The Milltown Reservoir Sediments site sits near Missoula, Montana, where more than a century of upstream mining deposited about 6.6 million cubic yards of contaminated sediments behind the Milltown Dam. The site covers roughly 120 miles of the Clark Fork River and has been on the National Priorities List since September 1983. Cleanup is divided into four operable units covering the water system, the reservoir, the mainstem river, and sitewide activities. Construction is not yet complete across the site.
Arsenic is the most widespread contaminant, found in soil, groundwater, surface water, and solid waste across all three operational areas. Cadmium, copper, lead, and zinc are concentrated mainly in the mainstem river area but also appear in the reservoir. The greatest health risks come from drinking contaminated groundwater and direct contact with arsenic in soils and tailings. Groundwater migration is under control, meaning contaminated groundwater is stabilized and poses no unacceptable risk to surface water. However, human exposure is not yet under control across the site.
Remedial action on the water system operable unit finished in September 1984. Work in the reservoir operable unit has been underway since February 2006 and is in long-term operation and maintenance, with the 2021 EPA five-year review finding it currently protective of human health and the environment. In the mainstem river operable unit, the Montana Department of Environmental Quality is leading contaminated soil and tailings removal across 43 river miles in 22 construction phases. A removal action that started in December 2023 finished in September 2024, and remedial design and action work is estimated to continue through 2026 and into 2027. EPA recommends finalizing an operation and maintenance plan, establishing long-term institutional controls, and collecting additional groundwater data to verify cleanup goals are being met.
Beyond cleanup, portions of the site have been transformed. Montana acquired sections of the site in 2010, and today a 500-acre state park offers trails, river access, and wildlife habitat at the confluence of the Clark Fork and Blackfoot Rivers. The Clark Fork Coalition has managed a sustainable cattle ranch on the Clark Fork River portion since 2005. Between 2011 and 2012, the floodplain was restored to function as a naturally self-sustaining river ecosystem.
EPA is now conducting a fourth five-year review to assess whether remedies continue to protect human health and the environment. Community members with relevant information can contact EPA Community Involvement Coordinator Beth Archer to be interviewed for the review. The Clark Fork River Technical Assistance Committee (CFRTAC) is a volunteer citizens' organization that helps local communities engage in Superfund remediation and restoration efforts along the Clark Fork River and provides additional resources for community involvement.
