Environmental geochemistry of abandoned mercury mines in West-Central Nevada, USA

The Humboldt River is a closed basin and is the longest river in Nevada. Numerous abandoned Hg mines are located within the basin, and because Hg is a toxic heavy metal, the potential transport of Hg from these mines into surrounding ecosystems, including the Humboldt River, is of environmental concern. Samples of ore, sediment, water, calcines (roasted ore), and leachates of the calcines were analyzed for Hg and other heavy metals to evaluate geochemical dispersion from the mines. Cinnabar-bearing ore samples collected from the mines contain highly elevated Hg concentrations, up to 6.9 %, whereas calcines collected from the mines contain up to 2000 mg Hg/kg. Stream-sediment samples collected within 1 km of the mines contain as much as 170 mg Hg/kg, but those collected distal from the mines (>5 km) contain <1 mg Hg/kg, indicating significant geochemical dispersion of Hg downstream from the mines. Sediment samples collected from the Humboldt River basin contain less-than-or-equals, slant0.28 mg Hg/kg. Leachate samples of the calcines obtained by using a synthetic water leaching technique contain as much as 1500 μg Hg/l, suggesting that some calcines contain soluble Hg compounds. However, much lower Hg concentrations were found in water samples collected from the Humboldt River system, and these were below the 0.012 μg/l Hg standard used to protect against chronic effects to aquatic wildlife. Mercury transference from these mines to the Humboldt River basin is generally minor because the mines are typically >8 km from the Humboldt River, and Hg is transported and diluted through a large volume of pediment before it reaches the Humboldt River.