Print this pageMercury mining on the shores of Clear Lake, California has contaminated the lake sediments with inorganic mercury (cinnabar). Despite the low solubility of cinnabar (HgS), mercury has entered the food chain and is bioaccumulating in the highest trophic levels. A key reaction leading to the bioavailability of mercury is methylization, which is partially mediated by sulfate-reducing bacteria in organic-rich, but sulfatepoor, lake sediments. Low pH, sulfate-rich fluids have recently been documented to be discharging out of the lake bottom near the Sulphur Bank Mercury Mine, an EPA Superfund site. Three possible end-member fluid sources have been identified: (1) acid rock drainage from mine waste; (2) seepage from an abandoned open pit mine pond; and (3) hydrothermal fluids derived either from hot springs that vent into the mine pit or leakage from geothermal test wells that were drilled at the mine site. The endmember fluids can be distinguished by their chemical and isotopic composition.
This study will use a combination of conservative and nonconservative tracers (e.g. B/Cl, SO4/Cl, NH4/K, d18O, dD, and d34S) to identify the source of acid-sulfate waters into Clear Lake. Vertical pore-fluid profiles will allow estimation of the diffusive and advective flux of fluids into the lake. These data will be useful in determining the proper siting of wells to determine the hydrological coupling of the mine site and lake. They will guide the choice of appropriate mine remediation strategies and will establish a baseline to evaluate the effectiveness of the selected remedial action(s).
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid