Carson River Mercury Site — Overview
The Carson River basin, from New Empire to Stillwater and the Carson Sink, was designated a National Priority Listed (NPL) site under the Comprehensive Environmental Response Compensation and Liability Act (CERCLA or Superfund) in August, 1990. This is Nevada's only NPL site and is being jointly managed by NDEP and the U.S. Environmental Protection Agency 9 (EPA), Region IX, in San Francisco.
Mercury is toxic to humans and other organisms. It affects the central nervous system as well as the brain, kidneys and developing fetus. Mercury can also bioaccumulate; that is, increase in concentration through the food chain.
A significant amount of characterization work has been accomplished in the site area to quantify the extent and magnitude of the mercury contamination. This has included the identification of historic mill sites, soil sampling for mercury contamination, development of a health based numerical remediation standard (80 mg/kg total mercury) and an epidemiological survey of the health of residents in the area. The 80 mg/kg level was established by a EPA using a risk assessment methodology for ingestion and is a conservative threshold level for long-term exposure of a child up to 6 years old. The rationale is that children beyond 6 years of age will no longer ingest surface soil incidental in their outside play. Inhalation exposure has been determined to not be a health concern in the area.
HISTORIC MINING — Mining in the Carson River drainage basin commenced in 1850 when placer gold deposits were discovered near Dayton at the mouth of Gold Canyon. Throughout the 1850s, mining consisted of working placer deposits for gold in Gold Canyon and Sixmile Canyon. These ore deposits became known as the Comstock Lode.
The initial ore discovered was extremely rich in gold and silver; gold was more abundant in Gold Canyon while silver was more abundant in Sixmile Canyon. The early mining methods concentrated on exposing as much of the lode as was possible in wide trenches. Throughout 1859, ore was shipped to San Francisco for processing.
After local ore processing began in 1860, most major mining companies operated their own mills, but there were also a large number of smaller private mills. Initial ore processing techniques were slow and inefficient and a fair amount of trial and error experimentation went into the development of an effective ore-processing technique. Refinements were aimed primarily at increasing the speed of gold and silver recovery, increasing the percentage of gold and silver recovered, and decreasing the amount of gold and silver discarded in tailings piles. The general milling process employed before 1900 involved pulverizing ore with stamp mills, creating a slurry, and adding mercury to the mixture. Mercury forms an amalgam with the precious metals which is then separated from the solution and retorted. After 1900, cyanide leaching and flotation processes replaced amalgamation.