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THE HERMAN PIT AND ITS ROLE IN MERCURY TRANSPORT AT THE SULPHUR BANK MERCURY MINE SUPERFUND SITE, CLEAR LAKE, CALIFORNIA
Citation:
Jewett*, D G., E. Manges, AND P. Lechler. THE HERMAN PIT AND ITS ROLE IN MERCURY TRANSPORT AT THE SULPHUR BANK MERCURY MINE SUPERFUND SITE, CLEAR LAKE, CALIFORNIA. Presented at NRMRL's Workshop on Characterization, Modeling, Remediation, and Monitoring of Mining-Impacted Pit Lakes, Reno, NV, 4/4-6-00.
Description:
The Sulphur Bank Mercury Mine (SBMM) is an abandoned sulphur and cinnabar mine located on the eastern shore of the Oaks Arm of Clear Lake, Lake County, California. SBMM was one of the largest mercury producers in California and has been described as one of the most productive shallow hot spring mercury deposits in the world. The hydrothermally altered andesite deposit was discovered in 1857 and was mined for sulphur from 1865 to 1871. In 1872, as mercury became a valuable mineral, underground mining for cinnabar began and continued until shaft-mining operations were abandoned in 1904. The mine reopened in 1915 as an open pit mine and on-site processing of ore began in 1918. Mining on a large scale continued until 1946 and intermittently thereafter until all operations ceased in 1957. An estimated 4.7 x 106 kg of Hg were produced from SBMM over its lifetime.
The open-pit mining operations created a 9.3 ha open pit (~30 m in depth) surrounded by 1.1 x 106 metric tons of waste rock and tailings. The pit, now referred to as Herman Pit, is a hydrologic sink collecting both surface and ground water from the local watershed and it filled with water after mining ceased. Herman Pit also receives gas and water input from local geothermal activity. Oxidation of sulfide minerals and H2S gas has generated acidic water in the pit (pH ~2.9). Because the hydraulic head in the pit is approximately 4 m higher than that of Clear Lake, subsurface outflow from the pit passes through waste rock piles before entering Clear Lake. Although the pit water is relatively low in Hg, the acidic outflow leaches Hg as it migrates through the waste piles and transports it to Clear Lake where it is then dispersed throughout the ecosystem. Elevated Hg levels in fish were discovered in the late 1970's ? early 1980's and SBMM was placed on the Superfund National Priorities List in 1990.
By concentrating on the geochemistry of mine waste and waters at SBMM, their spatial variability, and the reactions governing the solubility of Hg phases, progress has been made towards understanding this water-rock system. While the concentration of Hg in Herman Pit water is substantial (300 ng/L), it does not exceed USEPA drinking water standards (2 mg/L). Once the pit and meteoric waters pass through mine wastes, however, their quality is further degraded and even Hg (17 mg/L) exceeds the drinking water standard. O2 and Cl- availability are important parameters promoting Hg solubility in SBMM wastes. Effectively limiting O2 and Cl- availability to mine wastes may be important in reducing the Hg supply to Clear Lake by suppressing undesirable reactions. Additional investigations are necessary to provide a more detailed characterization of subsurface flow paths, spatial distribution of Hg, and distribution of reaction controlling environmental conditions.