In the late 1800's, mercury amalgamation was used in milling gold and silver ore from the Comstock Mining District in and around Virginia City in west-central Nevada.  At least 7,500 tons of elemental mercury were lost during these operations.  Much of this mercury was discarded in mill tailings or discharged to the Carson River or its tributaries in mill effluent.  Mercury has since become widely distributed in the lower Carson River basin.  Previous investigations documented extensive mercury contamination of water, sediment, food chain organisms, fish, and migratory birds in Lahontan Valley, including some wetlands on Stillwater National Wildlife Refuge (NWR).  The concentrations often exceeded levels associated with adverse effects.   In 1999, the Fish and Wildlife Service and the Environmental Protection Agency initiated a joint investigation to evaluate mercury contamination in Lahontan Valley.  The goal of this effort was to identify wetlands in Lahontan Valley where fish and wildlife were at high risk of mercury-related effects.  The study objectives were to evaluate total mercury and methyl mercury concentrations in water, sediment, and aquatic invertebrates in major wetlands on Stillwater NWR, and to assess the biological availability and severity of contamination in major wetlands in Lahontan Valley.  Mercury concentrations in water, sediment, and invertebrates present a risk to fish and wildlife on Stillwater NWR, Fallon NWR, and other Lahontan Valley wetlands.  In 1999, methylmercury concentrations in about half of the invertebrate samples exceeded concentrations associated with long-term reductions in avian productivity; however, concentrations were well below levels associated with major toxic effects.

Based on the findings of this and other recent studies, several points relevant to the management of Stillwater NWR and other Lahontan Valley wetlands are available. 

• Substantial amounts of mercury were deposited in Lahontan Valley and transport of mercury via water movement between wetlands is continuing.  
• Flood conditions in the Carson River upstream of Lahontan Reservoir may substantially increase the load of total mercury transported to Lahontan Valley wetlands.  Selective management of flood waters may reduce total mercury deposition in selected wetlands. 
• Availability of mercury to aquatic invertebrates is controlled by methyl mercury levels in sediment. Total mercury levels in water and sediment do not appear to provide a suitable indicator of biological risk of mercury and wetland management based on total concentrations in sediment does not appear to be an effective remedial strategy. 
• The proportions of methyl mercury to total mercury in sediment and methyl mercury availability to aquatic invertebrates were greater in constructed wetlands despite having lower total mercury concentrations.  As a result, mercury risk to fish and wildlife appears to be greater in constructed wetlands.

Results from this study are being used to determine which wetlands pose the most significant hazard from mercury contamination and therefore possibly should be dried up; to determine where new wetlands might be established to reduce mercury exposure to fish and wildlife; and to plan additional studies that will determine implications of changes in water conditions to mercury availability and ecological risk in Lahontan Valley.

Learn More by Reading the Full Report: Tuttle, P.L., Higgins, D.K., and Quashnick, J. , Mercury Characterization in Lahontan Valley Wetlands, USFWS, Div. of Env. Contaminants, Reno, NV, 1999.

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