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WATER-ROCK INTERACTIONS INFLUENCING MERCURY FATE AND TRANSPORT FROM AN ABANDONED MINE SITE TO AN ADJACENT AQUATIC ECOSYSTEM
Citation:
Jewett*, D G., E. Manges, G. J. Reller, M. A. Engle, AND J. B. Bauman. WATER-ROCK INTERACTIONS INFLUENCING MERCURY FATE AND TRANSPORT FROM AN ABANDONED MINE SITE TO AN ADJACENT AQUATIC ECOSYSTEM. Presented at Amer. Geophysical Union Conference, San Francisco, CA, 12/10-14/2001.
Description:
Clear Lake, located 150 km north of San Francisco, is one of the largest fresh water lakes in California and is an important economic resource for the region. Elevated mercury levels in fish in Clear Lake were identified in the late 1970s, resulting in a fish consumption advisory for the lake. Although Clear Lake is located in a region of naturally occurring mercury deposits, the Sulphur Bank Mercury Mine (SBMM) Superfund Site, an abandoned mine adjacent to the lake, also is a source for a modern-day mercury flux to the local aquatic ecosystem. An investigation to characterize the hydrogeologic and geochemical setting at the SBMM and understand water-rock interactions and their influence on mercury migration into Clear Lake is in progress. Historic mining operations produced 4.7 x 106 kg of Hg from the SBMM, leaving a 9.3 ha open pit surrounded by 49 ha of mine waste. The pit, which receives water from the underlying hydrothermal system and surface and ground waters from the immediate watershed, filled when mining ceased. Water in the pit is acidic due to the oxidation of H2S gas and sulfur-bearing minerals. Clear Lake and the open pit are separated by a distance of about 250 m, but the water level in the pit is 3-4 m greater than that of the lake. Subsurface outflow is a major component of discharge from the pit with ground water flowing to Clear Lake through the mercury-laden waste rock and overburden. Ground-water outflow from the pit through mine wastes is greatest when water levels create a combination of near-maximum hydraulic gradients and increased saturated thickness. Major ion and trace element analyses have identified several distinct water types at the SBMM, which, with the exception of Hg, are stable at most locations over time. Elevated Hg concentrations are associated with low pH (~pH 3), high Eh (typically >400 mV) waters, with the highest concentrations (up to 350 ug/L) reported for water samples collected from wells located between the pit and Clear Lake and screened in waste rock material. These high Hg concentrations are present in a mixture of pit outflow and infiltrating meteoric water based on conservative, dissolved constituent and stable isotope data. Results of filtration sampling experiments indicate that while particulate transport of mercury at the site does occur, it may not be a significant component of mercury flux through ground water at SBMM. (This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.)