INTERDISCIPLINARY ASSESSMENT OF MERCURY AND ARSENIC FROM A CARBONATE-HOSTED ORE DEPOSIT, CENTRAL IDAHO, U.S.A

The Cinnabar mine site in central Idaho is a primary source area for mercury and arsenic entering the East Fork South Fork of the Salmon River, a critical spawning habitat for bull trout and Chinook salmon. An interdisciplinary study was conducted to determine geologic background concentrations for arsenic and mercury, evaluate primary source vectors (e.g., groundwater flow or sediment transport) for these metals, and to determine the extent to which historical mining activity affects the aquatic ecosystem.

Mercury mineralization at the Cinnabar mine site is hosted by carbonate rocks, which generate waters dominated by Ca²⁺ and HCO₃^- at pH 7 to 9. Cinnabar Creek, which flows through the mining area, had 8 to 14 ng Hg L^-1 and 4.8 to 9.5 µg As L^-1 in unfiltered water above the mine site, with concentrations increasing to 257 ng Hg L^-1 and 20.6 µg As L^-1 in Cinnabar Creek above its confluence with Sugar Creek. Sugar Creek, which does not include mineralized carbonate rocks in its upper watershed, had background concentrations of 3.6 ng Hg L^-1 and <5 µg As L^-1. Below the confluence with Cinnabar Creek, concentrations for these metals in Sugar Creek increased to 13.6 ng Hg L^-1 and 5.5 µg As L^-1. Sculpin, fish which tend to stay within a single stream reach, were not found in Cinnabar Creek, but in Sugar Creek, had of magnitude increase in Hg concentrations in muscle tissue between individuals collected above and below Cinnabar Creek, demonstrating that the fish are bioaccumulating mercury. Mercury isotope data are being applied to quantify the bioaccumulation of mercury from stream water and sediment into benthic invertebrates, and ultimately into fish. Combining ecotoxicology data with geochemistry provides information on geological and mining sources for regional mercury inputs to the Salmon River, and helps inform remediation goals for this watershed.