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The Carson River Mercury site (CRMS) 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 130-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. During the minin era, an estimated 7,500 tons of mercury were discharged into the Carson River drainage.

Today, mine tailings with elevated mercury levels are still present at and around the historic mill sites.  In addition, the mercury is in the sediments and adjacent floodplain of the Carson River, including the historic flow path, the network of agricultural drains and canals, the Lahontan Reservoir, Carson Lake, Little and Big Washoe Lakes, Stillwater Wildlife Refuge and Indian Lakes. This mercury-contaminted sediments are the source of high mercury levels in nearby fish and wildlife. 

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?

CRMS 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 and adjacent floodplain from the area of New Empire to its terminus in the Carson Sink.

Carson River Operable Unit 1 (OU-1): In 1995, EPA published the Record of Decision (ROD) for this area, which is a document where EPA establishes the remedy (long-term action to remove, clean up, or manage site contamination).  The remedy for OU1 included excavation of contaminated soils, backfilling with clean soil and off-site disposal of the contaminated soil. In one area, the remedy also included placement of clean soil on top of the contaminated soil in lieu of excavation and backfilling. Both approaches reduce risks to human health by limiting contact with soils containing high levels of mercury. The remedy also included restoration and landscaping of contaminated areas after excavation and backfilling.

During cleanup work in the 1990s, EPA excavated about 9,000 cubic yards of contaminated soil. Most of the soils were disposed of at a nearby landfill. EPA transported about 500 cubic yards of “highly contaminated mercury” soils out of state for treatment at an approved thermal treatment facility. After excavation, EPA brought in clean fill , reseeded and landscaped the properties, and put measures in place to control erosion. Finally, EPA replaced and restored pipelines, fences, walls and other utilities, and, in some cases, improved drainage.

Cleanup work also included development of a “Long-Term Sampling and Response Plan” (LTSRP or "Plan"). The Plan describes EPA and the Nevada Division of Environmental Protection (NDEP) efforts to address risks in undeveloped, uncharacterized residential areas with high levels of mercury (and arsenic and lead found with the mercury) in soilsin Lyon County. The Plan addresses risks that could result from long-term contact with mercury-contaminated soils. Long-term contact is most likely to occur 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. The Plan was extensively revised in September 2018, addressing whether and how sampling and potential mitigation would occur.

Carson River Operable Unit 2 (OU2):  Mercury is in the sediments and adjacent floodplain in the Carson River, including the historic flow path, the network of agricultural drains and canals, and the Lahontan Reservoir, Carson Lake, Little and Big Washoe Lakes, Stillwater Wildlife Refuge and Indian Lakes. These mercury-contaminated sediments are the cause of high levels of mercury in nearby fish and wildlife. The contamination presents a health risk to those who consume mercury-contaminated fish. EPA has completed site investigations and published the Remedial Investigation Report in April 2018. EPA will complete the Feasibility Study in late-2018, after agency peer-review. 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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Activity and Use Limitations

At this site, activity and use limitations that EPA calls institutional controls are in place. Institutional controls play an important role in site remedies because they reduce exposure to contamination by limiting land or resource use. They also guide human behavior. For instance, zoning restrictions prevent land uses – such as residential uses – that are not consistent with the level of cleanup.

For more background, see Institutional Controls.

It is not safe to eat mercury-contaminated fish caught in the area.  The contamination presents a serious health risk to those who consume mercury-contaminated fish.  Mercury can cause permanent damage to the nervous system and serious disabilities for developing fetuses and children.

The Nevada Division of Health (NDOH) has issued health advisories for nearby water bodies.  NDOH recommends no consumption of some fish species from Big and Little Washoe Lakes.  NDOH recommends no consumption of some any fish from the Lahontan Reservoir and the Carson River from Dayton downstream to the reservoir.  It is not safe to eat any fish caught in these areas.  For more information, visit:

Catch and release, swimming, and recreation are safe activities.

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Sampling and Monitoring

In 1993, EPA undertook an extensive effort to sample for mercury and other compounds in Csoil, sediment, surface water, groundwater, and biota located within the Site. In December of 1994, EPA published the results in the Human Health Risk Assessment and Remedial Investigation Report. [ADD LINK TO REPORT] In 1992, EPA began the assessment of the ecological impacts of mercury contamination in Lahontan Reservoir and upstream portions of the Carson River. 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 confirm the findings of the 1994-1995 assessment and look for more direct evidence of mercury-related adverse impacts. This ecological effects study was developed with researchers at the USGS Forest and Rangeland Ecosystem Science Center in Corvallis, Oregon and the Patuxent Wildlife Research Center in Laurel, Maryland.  The study 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. In addition, 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. Additional 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 human health and ecological risk assessments at CRMShas 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. After the Feasibility Study is 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.

Meanwhile, the Long-Term Sampling and Response Plan (LTSRP or "Plan"), a living document to address potential soil contamination within the large footprint of the Site impacting future residential developments or contruction activities at individual residences, was extensively revised in September 2018. The document includes a general Sampling and Analysis Plan to guide where and how samples are collected for residences near and downstream of the historic millsites to detect whether unacceptable levels of mercury, arsenic and lead may be present and further spread. Addititionally, EPA is working closely with the Nevada State Historic Preservation Office (SHPO) to ensure ongoing compliance with Section 106 of the National Historic Preservation Act to protect cultural resources. EPA will negotiate a Programmatic Agreement with the SHPO and consulting agencies before undertaking broadscale sampling and potential remediation actions as part of the LTSRP.

In spring 2019, EPA and NDEP plan to conduct a pilot soil sampling study to assess a new, rapid approach to sampling and characterizing soil.  This sampling approach will help EPA and NDEP determine how the mercury, arsenic, and lead has spread across the Site over time, away from the historic mills. We have selected properties around  historic mill sites in Virginia City and one in the drainage below Sixmile Canyon. EPA is working with archaeologists and the Nevada State Historic Preservation Office (SHPO) specifically about these properties to monitor cultural resources and ensure Section 106 compliance.

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Emergency Response and Removal

EPA's ite cleanup actions 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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