Investigations of wetlands in Lahontan Valley have documented concentrations of inorganic contaminants in water, sediment, and biological samples in excess of concentrations associated with adverse effects to fish and wildlife. Under the auspices of the Truckee-Carson-Pyramid Lake Water Settlement Agreement, the Department of the Interior has implemented a program to acquire rights for water to restore and maintain a portion of the historic wetlands in Lahontan Valley. Although inflow to wetlands will be partially restored, the effects of increased inflow to wetland contamination is uncertain.

In 1990, the Nevada State Office, in conjunction with Stillwater National Wildlife Refuge (NWR), instituted a monitoring program on Stillwater NWR to assess implications of inorganic contaminants to fish, wildlife, and human health and to assess changes in inorganic contaminant concentrations in wetlands maintained With freshwater and agricultural drainage water. This program entailed the measurement of water quality parameters and the determination of trace element concentrations in water, sediment, and biological matrices. These data and the data tables provide results of the Stillwater National Wildlife Refuge, Indian Lakes and Fallon National Wildlife Refuge, focused on the wetlands in order to measure concentrations of Hg to evaluate potential human health and ecological exposure pathways.

The original sources of Hg contamination in the Carson River Basin are from historic gold and silver mining and associated milling of the Comstock Lode near Virginia City, Nevada. Runoff and erosion from an estimated 236 'stamp mills', driven by flumes, resulted in a cummulative release of an estimated 7,500 Tons of elemental mercury into the Carson River Basin. The elemental mercury, imported from mines in California and used to almalgamate the ore at the stamp mills, contaminated sediments throughout the Basin from the source area situated approximately between Carson City and Dayton, to the closed terminal wetlands in the Carson Sink. This area is the primary source of Hg pollution in the Basin, considering the naturally occurring mercury concentrations are close to the crustal average. During runoff and flood events, the River laterally cuts through the contaminated sediments in the overbanks and transports Hg with suspended sediments, and with concentrations rising with higher flow.

When Lahontan Reservoir was built in 1915, it became a settling basin for suspended Hg from the Carson River, and while it retains up to 90 percent of influent sediments, the reservoir continues to pass significant concentrations of suspended and dissolved inorganic Hg and methylmercury (Me-Hg) downstream to the Carson Sink. The EPA is publishing this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU2 Remedial Investigation Report (EPA, 2017), which describes the nature and extent of contamination from the Site. Literature and other source Hg data are summarized in the RI, for surface waters, sediments, and biological tissues. The report contains the Human Health Risk Assessment and Ecological Risk Assessment.