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USE OF ENVIRONMENTAL ISOTOPES TO DIFFERENTIATE WATER SOURCES AND CONSTRAIN THE WATER BUDGET AT THE SULPHUR BANK MERCURY MINE, CLEAR LAKE, CALIFORNIA
Citation:
Engle, M. A., G. J. Reller, J. B. Bauman, F. Goff, E. Manges, AND D G. Jewett*. USE OF ENVIRONMENTAL ISOTOPES TO DIFFERENTIATE WATER SOURCES AND CONSTRAIN THE WATER BUDGET AT THE SULPHUR BANK MERCURY MINE, CLEAR LAKE, CALIFORNIA. US EPA Conf. Hardrock Mining 2002: Issues Shaping the Industry, Denver, CO, 05/7-9/2002. U.S. Environmental Protection Agency, Washington, DC, 2002.
Description:
The Sulphur Bank Mercury Mine (SBMM) is a 65 ha site located on the eastern shore of the Oaks Arm of Clear Lake, Lake County, California. Between 1864 and 1957, SBMM was the site of underground and open pit mining operations for S and Hg, coinciding with past and present hot springs and fumarole emissions (T<70?C). When mining ceased in 1957, a 9.3 ha open pit (Herman Impoundment, HI) filled with water derived from hydrothermal fluids, groundwater, and surface runoff. Because of mining disruptions, the site now releases Hg to the surrounding ecosystem. Water leaking from HI transports Hg into adjacent Clear Lake during passage through a barrier of mine waste, known as the waste rock dam, where it picks up Hg by water-rock interaction. To differentiate water sources found at SBMM and to constrain a water budget for HI, environmental isotope analysis was conducted on SBMM waters.
Stable isotopes of oxygen and hydrogen, collected in December 2000, identified relative contributions from meteoric, evaporated meteoric, and hydrothermal end-members to 8 different water types at SBMM. Processes including evaporation, groundwater mixing, and isotopic exchange with CO2-CH4-H2S-bearing hydrothermal gases were also discerned. Mixing calculations from this data indicate that HI is roughly one-third hydrothermal water and two-thirds evaporated meteoric water. These results support previous calculations from conservative ion balances (Cl, B, etc.) used for determination of the HI water budget
Tritium activity was measured in waters at 17 locations to determine groundwater ages, travel times, and mixing proportions of SBMM waters. Tritium data were also used to examine groundwater flow along three groundwater flow paths. In one case, a travel time was calculated based on tritium levels in the water along the transect to constrain groundwater velocities in the HI water budget. The tritium-derived travel times (2.14 to 14.6 ft/yr) were found to be more reasonable than velocities calculated using Darcy's Law (205 to 4,801 ft/yr).