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HYDROGEOLOGICAL AND GEOCHEMICAL FACTORS INFLUENCING MERCURY FATE AND TRANSPORT AT THE SULPHUR BANK MERCURY MINE, LAKE COUNTY, CALIFORNIA
Citation:
Jewett*, D G., E. Manges, G. J. Reller, P. Lechler, AND E R. Bates*. HYDROGEOLOGICAL AND GEOCHEMICAL FACTORS INFLUENCING MERCURY FATE AND TRANSPORT AT THE SULPHUR BANK MERCURY MINE, LAKE COUNTY, CALIFORNIA. Presented at Workshop on the Fate, Transport, and Transformation of Mercury in Aquatic and Terrestrial Environments, West Palm Beach, FL, May 8-10, 2001.
Description:
Clear Lake, located approximately 150 km north of San Francisco in Lake County, is one of the largest fresh water lakes in the California. Elevated mercury levels were first identified in fish from Clear Lake in the late 1970s and early 1980s. Although naturally occurring mercury deposits are common in this region, the Sulphur Bank Mercury Mine (SBMM), located adjacent to Clear Lake, also is a potential source for a modern-day mercury flux to the local aquatic ecosystem. The Sulphur Bank hot springs mineral deposit was discovered in 1857 and the hydrothermal system responsible for depositing the mercury ore is still active. Subsurface and surface mining operations produced an estimated 4.7 x 106 kg of mercury from the SBMM. However, surface mining also created the Herman Pit (a 9.3 ha, 30 m deep, open pit) surrounded by over 1.1 x 106 t of waste rock, tailings, and overburden. Mining ceased in 1957 and the open pit filled with water forming the Herman Impoundment. Herman Impoundment is a hydrologic sink for surface and ground waters in the immediate watershed. The impoundment also receives water upwelling from the underlying hydrothermal system. The water in the impoundment is acidic (pH~3) due to the oxidation of H2S gas and sulfide minerals. Herman Impoundment and Clear Lake are separated by a distance of about 250 m, but the hydraulic head of the impoundment is approximately 3.8 m greater than that of the lake. Subsurface outflow is a major component of discharge from the impoundment with ground water migrating to Clear Lake through the mercury-laden waste rock and overburden.
The U.S. EPA has been conducting a comprehensive site investigation to characterize the hydrogeologic and geochemical setting at the SBMM. Results from this investigation indicate that the majority of ground water discharge from the site to Clear Lake occurs through the waste rock/upper lake sediments unit (2,000 - 3,000 ft3/day, or 65% of the total discharge). The remaining subsurface discharge passes through the andesite (900-1,300 ft3/day) and lower lake sediments units (150-250 ft3/day). Surface and ground water quality samples have been collected and analyzed for a variety of constituents. Mercury concentrations (dissolved and solid phase) are greatest in the waste rock/upper lake sediments unit and the subsurface mercury flux from the impoundment to Clear Lake occurs almost entirely through this unit (99.8% of the total Hg flux). Water chemistry results suggest that water-rock interaction at a lower pH and an increased oxidation state within the waste rock and overburden is supplying more mercury to Clear Lake than the hydrothermal system beneath Herman Impoundment. These observations make it possible to entertain the concept of remediating this site in the presence of an active hydrothermal system, which itself is a natural mercury flux to the local ecosystem.