Monitoring Mercury and Suspended Sediment in the Carson River, Nevada: Continuation of a Unique, Long-Term Dataset
The USGS Nevada Water Science Center (NVWSC), in collaboration with the U.S. Environmental Protection Agency (EPA), monitored mercury in the Carson River from 1997 to 2013. To preserve this long‑term dataset and to better understand chemical conditions associated with mercury methylation, the NVWSC resumed monitoring mercury concentrations and suspended sediment in the Carson River above and below Lahontan Reservoir in 2017 with the Nevada Division of Environmental Protection (NDEP). The monitoring program continues today.
Background
During the 40-year period of intense gold and silver mining in the Comstock Mining District released in the late 1800s, an estimated 7,500 tons of mercury were lost to the Carson River 1. This mercury was used to extract gold and silver from the crushed ore. Much of this mercury ended up in mill tailings that later eroded into the river system and was redeposited as bank material and bed sediment.
Previous studies have shown that mercury moves through the river attached to suspended sediment 2, and about 90 percent of the mercury-bearing sediment becomes trapped in Lahontan Reservoir 3.
In 1990, the EPA listed over 50 miles of the lower Carson River system on the National Priorities List as the Carson River Mercury Site 4. From 1997 to 2013, the NVWSC and EPA collected and analyzed surface-water and suspended sediment samples for mercury and methylmercury at two long-term monitoring sites located about four miles upstream and one mile downstream of Lahontan Reservoir. The joint dataset is one of the longest continuous records of mercury and methylmercury for any river system 2.
References
1 Smith, G.A., Tingley, J.V., 1943. The history of the Comstock Lode, 1850-1920. Univ. Nev. Bull. 37, 305 pp.
2 Morway, E.D., Thodal, C.E., Marvin-DiPasquale, Mark, 2017, Long-term trends of surface-water mercury and methylmercury concentrations downstream of historic mining within the Carson River watershed: Environmental Pollution, vol. 229, p. 1006-1018.
3 Hoffman, R.J. and Taylor, R.L., 1998, Mercury and suspended sediment, Carson River Basin, Nevada: Loads to and from Lahontan Reservoir in flood year 1997 and deposition in reservoir prior to 1983: U.S. Geological Survey Fact Sheet FS-001-98.
4 U.S. Environmental Protection Agency, 2023, Carson River Mercury Superfund Site at a Glance, accessed February 19, 2026, at https://semspub.epa.gov/work/09/100038708.pdf.
The USGS Nevada Water Science Center (NVWSC), in collaboration with the U.S. Environmental Protection Agency (EPA), monitored mercury in the Carson River from 1997 to 2013. To preserve this long‑term dataset and to better understand chemical conditions associated with mercury methylation, the NVWSC resumed monitoring mercury concentrations and suspended sediment in the Carson River above and below Lahontan Reservoir in 2017 with the Nevada Division of Environmental Protection (NDEP). The monitoring program continues today.
Background
During the 40-year period of intense gold and silver mining in the Comstock Mining District released in the late 1800s, an estimated 7,500 tons of mercury were lost to the Carson River 1. This mercury was used to extract gold and silver from the crushed ore. Much of this mercury ended up in mill tailings that later eroded into the river system and was redeposited as bank material and bed sediment.
Previous studies have shown that mercury moves through the river attached to suspended sediment 2, and about 90 percent of the mercury-bearing sediment becomes trapped in Lahontan Reservoir 3.
In 1990, the EPA listed over 50 miles of the lower Carson River system on the National Priorities List as the Carson River Mercury Site 4. From 1997 to 2013, the NVWSC and EPA collected and analyzed surface-water and suspended sediment samples for mercury and methylmercury at two long-term monitoring sites located about four miles upstream and one mile downstream of Lahontan Reservoir. The joint dataset is one of the longest continuous records of mercury and methylmercury for any river system 2.
References
1 Smith, G.A., Tingley, J.V., 1943. The history of the Comstock Lode, 1850-1920. Univ. Nev. Bull. 37, 305 pp.
2 Morway, E.D., Thodal, C.E., Marvin-DiPasquale, Mark, 2017, Long-term trends of surface-water mercury and methylmercury concentrations downstream of historic mining within the Carson River watershed: Environmental Pollution, vol. 229, p. 1006-1018.
3 Hoffman, R.J. and Taylor, R.L., 1998, Mercury and suspended sediment, Carson River Basin, Nevada: Loads to and from Lahontan Reservoir in flood year 1997 and deposition in reservoir prior to 1983: U.S. Geological Survey Fact Sheet FS-001-98.
4 U.S. Environmental Protection Agency, 2023, Carson River Mercury Superfund Site at a Glance, accessed February 19, 2026, at https://semspub.epa.gov/work/09/100038708.pdf.