Increased Mercury, Reduced Insect Diversity, and Food Web impacts from Historical Mercury Mining

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Headwater streams influence downstream ecosystems and represent ecologically important areas where aquatic health is heavily influenced by surrounding land use. Historical mining activity in the western United States left a legacy of abandoned mines and waste rock in headwaters of major rivers. In sites where mercury was mined or used to process metals like gold or silver, both inorganic and organic forms of mercury can be found in mine waste and can be transported to headwaters through weathering, runoff, and atmospheric deposition, posing a risk for contamination. The biologically available form of mercury, methylmercury (MeHg), is a potent neurotoxin, which can impact biota exposed directly, and organisms higher in the food-web.

To understand potential food-web and other ecological impacts of historical mercury mining on headwater ecosystems, sampling was conducted up and downstream of the Cinnabar mercury mine site in the headwaters of the Salmon River in central Idaho where historical mining activities resulted in elevated mercury and arsenic concentrations in nearby sediments and water. Streams within the Salmon River watershed provide critical habitat for threatened anadromous fish including Chinook salmon (Oncorhynchus tshawytscha), steelhead trout (Oncorhynchus mykiss), and bull trout (Salvelinus confluentus). Sediment, seston (particles suspended in water such as cells, leaf fragments, algae, and other organic matter), aquatic insect larvae and adults, riparian spiders known to eat mainly adult aquatic insects, and fish were collected up and downstream of the mine site inputs to the watershed. Total mercury (THg) and MeHg concentrations were determined in all samples, while density, abundance, and diversity were assessed for aquatic insects.

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Results of the study found elevated total mercury (THg) levels in seston, insects, spiders and fish sampled up to 7.6 km downstream of the mine site compared with upstream samples. Insect communities displayed a reduced diversity of insects and extremely elevated mass (10–1778‐fold higher) of THg and MeHg in benthic aquatic insects downstream from the mine site. Concentrations of MeHg in downstream bull trout were in ranges known to have reproductive effects, and MeHg concentrations in riparian spiders were found at levels known to have negative effects when eaten by birds and other fish species. Mercury concentrations in bull trout were within the range where a human health consumption advisory would apply. These advisories are used provide warnings about eating fish from affected areas to prevent negative human health effects.

This research increases our understanding of the ongoing impacts of historical mining activities on streams and headwater ecosystems in the United States. Previous mercury mining and mercury use in mine processing can reduce current biological diversity, increase total and methyl mercury in aquatic insects, and have impacts up the food web to birds, piscivorous fish, and humans.

This research was supported by the USGS Ecosystems Mission Area, through the Environmental Health Program (Contaminant Biology Program) and the USGS Mineral Resources Program to provide an understanding of the ongoing impacts of active and abandoned mines to ecosystem health.