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SEASONAL VARIATIONS OF DISSOLVED MERCURY CONCENTRATIONS AT THE SULPHUR BANK MERCURY MINE, CLEAR LAKE, CALIFORNIA: IMPLICATIONS FOR MINE DRAINAGE MONITORING
Citation:
Bauman, J. B., M. A. Engle, G. J. Reller, D G. Jewett*, AND E. Manges. SEASONAL VARIATIONS OF DISSOLVED MERCURY CONCENTRATIONS AT THE SULPHUR BANK MERCURY MINE, CLEAR LAKE, CALIFORNIA: IMPLICATIONS FOR MINE DRAINAGE MONITORING. US EPA Conf. Hardrock Mining 2002: Issues Shaing the Industry, Denver, CO, 05/7-9/2002. U.S. Environmental Protection Agency, Washington, DC, 2002.
Description:
The Sulphur Bank Mercury Mine in Lake County, California (SBMM) was operated from the 1860s through the 1950s. Mining for sulfur started with surface operations and then progressed to shaft and later open pit techniques to obtain mercury. SBMM is located adjacent to the shore of Clear Lake and has been identified as a significant source of mercury to Clear Lake. Based on results of groundwater sampling, the dissolved mercury concentrations can vary by three orders of magnitude at the same sampling point over time. Understanding the cause of this mercury concentration variability is significant in supporting the evaluation of remediation efforts at SBMM, and has implications for mercury monitoring at other sites containing mercury minerals. U. S. EPA recently completed six monthly sample collection events at 6 wells passing water from each well through various sized filters (filtration sampling). Filtration sampling results in conjunction with major ion and environmental isotope analytical results have identified the most likely source of mercury concentration variation as interaction of high Eh, low pH water with mercury-containing rock. Filtration sampling occurred over one-half of the SBMM hydrologic cycle from December 2000 through June 2001 as groundwater stages changed from low to high elevations due to seasonal precipitation. Mercury concentrations were observed to be low despite favorable chemical conditions (high Eh, low pH) until the mine wastes became saturated during high groundwater stages. Upon contact of high Eh, low pH groundwater with mercury-bearing mine wastes, dissolved mercury concentrations increased. Particulate bound mercury transport was found to be insignificant at SBMM.
Implications for monitoring mercury at other locations with mine wastes containing mercury minerals are:
1. One-time sampling is not likely to represent aqueous mercury concentrations adequately to support evaluation of mercury flux.
2. Mercury concentrations in water vary with chemical and hydrologic conditions.
3. Site hydrology and geochemical factors must be understood to correctly interpret results of aqueous mercury analyses.
Without a clear understanding of site hydrology and geochemistry, the potential for mercury mobilization, and mass of mercury in the hydrologic system will not be accurately evaluated.