Geophysical Investigation of the Sulphur Bank Mercury Mine Superfund Site, Lake County, California

Airborne geophysical reconnaissance was used to identify potential flow paths for mercury-rich, acidic water entering Clear Lake near the Sulphur Bank Mercury Mine. Airborne magnetic and electromagnetic conductivity surveys were conducted over a 12.3 km2 (4.75 mi2) area that included the Oaks Arm of Clear Lake and the old mine. These surveys identified four magnetic and/or conductive anomalies that may represent groundwater conduits towards or away from the Herman Impoundment. An anomaly that extended from Herman Impoundment through a waste rock dam and into Clear Lake was selected for a more detailed ground electromagnetic conductivity survey. The combined results of the airborne and ground surveys provided a detailed, lateral depiction of conductive zones, the most probable pathways for groundwater flow. These surveys also identified near-surface areas that may contain elevated concentrations of sulfide minerals that weather to produce acid groundwater.