PERSPECTIVES ON MERCURY CONTRIBUTIONS TO WATERSHEDS FROM HISTORIC MERCURY MINES AND NON-MINE MERCURY SOURCES: EXAMPLES FROM THE SULPHUR CREEK MINING DISTRICT

Calcined tailings are generally perceived as the most important environmental concern at historic mercury mine sites. However, other mine site and non-mine mercury sources exist that may contribute significant, if not greater amounts of mercury to watersheds. The Sulphur Creek mining district, east of Clear Lake, California, provides good examples of the diversity of these different mercury sources. Mine site sources here include: calcined tailings, ore piles, furnace site soils, waste rock piles, and disturbed areas of soil and rock with naturally elevated background mercury. Except for ore, these materials typically contain between 1 and 300 ppm Hg with most mercury probably present as cinnabar or metacinnabar. Currently, the Sulphur Creek district mine sites are estimated to contribute between 5 and 28 kg mercury per year to the Cache Creek watershed. The relative contributions from the different sources are: calcined tailings 16 percent; ore, 14 percent; waste rock, 3 percent; and surface cuts/disturbed areas 67 percent.

Non-mine site mercury sources within the Sulphur Creek district include: thermal springs; stream sediment below mines and thermal springs, naturally enriched mercury soils, background mercury soils and mercury from atmospheric deposition. Thermal springs contribute dissolved and precipitate mercury directly to streams, and mercury to the atmosphere (some of which may be deposited locally). Sediment deposits below mines and thermal springs, and undisturbed soils from hydrothermally altered areas, often contain between 1 and several 100 ppm mercury. Background soil away from mines typically contains 0.1 to 0.15 ppm. Sulphur Creek mine materials probably contribute mercury to the atmosphere at flux rates similar to those published for the nearby McLaughlin gold mine, which range from about 100 to 10,000 ng/m2/hr. Atmospheric mercury deposition rates may be 1 or 2 ng/m2/hr. The total annual mercury contribution of these sources to Cache Creek is uncertain, but preliminary estimates suggest the amount may equal or exceed the annual mine site mercury contributions. If these estimates are correct, strategies to reduce mercury loads and methyl mercury in Cache Creek and other watersheds may require reductions in both non-mine and mine related mercury contributions to be successful.