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GEOCHEMICAL EVOLUTION OF GROUND WATER AND TRANSPORT OF MERCURY AT THE SULPHUR BANK MERCURY MINE SUPERFUND SITE IN NORTHERN CALIFORNIA, USA
Citation:
Lechler, P. AND D G. Jewett*. GEOCHEMICAL EVOLUTION OF GROUND WATER AND TRANSPORT OF MERCURY AT THE SULPHUR BANK MERCURY MINE SUPERFUND SITE IN NORTHERN CALIFORNIA, USA. Presented at 31st International Geological Congress, Rio de Janeiro, Brazil, 8/6-17/2000.
Description:
The Sulphur Bank Mercury Mine, located on the shore of Clear Lake, Lake County, California, is a potential source for a modern-day mercury flux into the local aquatic ecosystem. Surface mining created the Herman Pit, a 9.3 ha open pit with a depth > 30 m, while overburden and processed tailings (estimated at over 1.1 million metric tons) containing elevated mercury concentrations were disposed of in piles around the pit. Mining ceased in 1957 and the open pit filled with water creating Herman Impoundment.
Herman Impoundment is a sink for ground water migrating through the upper portion of the watershed. The impoundment also receives geothermal waters upwelling through fractured bedrock and some surface drainage. The water in the pit is acidic due largely to the oxidation
of hydrogen sulfide and sulfide minerals. Herman Impoundment and Clear Lake are separated by about 250 m of waste rock piles, however, the elevation of the water surface in the impoundment is approximately 3.8 m greater than that of the lake. Subsurface outflow is a major component of discharge from the pit with ground water migrating to Clear Lake through the mercury-enriched waste piles.
Water quality samples were collected along a hypothetical flowpath: (1) Herman Impoundment surface water in the vicinity of spring discharges (east end), (2) Herman Impoundment surface water (west end), (3) ground water sample from well MW-13 proximal to Clear Lake. Water pH values decrease from 3.18 at (1) to 3.08 at (2) to less than 3 in the monitoring wells while sulfate increases from 2390 to 4260 mg/L. Substantial ammonium (83 mg/L) is also being added to the system from the subaqueous springs in Herman Impoundment. Ammonium decreases down the flowpath as nitrate increases, again reflecting increasing oxidation of the waters. Of particular importance, dissolved Hg increases from < 1 ug/L near the spring discharge area of Herman Impoundment to 17 ug/L in the monitoring well nearest to Clear Lake.