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Mercury methylation at mercury mines in the Humboldt River Basin, Nevada, USA

¹ US Geological Survey, P.O. Box 25046, Federal Center, MS 973, Denver, CO 80225, USA (e-mail: JGRAY{at}USGS.GOV)
² Battelle Marine Sciences Laboratory, 1529 West Sequim Bay Road, Sequim, WA 98382, USA

Total Hg and methylmercury concentrations were measured in mine-waste calcines (retorted ore), sediment, and water samples collected in and around abandoned mercury mines in western Nevada to evaluate Hg methylation at the mines and in the Humboldt River Basin. Mine-waste calcines contain total Hg concentrations as high as 14 000 µg g^–1. Stream-sediment samples collected within 1 km of the mercury mines contain total Hg concentrations as high as 170 µg g^–1, whereas stream sediments collected at a distance >5 km from the mines, and those collected from the Humboldt River and regional baseline sites, contain total Hg concentrations <0.5 µg g^–1. Similarly, methylmercury concentrations in mine-waste calcines are locally as high as 96 ng g^–1, but methylmercury contents in stream sediments collected downstream from the mines and from the Humboldt River are lower, ranging from <0.05 to 0.95 ng g^–1. Stream-water samples collected downstream from two mines studied contain total Hg concentrations ranging from 6 to 2000 ng l^–1, whereas total Hg in Humboldt River water was generally lower, ranging from 2.1 to 9.0 ng l^–1. Methylmercury concentrations in the Humboldt River water were the lowest in this study (<0.02–0.27 ng l^–1). Although total Hg and methylmercury concentrations are locally high in mine-waste calcines, there is significant dilution of Hg and there is also lower Hg methylation down gradient from the mines, especially in the sediments and water collected from the Humboldt River, which is >8 km from the nearest mercury mines. Our data indicate little transference of Hg and methylmercury from the sediment to the water column due to the lack of mine runoff in this desert climate.