EPA ID# WAD980722789
EPA Region 10
Okanogan County
Horse Springs Coulee

4th Congressional District

Other Names:
Last Update: September, 2012

Hide details for Site DescriptionSite Description

The Silver Mountain Mine site is an abandoned silver and gold mine located in Horse Springs Coulee, approximately six miles northwest of Tonasket, Washington. The area around the site is semi-arid with scrub vegetation and is primarily used for cattle grazing. The site covers five acres. The mine operated sporadically from 1928 to the 1960s. In the early 1980s, cyanide was used to extract metals from mine tailings. In this process, a solution of sodium cyanide was pumped over the tailings and drained into a collection basin where metals were extracted from the solution. By 1983, the site was abandoned, and the mine tailings and holding basin, which contained cyanide-contaminated water, were left behind. A leachate collection trench associated with the ore extraction was contaminated with cyanide and arsenic. The nearest residence is approximately three miles away, and fewer than five people live within three miles. Private wells are located within three miles and are used for domestic purposes, irrigation, and livestock watering.

Site Responsibility: This site has been addressed through federal and state actions. The Washington Department of Ecology will perform site maintenance.

NPL Listing HistoryDates
Proposed Date:10/15/1984
Removed Date:
Withdrawal Date:
Final Date:06/10/1986
Deleted Date:09/22/1997

Hide details for Threats and ContaminantsThreats and Contaminants

Media Affected: Groundwater
The primary contaminants of concern were arsenic and cyanide. Arsenic is a component of the native rock in the area. Cyanide was brought to the site and sprayed on the heap of previously mined materials. Three potential sources of contaminants were identified at the site: the heap leach pile, the unprocessed rock, and the mine drainage water. Key exposure pathways were identified as on-site soils, on-site surface water, on-site groundwater in a shallow aquifer and off-site regional groundwater.

Hide details for Cleanup ProgressCleanup Progress

Early Actions: In 1982, the State conducted an action which included using sodium hypochlorite to neutralize the cyanide in the pond and in the mine tailings. In 1985, the State removed contaminated water in the leachate collection trench from the site and covered the trench with a liner.

Long-Term Actions: Cleanup was completed by EPA in 1992. Approximately 7,000 cubic yards of mine tailings were consolidated and capped, the mine entrance was closed, and the site was revegetated and fenced. Deed restrictions were implemented to protect the cap. In an October 1994 Explanation of Significant Differences, EPA determined not to monitor the groundwater due to the lack of groundwater at this semi-arid site. The shallow aquifer was not found above the bedrock formation where the groundwater was previously thought to be located. EPA deleted this site from the National Priorities List (NPL) in September 1997.

Operation and Maintenance: The site cleanup was designed to require very little maintenance. The site is in a remote area with semi-arid conditions. Under the State Superfund Contract, Washington Department of Ecology will provide maintenance with a minimal amount of work.

Five-year Review: A fourth Five-Year Review for Silver Mountain Mine was completed in September 2012. Consistent with the finding of the previous FYR completed in 2007, the remedy is performing as designed, remains protective and the cap remains in excellent condition. No additional remedial actions are required. The Washington State Department of Ecology (Ecology) will remain responsible for enforcing future maintenance activities at the Site. Prior to the next FYR, Ecology and EPA will work with the current property owner to develop and record a new environmental covenant that follows the guidelines of Uniform Environmental Covenants Act. During site inspections, fencing installed by adjacent property owner will be inspected to confirm that it remains in place and undamaged. If the fence is damaged or removed, Ecology will require Site property owner to replace the fence to ensure access to the Site remains controlled.

Hide details for Regional ContactsRegional Contacts

SITE MANAGER(S):William Ryan
PHONE NUMBER:206-553-8561
Information pertaining to this site is housed at the following location(s):
Okanogan County Health District (Administrative Record)
Environmental Health Section
149 North 3rd Street
Okanogan, WA 98840

EPA Region 10 Superfund Records Center (Administrative Record)
1200 Sixth Avenue, ECL-076
Seattle, WA 98101