Mercury in the Carson River

During the gold and silver rush near Virginia City, Nevada in the mid to late 1800’s, 75 ore-processing mills were located along the Carson River and three of its tributaries (Six Mile Canyon, Seven Mile Canyon, and Gold Canyon). At most of the mills, gold was recovered from ore using mercury amalgamation. This process of gold recovery resulted in the loss of 6 to 7 tons of elemental mercury to the environment, mostly incorporated in mill tailings (Smith, 1943). Over 100 years of weathering and erosion have redistributed these mill tailings throughout the channel and flood plains of the lower Carson River Basin. Since 1915, Lahontan Reservoir has received and retained most of the mercury contaminated sediments transported in the Carson River.

Elevated concentrations of mercury in the Carson River Basin, Nevada were first reported by the USGS in 1973 (Van Denburgh, 1973).  The USGS study showed high concentrations of mercury in water and bottom sediment downstream from sites where mercury was used in the late 19th century in the milling of gold and silver ore.  Subsequent investigations have further documented the extent and severity of the contamination.  The elevated levels of mercury have prompted the issuance of public-health warnings prohibiting the consumption of gamefish and carp from the Carson River downstream of Dayton, in Lahontan Reservoir, and in Lahontan Valley.

In August 1990, the U.S. Environmental Protection Agency (USEPA), listed the Carson River in West-Central Nevada, as a site for possible remediation of mercury contamination under the auspices of the Comprehensive Environmental Response, Contamination, and Liability Act of 1980, also known as CERCLA, or Superfund. Subsequently, USEPA coined the term Carson River Mercury Site (CRMS) to identify the 100-mile reach of river designated for site investigation. This reach begins in Carson City and ends at the Stillwater National Wildlife Refuge in the Carson Desert. As part of the initial CRMS investigation, USEPA is required to do a Remedial Investigation and Feasibility Study (RI/FS) in order to characterize the scope of contamination of mercury which poses a potential risk to human health and to the environment of the affected area. The results of the RI/FS will assist resource managers to develop sound plans for remediating the potential risks. Recognizing the importance of fluvial transport of mercury-contaminated sediments to Lahontan Reservoir, USEPA-Region 9 requested in March 1997 that the USGS Nevada Water Science Center collect mercury and mercury-related data on the Carson River.

Objectives

Data on concentrations of total mercury, total methyl mercury, and suspended sediment will be collected at two USGS gaging stations, Carson River near Fort Churchill, NV (10312000), and Carson River below Lahontan Reservoir, NV (10312150). These data subsequently can be used to determine seasonal loads of total mercury and methylmercury in and from Lahontan Reservoir.

Strategy and Approach

Field measurements of streamflow, water temperature, specific conductance, dissolved oxygen (DO) and pH will be recorded each time that water samples are collected. Streamflow measurements will be made. Field measurements of water temperature, specific conductance, DO, and pH will be made with instruments calibrated on-site. Water samples will be collected along the cross section of the river using depth-integrating techniques. At a minimum, five vertical profiles of the water column along the cross section of the river will be sampled and composited. Data collection will occur weekly during the rising stage of the spring snowmelt hydrograph, then every other week during the falling stage of the snowmelt hydrograph, and monthly thereafter, during low- flow conditions. The data collection program described herein also incorporates two "event measurements", e.g., runoff from thunderstorms, assuming that these events occur. Each event will be sampled once on the rising stage of the hydrograph, once near the peak, and once on the falling stage. These event data will document the movement of mercury into Lahontan Reservoir during non-snowmelt runoff periods.

Data availability

USGS National Water Information System website (NWISweb): Carson River near Fort Churchill, NV (10312000) || Carson River below Lahontan Reservoir, NV (10312150)

Annual Data Report: Carson River near Fort Churchill, NV (10312000) || Carson River below Lahontan Reservoir, NV (10312150)

Relevance and Benefits

An important part of the USGS mission is to provide scientific information to manage the water resources of the Nation. To effectively assess the quality of the surface-water and ground-water resources in the Carson River in Nevada, the USGS collects water-quality data from the river. Water-quality data are published in annual data reports, and an increasing amount of real-time and historical water-quality data are available on the World Wide Web. The data, collected using standardized instruments and procedures, contribute to a nationally consistent data set for assessment of the water quality of the Nation. Long-term water-quality data characterize the physical, chemical, and biological changes in our water resources in response to natural processes including climatic variations, storms, floods, droughts, and a variety of human activities that exert an influence on water-quality conditions. The data are useful in designing programs to protect watersheds, sensitive aquatic habitats, biota, and drinking water sources from urban and agricultural runoff, sewage, and industrial and mining wastes. The water-quality data collected from the Carson River are an integral part of the nationwide water-quality data program.

Publications

Hoffman, R.J., and Thomas, K.A., 2000, Methylmercury in water and bottom sediment along the Carson River system, Nevada and California, September 1998: U.S. Geological Survey Water-Resources Investigations Report 00-4013, 17 p.

Thomas, K.A., 2000, Methylmercury in water and bottom sediment along the Carson River System, Nevada and California, September 1998 [abs.]: Assessing and Managing Mercury from Historic and Current Mining Activities, U.S. Environmental Protection Agency, San Francisco, Calif., November 2000, p. 93.

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, 6 p.