activities affecting sediment budgets could substantially
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