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The Carson River Mercury site includes mercury-contaminated soils at former mill sites, mercury contamination in waterways adjacent to the mill sites, and mercury contamination in sediments, fish and wildlife. The site extends over more than a 50-mile length of the Carson River beginning near Carson City, Nevada, and extending downstream to the Lahontan Valley. Contamination at the site is a legacy of the Comstock mining era of the late 1800s, when mercury was imported to the area for processing of gold and silver ore. Today, the mercury is in the sediments and adjacent floodplain of the Carson River and in the sediments of Lahontan Reservoir, Carson Lake, Stillwater Wildlife Refuge and Indian Lakes. Following immediate actions to protect human health and the environment, site investigations and cleanup planning are ongoing.

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What Has Been Done to Clean Up the Site?

The site is being addressed through federal and state actions.

Excavation and removal of mercury-contaminated tailings and soils from the site have reduced the potential for exposure to contaminated soil while further studies are taking place.

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What Is the Current Site Status?

To better manage site investigations and cleanup, EPA established two operable units (OUs) for the site. OU-1 consists of the old mill sites and related tailings. OU-2 consists of the Carson River from the area of New Empire to its terminus in the Carson Sink.

Carson River Operable Unit 1 (OU-1): The final remedy included excavation of contaminated soils, backfilling with clean soil and off-site disposal of the contaminated soil. In one of the four OU areas, the remedy also included placement of clean soil on top of the contaminated soil in lieu of excavation and backfilling. Both approaches reduce risks by limiting contact with soils containing elevated levels of mercury. The remedy also included restoration and landscaping of contaminated areas after excavation and backfilling.

During cleanup, about 9,000 cubic yards of contaminated soil were excavated. Most of the soils were disposed of at a nearby landfill. About 500 cubic yards of “high mercury” soils were transported out of state for treatment at an approved thermal treatment facility. After excavation, clean fill was brought in, reseeding and landscaping were completed, and measures put in place to control erosion and temporarily irrigate reseeded areas. Finally, pipelines, fences, walls and other utilities were replaced or restored, and drainage improvements were made.

Cleanup work also included development of a “Long-Term Sampling and Response Plan” to address risks in undeveloped, uncharacterized areas with elevated levels of mercury. The Plan describes EPA and the Nevada Division of Environmental Protection (NDEP) efforts to address risks to public health and the environment from mercury-contaminated soils at the CRMS in Lyon County. The Plan addresses risks that could result from long-term direct contact with soils having elevated levels of mercury. Long-term contact is most likely in new, uncharacterized, residential development settings. NDEP has worked on more than 70 development/project proposals, required analyses of soil samples for mercury at about 26 developments, and worked with the developer on mitigation at several developments. Mitigation involved covering or capping contaminated soils. EPA's role has been to provide technical assistance to NDEP and occasionally to work directly with developers and their consultants.

Carson River Operable Unit 2 (OU2): Mercury-contaminated sediments in the Carson River, Lahontan Reservoir, Carson Lake and Stillwater National Wildlife Refuge are the cause of elevated levels of mercury in fish and wildlife in and near the contaminated areas. The contamination presents a health risk to those who consume mercury-contaminated fish. Site investigations are ongoing, and are expected to continue through at least 2015. After the studies are complete, EPA will evaluate the costs and benefits of cleaning up mercury contamination in the river, reservoir and wetlands and determine what type of cleanup, if any, is warranted.

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Sampling and Monitoring

In 1993, EPA undertook an extensive effort to sample for mercury and other compounds in CRMS soil, sediment, surface water, groundwater, and biota. The results were published in December 1994 in the Human Health Risk Assessment and Remedial Investigation Report. [ADD LINK TO REPORT] Assessment of the ecological impacts of mercury contamination in Lahontan Reservoir and upstream portions of the Carson River began in 1992. The results of the initial ecological assessment are summarized in a report titled “Ecological Risk Assessment Carson River Mercury Site Upstream of Lahontan Dam” (May 1998). That study includes the results of analyses of surface water, sediment pore water, sediment, zooplankton, benthic invertebrates (midges), fish (Sacramento blackfish, carp, walleye), and bird blood and feathers (double-crested cormorants, bank swallows) collected from Lahontan Reservoir in 1994 and 1995. One of the findings of the assessment was that fish-eating birds and possibly other wildlife at the site were exposed to levels of mercury contamination shown to cause harm to wildlife in other studies. In 1997, EPA began a study to test the findings of the 1994-1995 assessment and look for more direct evidence of mercury-related adverse impacts. This ecological effects study, carried out with researchers at the USGS Forest and Rangeland Ecosystem Science Center in Corvallis, Oregon and the Patuxent Wildlife Research Center in Laurel, Maryland, examined the effects of mercury-contaminated water, sediment, and prey on the reproductive success and health of three species of fish-eating birds nesting at the site. The study focused on fish-eating birds because mercury bioconcentrates, reaching the highest levels in organisms at the top of the food web. Eggs and blood samples were collected over several years from areas of the site where exposure to mercury was most likely to exceed safe levels, to evaluate year-to-year variability in exposure and to look for relationships between mercury exposure and nesting success. In 1997 and 1998, young nestlings and adult birds were also collected and examined to identify any sublethal effects of mercury exposure in vital organs and tissues. The effort to correlate the reproductive success of egrets and herons with their exposure to mercury has been inconclusive, due to limited sample size and the greater importance of other factors on reproductive success. Measurements of biochemical markers associated with mercury exposure and histopathological examinations have, however, revealed potentially adverse effects on young birds associated with mercury exposure. The significance of the observed changes is currently being evaluated. Since 1997, most of the investigation work has continued though agreements with the US Geological Survey, the US Fish and Wildlife Service, and university researchers. In addition to the ecological effects study carried out in cooperation with the USGS, studies have been completed to examine: 1) the formation and degradation of methylmercury in contaminated sediments; 2) whether contaminated sediments in Lahontan Reservoir are a significant source of mercury to wildlife; 3) the transport of mercury in Stillwater National Wildlife Refuge; and 4) loadings of mercury into and from the Lahontan Reservoir. The ecological risk assessment at the Carson River site has been more extensive than is typical at Superfund sites, in part because of the absence of any simple or inexpensive cleanup options for the contaminated sediments. Investigations are expected to continue through at least 2014. After the studies are complete, EPA will evaluate the costs and benefits of cleaning up mercury contamination in the river, reservoir, and wetlands and determine what type of cleanup, if any, is warranted.

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Emergency Response and Removal

Site cleanup has also included removal actions, or short-term cleanups, to address immediate threats to human health and the environment. In 1991, concerns over the possible exposure of vehicle users to contaminated materials prompted the removal of mercury-contaminated tailings near Dayton, Nevada. Also in 1991, a second removal action took place at a park in Dayton. Contaminated materials were taken to a mineral resource recovery facility.

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