CARSON RIVER, NV
Virginia City Nevada
Photographs for the site were taken just outside Virginia City at a small park. The area itself is beautiful, but as the sign notes the water and river sediment contain dangerous amounts of PCBs.
The Carson River Mercury Site includes mercury-arsenic-lead-contaminated soils at former mill sites, mercury contamination in waterways adjacent to the mill sites, and mercury contamination in sediments, fish and wildlife over more than a 80 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. Ore mined from the Comstock Lode was transported to mill sites, where it was crushed and mixed with mercury to amalgamate the precious metals. The mills were 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 Carson River drainage, primarily in the form of mercury-contaminated tailings.
Currently the mercury is in the sediments and adjacent flood plain of the Carson River and in the sediments of Lahontan Reservoir, Carson Lake, Stillwater Wildlife Refuge, and Indian Lakes. Nevada State Health Division advisories recommend limited or no consumption of fish and ducks at the Site due to high levels of mercury. In addition, tailings with elevated mercury levels are still present at and around the historic mill sites, particularly in Six Mile Canyon. EPA, the U.S. Geological Survey, the U.S. Fish and Wildlife Service, the Nevada Division of Environmental Protection (NDEP), University of Nevada researchers, and others have carried out studies to determine the extent of contamination, evaluate the human health and ecological risks, and better understand the processes that govern the movement and toxicity of the mercury. To better manage the site investigation and cleanup, EPA has established two Operable Units (OU) for the site. OU 1 consists of the old mill sites and related tailings above the high water mark. OU2 consists of the Carson River from the area of New Empire Mill to its terminus in the Carson Sink.
Surface water, sediment, soil, fish and wildlife at the site are highly contaminated with mercury. The primary human health threats are to children in long-term direct contact with highly contaminated soils found in tailings piles or at former mill sites, and individuals who consume contaminated fish or wildlife. The Nevada Health Division advises that no gamefish or carp be consumed from most of the site. Risks to wildlife are also significant, particularly to fish-eating birds at the top of the food web.
Mercury levels in gamefish in Lahontan Reservoir (e.g., walleye, white bass) routinely exceed the Food and Drug Administration action level of 1 part per million (ppm). In 1998, a walleye had a record-high 16 ppm mercury present in its tissue.
Due to elevated levels of methyl mercury in fish, the Nevada State Health Division has issued health advisories recommending no consumption of any fish from Big and Little Washoe Lakes, Lahontan Reservoir, and the Carson River from Dayton downstream to the reservoir. Mercury can cause permanent damage to the nervous system and serious disabilities for developing fetuses and children. Catch and release, swimming, and recreation are safe.
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.
Four areas in Dayton and Silver City, Nevada were found to exceed the 80 ppm soil cleanup level. Approximately 12 homes were located on or adjacent to the contaminated soils. In 1994, EPA developed a proposal to address the risks posed by the contaminated soils in the four areas and asked the public to evaluate the proposal. After considering public comments provided before, during, and after a public meeting held in 1995, EPA adopted its final cleanup plan (i.e., the “Record of Decision”). The plan called for excavation of the contaminated soils to a maximum depth of two feet, backfilling with clean soil, and offsite disposal of the contaminated soil. In one of the four 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.
From August 1998 through December 1999, EPA’s contractors carried out cleanup work in Dayton and Silver City. Initial activities consisted of clearing and grubbing of brush, downed tree limbs, personal property, and debris. Ultimately, approximately 9,000 cubic yards of contaminated soil were excavated Most of the soils were disposed at a nearby landfill. In some of the soil, however, the mercury was less tightly bound to the soil and samples failed the Toxicity Characteristic Leaching Procedure (TCLP) test. These “high mercury” soils, comprising approximately 500 cubic yards, were transported our of state for treatment at an approved thermal treatment facility. After excavation, clean fill was brought in, re-seeding and landscaping were completed, and measures implemented to control erosion and temporarily irrigate re-seeded areas. Finally, pipelines, fences, walls, and other utilities were replaced or restored, and needed drainage improvements made.
During cleanup work in one of the contaminated areas, the archaeologist monitoring excavation observed large timbers later identified as part of the foundation of the 19th century mill building. The surrounding soil, as well as the timbers themselves, contained small pools of elemental mercury. This discovery resulted in a seven month suspension in the cleanup as arrangements were made to handle the pools of mercury and the “high mercury” soils. The cleanup work is described in detail in the “Remedial Action Report, Carson River Mercury Site” (September 2000). Additionally, the cleanup work also includes the development of a “Long Term Sampling and Response Plan” to address risks in undeveloped, un-characterized areas with elevated levels of mercury.
In 1997, EPA began a study to test the findings of the 1994-95 assessments and to 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, has 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 focuses on fish-eating birds because mercury bioconcentrates, reaching the highest levels in organisms at the top of the food web. Eggs and blood samples have been collected annually from areas of the site where exposure to mercury is 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.
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: i) the formation and degradation of methylmercury in contaminated sediments; ii) whether contaminated sediments in Lahontan Reservoir are a significant source of mercury to wildlife; iii) the transport of mercury in Stillwater National Wildlife Refuge; and iv) loadings of mercury into and from the Lahontan Reservoir.
In 2013, EPA signed an Explanation of Significant Differences (ESD) adding arsenic and lead to the list of contaminants to be sampled and potentially remediated.
In 2013-2014, EPA conducted an Optimization Review of the years of investigation data to prioritize and identify significant data gaps. This was needed prior to concluding the characterization and starting the final Remedial Investigation / Feasibility Study (RI/FS) report. Investigations are expected to continue through at least 2016. 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.