IMPACTS OF HISTORICAL MERCURY AND GOLD MINING ON FRESHWATER FISHERIES HABITAT AND HUMAN HEALTH

We assessed impacts on fisheries habitat with human health implications for the Cinnabar mine site, Idaho, active from ~1917 through the 1970s. Extraction and on-site processing of cinnabar and metacinnabar bearing rock left a footprint of tailings and subsurface workings flooded with ground water. A synoptic study of stream water and sediment chemistry, fish community structure, and ecotoxicology of benthic invertebrates conducted on Cinnabar Creek and Sugar Creek demonstrated that the in-stream and riparian ecosystem was disrupted downstream from the Cinnabar mine site. Mercury (Hg) and arsenic (As) were co-contaminants to downstream food webs. Concentrations of 257 ng Hg L^-1 and 20.6 µg As L^-1 were measured in unfiltered stream water collected approximately 4 km downstream from Cinnabar mine. Seston and periphyton acted as biologically reactive surfaces where Hg was methylated, resulting in increased Hg accumulation and decreased insect and fish biodiversity. Total Hg accumulated in seston, aquatic insect larvae, adult aquatic insects, riparian spiders, and fish at sites downstream of mining. Total Hg concentrations in these receptors were much higher (1.3 to 11.3-fold) and isotopically distinct compared with sites immediately upstream of mining inputs. Methylmercury (MeHg) concentrations in bull trout and riparian spiders (732-918 and 347-1,140 ng MeHg g^-1 dry weight, respectively) were at levels concerning for consumption by humans, birds, and piscivorous fish. The diversity of benthic insect communities was locally depressed by 12-20% immediately downstream of mining inputs and was accompanied by an absence of metal-sensitive fish species (sculpin and juvenile steelhead trout), and elevated stream water Hg and As concentrations persist into the East Fork South Fork Salmon River watershed. Legacy mining activity in the upper Salmon River Mountains can have ecologically important effects on downstream communities, including reduced biodiversity and increased mercury exposure of higher order consumers, including fish, birds, and humans. The Nez Perce Tribe holds fishing rights under the 1855 treaty. Elderly and low-income members of the Tribe rely on ceded fishing rights, making the compromise of fisheries through legacy mining an environmental justice issue.