The Upper Tenmile Creek Mining Area covers 53 square miles southwest of Helena in Lewis and Clark County, Montana. It contains about 150 active and abandoned mines left over from the Rimini Mining District, which operated from the 1870s through the 1930s. The site was added to the National Priorities List in October 1999 and is divided into eight operable units. Cleanup is still underway, and the site has not reached construction completion or been removed from the list.
Gold, lead, copper, and zinc mining left behind waste rock and tailings scattered across roads, yards, and waterways. EPA identified arsenic, cadmium, copper, lead, manganese, mercury, and zinc as the primary contaminants of concern. Arsenic and lead are each found in six different media types, including soil, groundwater, surface water, sediment, leachate, and solid waste. Iron and aluminum have also been detected. The greatest health risk comes from ingesting or having direct contact with contaminated soil or groundwater. Exposure pathways are being controlled in the short term, but human exposure is not yet fully under control across the entire site. Contaminated groundwater is still moving and has not been contained.
Because EPA has not identified a responsible party, cleanup on private land is funded 90 percent by federal dollars and 10 percent by the state. The U.S. Forest Service leads work on federal lands. Key cleanup actions include building a public water system for Rimini, excavating contaminated roadway and residential soil, disposing of mine waste at the Lutrell Repository, and capping mine shafts. Removal actions at the mine sites wrapped up between 2001 and 2002. A remedy was selected in June 2002 and updated by amendment in September 2008 to focus on institutional controls, excavation, and a permanent replacement water supply. Remedial action began in August 2002. Design for work along Minnehaha Creek and remaining residential areas was completed in 2015, with remedial action waiting on federal funding. The site has received support through the Infrastructure Investment and Jobs Act.
A new remedial investigation and feasibility study for the Watershed operable unit started in September 2024, with completion estimated between December 2026 and February 2027. EPA completed its most recent five-year review in August 2024. Community members can review site records at the EPA Superfund Records Center in Helena, located at 10 West 15th Street, Suite 3200, open Monday through Friday from 8:00 a.m. to 4:30 p.m. The center can be reached at 406-457-5046 or toll-free at 866-457-2690.
