MERCURY SOURCES AND MIGRATION AT SULPHUR BANK MERCURY MINE, LAKE COUNTY CALIFORNIA

The Sulphur Bank Mercury Mine (SBMM) in Lake County, California has been identified as a significant source of mercury to Clear Lake. Mining has resulted in the creation of a pit currently filled with water (the Herman Impoundment), and approximately 2 million cubic yards of mine wastes consisting of waste rock, ore, and tailings located in piles at the site. Sources of mercury for off site transport are hydrothermal gas and mine wastes.

Recent investigations have evaluated the off site migration of mercury through groundwater, surface water and air from SBMM. Significant findings from these investigations include: 1) Groundwater flow along fractures is not responsible for transporting large quantities of water and mercury away from the site. 2) Near surface hydrothermal solutions do not contain significant quantities of mercury. 3) Acid rock drainage transports a small quantity of mercury. 4) Chemical conditions within Herman Impoundment precipitate aqueous mercury. 5) Interaction of water and waste rock between the Herman Impoundment and Clear Lake transports 97% of the mercury migrating from the site in water. 6) Mine wastes and undisturbed geologic units emit mercury to the atmosphere. 7) Hydrothermal vents emit mercury to the atmosphere. 8) Surface controls implemented in the 1990s have significantly reduced the quantity of mercury migrating from the site in surface water.

The estimated mass of mercury migrating from SBMM to the environment is approximately 8.1 kg/year (1.9 kg/year in groundwater, 6.2 kg/year in air, and negligible in surface water).

Understanding the sources of mercury and locations of mercury migration away from the site supports evaluation of remediation measures that will significantly reduce the off site migration of mercury.