Silver Mountain Mine sits on about 5 acres roughly six miles northwest of Tonasket in Okanogan County, Washington. The site operated sporadically from 1928 to the 1960s to extract silver, copper, and gold. Between 1980 and 1981, operators used a sodium cyanide heap leach process to pull additional metals from old mine tailings. The operation was abandoned in 1981 without any cleanup, leaving behind cyanide-contaminated materials and mine tailings that fouled the groundwater. EPA added the site to the National Priorities List (NPL) in June 1986.
The contaminants of concern are antimony, arsenic, and cyanide. All three have been found in both groundwater and soil. EPA set cleanup standards at 200 milligrams per kilogram (mg/kg) for arsenic and 95 mg/kg for cyanide. The cleanup plan, finalized in March 1990, called for consolidating contaminated soil and mined rock, building a soil and clay cap over the consolidated materials, blocking the mine entrance, fencing the site, and placing deed restrictions on the property. Workers also neutralized the cyanide in the heap leach pile and pond and removed drums of hazardous materials. Construction wrapped up in late 1992.
Human exposure is currently under control. No unacceptable exposure pathways are present at the site. Physical construction of the cleanup is complete across the entire site, and all cleanup goals for current and reasonably anticipated future land uses have been met. All required land-use restrictions are in place. The site was deleted from the NPL on September 22, 1997, and achieved ready-for-anticipated-reuse status in July 2008. The land is currently used for cattle grazing. The Washington State Department of Ecology handles ongoing operation and maintenance, which is estimated to continue through November 2027.
EPA conducts a five-year review every five years to confirm that protective measures remain effective. The sixth such review was completed on August 8, 2022, and the next is estimated for August through October 2027. Past reviews have consistently confirmed that the cleanup continues to protect public health and the environment. Review reports are available to the public.
Community members who want more information can contact EPA's assigned staff or the Washington State Department of Ecology. Documents related to the site are held at the Washington State Department of Ecology Central Region Office in Union Gap and at the EPA Region 10 Superfund Record Center in Seattle. The EPA Record Center can be reached at 206-553-4494 or toll-free at 800-424-4372, extension 4494.
