EPA has collected soil, sediment, surface water and biota data to complete the remedial investigation (RI) at the Operable Unit (OU) No. 2 of the Carson River Mercury Superfund Site (CRMS). The goal of the study is to select a remedy that eliminates, reduces, or controls risks from site-related compounds of concern to human health and the environment. Specifically, the RI involves the investigation and study of the heavy metals contamination and the associated tributaries into the Carson River watershed resulting from site-related contaminant migration and to evaluate past and current releases and their impact to human health and the environment. The Site includes mercury-contaminated soils at former mill sites, mercury contamination in waterways adjacent to the mill sites, and mercury contamination in sediments and banks fish and wildlife over more than a 80 mile length of the River, beginning near Carson City, Nevada and extending downstream to the Lahontan Valley and ultimately at its terminus at the USFWS Stillwater National Wildlife Refuge. Contamination at the Site is a legacy of the Comstock mining era of the late 1860s, when mercury was imported to the area for processing of gold and silver ore. Ore mined from the Comstock Lode was transported to the mill sites, where it was crushed and mixed with mercury to amalgamate the precious metals (Washoe Process). The mills were primarily located in Virginia City, Silver City, Gold Hill, Dayton, Six Mile Canyon, Gold Canyon, and adjacent to the Carson River between New Empire and Dayton. During the mining era, an estimated 7,500 tons of mercury were discharged into the River drainage, primarily in the form of mercury-contaminated tailings.

EPA evaluated the total mercury representative of several surface and subsurface locations in OU1 for the purpose of the Human Health Risk Assessment investigation. EPA published this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994), which describes the nature and extent of contamination from the Site. The reports contain the Human Health Risk Assessment and Ecological Risk Assessment Reports. Literature and other source Hg data are summarized in the RI report for upland source areas.

EPA evaluated the total mercury representative of several surface and subsurface locations in OU1 for the purpose of the Human Health Risk Assessment investigation. EPA published this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994), which describes the nature and extent of contamination from the Site. The reports contain the Human Health Risk Assessment and Ecological Risk Assessment Reports. Literature and other source Hg data are summarized in the RI report for upland source areas.

For the Ecological Risk Assessment Report, EPA collected these samples, consisting of benthic invertebrates (crayfish), from three contaminated locations in the Carson River upstream from Lahontan Reservoir (Santiago Ruins, Dayton State Park, and Fort Churchill) and one background location (Ruhenstroth Dam). As a component of the ecological assessment for the Carson River Mercury Site, a survey of benthic macroinvertebrates was conducted to evaluate the potential adverse effects of mercury in surface water and sediment of the Carson River. Measures of benthic community response to mercury contamination derived from the benthic survey are presented in this the EcoRisk Assessment, Appendix C. A characterization of the existing environment at the sampling locations-including the habitat characteristics, fish and wildlife populations, and surrounding land use. Rapid BioAssessment Protocol II (RBP) involves the standardized collection, enumeration, and identification of major benthic taxa to the family level in riffle/run habitats. Standard data sheets are used to record results at each sampling. location. Collections are compared to a reference location, and an evaluation is made as to the impairment, or lack thereof, of the benthic community from stressors within the watershed. The biological condition of the community can be ranked according to three categories of impact: nonimpaired, moderately impaired, or severely impaired, where the degree of impairment implies less than optimal community structure, the loss of intolerant species, and dominance by a few tolerant taxa compared to background. RBP II also involves assessment of habitat characteristics such as stream flow, bottom conditions, and riparian vegetation. In addition, standard water quality parameters such as dissolved oxygen, temperature, pH, conductivity, color, and turbidity are determined. The RBP II was conducted on August 17 and 18, 1994 during a period of drought. The EPA is publishing this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994) as well as the OU2 Remedial Investigation Report (EPA, 2017), describing the nature and extent of contamination from the Site, primarily the Ecological Risk Assessment Report. The RI report also contains the Human Health Risk Assessment. Literature and other source Hg data are summarized in the RI for surface waters, sediments, and biological tissues.

For the Ecological Risk Assessment Report, EPA collected these samples, consisting of benthic invertebrates (crayfish), from three contaminated locations in the Carson River upstream from Lahontan Reservoir (Santiago Ruins, Dayton State Park, and Fort Churchill) and one background location (Ruhenstroth Dam). As a component of the ecological assessment for the Carson River Mercury Site, a survey of benthic macroinvertebrates was conducted to evaluate the potential adverse effects of mercury in surface water and sediment of the Carson River. Measures of benthic community response to mercury contamination derived from the benthic survey are presented in this the EcoRisk Assessment, Appendix C. A characterization of the existing environment at the sampling locations-including the habitat characteristics, fish and wildlife populations, and surrounding land use. Rapid BioAssessment Protocol II (RBP) involves the standardized collection, enumeration, and identification of major benthic taxa to the family level in riffle/run habitats. Standard data sheets are used to record results at each sampling. location. Collections are compared to a reference location, and an evaluation is made as to the impairment, or lack thereof, of the benthic community from stressors within the watershed. The biological condition of the community can be ranked according to three categories of impact: nonimpaired, moderately impaired, or severely impaired, where the degree of impairment implies less than optimal community structure, the loss of intolerant species, and dominance by a few tolerant taxa compared to background. RBP II also involves assessment of habitat characteristics such as stream flow, bottom conditions, and riparian vegetation. In addition, standard water quality parameters such as dissolved oxygen, temperature, pH, conductivity, color, and turbidity are determined. The RBP II was conducted on August 17 and 18, 1994 during a period of drought. The EPA is publishing this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994) as well as the OU2 Remedial Investigation Report (EPA, 2017), describing the nature and extent of contamination from the Site, primarily the Ecological Risk Assessment Report. The RI report also contains the Human Health Risk Assessment. Literature and other source Hg data are summarized in the RI for surface waters, sediments, and biological tissues.

EPA evaluated the total mercury representative of several subsurface locations in OU1 for the purpose of the Human Health Risk Assessment investigation. EPA published this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994), which describes the nature and extent of contamination from the Site. The reports contain the Human Health Risk Assessment and Ecological Risk Assessment Reports. Literature and other source Hg data are summarized in the RI report for upland source areas.

EPA evaluated the mercury fractions (species) representative of several locations in OU1 for the purpose of the Human Health Risk Assessment investigation. In particular, the mercuric chloride fraction was of interest to toxicologists, because this is most bioavailable form of the compound in the terrestrial food pathway. EPA published this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994), which describes the nature and extent of contamination from the Site. The reports contain the Human Health Risk Assessment and Ecological Risk Assessment Reports. Literature and other source Hg data are summarized in the RI report for upland source areas.

EPA evaluated the mercury fractions (species) representative of several locations in OU1 for the purpose of the Human Health Risk Assessment investigation. In particular, the mercuric chloride fraction was of interest to EPA toxicologists, because this is most bioavailable form of the compound in the terrestrial food pathway. EPA published this data in support of the Carson River Mercury NPL Site in Nevada. Data was compiled and evaluated for the OU1 Remedial Investigation Report (EPA, 1994), which describes the nature and extent of contamination from the Site. The reports contain the Human Health Risk Assessment and Ecological Risk Assessment Reports. Literature and other source Hg data are summarized in the RI report for upland source areas.

URS Corporation (URS) developed a Sampling and Analysis Plan in September 2011 that was consistent with the CRMS draft Long Term Sampling and Response Plan. URS submitted a draft SAP to NDEP for review and incorporated NDEP’s comments in the Final SAP. In October and November 2011, URS performed a Phase II ESA on the Site. The purpose of the investigation was to evaluate if impacts related to the CRMS are present on the 138 undeveloped residential lots and one partially developed lot. The lots subject to the Phase II ESA were owned by two separate entities and the Phase II ESA was divided accordingly. Section I: URS sampled lots 2 - 160, located in the westernmost section of the subdivision. Samples took place between 10/24/2011 and 11/2/2011. The sample date in the attribute table indicates the last date in the sampling period for this segment. Section 2: URS Corporation sampled lots M41 - M86, located in the easternmost section of the subdivision. Samples took place between 10/26/2011 and 11/1/2011. The sample date in the attribute table indicates the last date in the sampling period for this segment. Sample locations were collected using a Trimble GeoXT GPS unit. Analytical data for each sample point is contained with the related table, OndaVerde_SampleResults. Parameters sampled include mercury, arsenic, and lead. Detailed information on the sampling procedure and laboratory analysis can be found in the following documents: URS Corporation, Earl James Leaver, P.E., C.E.M., January 11, 2012, Phase II Environmental Site Investigation, ESA103278 Onda Verde L.L.C., North Side of Coleman Road and East of Highway 50, Fallon, NV 89510, letter report to Jack Yates, NDEP, Unpublished URS Corporation, Earl James Leaver, P.E., C.E.M., January 11, 2012, Phase II Environmental Site Investigation, ESA102893 Michael Munoz, 2160 Verona Drive, Fallon, NV 89510, letter report to Jack Yates, NDEP, Unpublished.

This is a summary of mercury (Hg) data from fish collected in the Lahontan Reservoir area of the Carson River Basin. NDOW collected the fish and the EPA Region 9 Laboratory analyzed the tissue for mercury (Hg) for EPA's Impaired Waters Program, water resources managers and researchers working in the Carson River Basin. These data and the data tables provide results of the Lahontan Reservoir area, used by recreationalists and fishermen, in order to measure concentrations of Hg to evaluate potential human health 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 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. The report contains the Human Health Risk Assessment and Ecological Risk Assessment. Literature and other source Hg data are summarized in the RI for surface waters, sediments, and biological tissues.