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RECENT GEOCHEMICAL SAMPLING AND MERCURY SOURCES AT SULPHUR BANK MERCURY MINE, LAKE COUNTY, CA
Citation:
Reller, G. J., E. Manges, AND D G. Jewett*. RECENT GEOCHEMICAL SAMPLING AND MERCURY SOURCES AT SULPHUR BANK MERCURY MINE, LAKE COUNTY, CA. Presented at U.S. EPA Workshop, San Francisco, CA, 11/28-30/2000.
Description:
The Sulphur Bank Mercury Mine (SBMM), located on the shore of Clear Lake in Lake County, California, has been identified as a significant source of mercury to the lake. Sulphur Bank was actively minded from the 1880's to the 1950's. Mining and processing operations at the Sulphur Bank Mercury Milne produced 107 lbs of mercury as well as the Herman Pit, a 20 acre, 90 ft deep pit surrounded by 106 tons of mercury-laden waste rock and tailings. The pit subsequently filled with water after mining ceased (creating Herman Impoundment) and the hydraulic gradient is from the pit to the lake, approximately 300 ft to the west. Mercury from waste rock-tailings disposal practices also impacts a fresh water marsh located approximately 1/8 mile to the north. Recent investigations conducted by the U.S. EPA have focused on characterizing the hydrogeology and water chemistry at the Sulphur Bank Mercury Mine.
As part of these investigations, the U.S. EPA sampled 65 wells, springs, and surface water bodies in the vicinity of the SBMM to identify sources of mercury and ground water pathways from the Sulphur Bank Mercury Mine to Clear Lake. The water samples were analyzed for metals and major ions including B, Br, and Li. Ion rations (B/C1, and Br/C1) indicate that geothermal solutions are present in nearly all of the on-site sampling points. The sources of mercury in ground water at the site are (in decreasing significance): acid rock drainage, the geothermal system, and Herman Impoundment waters. The geothermal solutions are diluted to varying degrees by surface water, and infiltrating precipitation. The chemical data also indicate that a significant source of mercury in ground water flowing off the site is due to mixing of geothermal fluids with acid rock drainage. This is consistent with independent research recently conducted off site.
During chemical analysis we realized that in samples with sufficient ionic strength, mercury values were biased low due to formation of complexes stable under the conditions of chemical analysis. Simple dilution of high ionic strength samples resulted in significant changes (up to 2 orders of magnitude) in the reported mercury concentrations. These observations imply that historical mercury analyses of high ionic strength waters may significantly underestimate the quantity of mercury present.