EPA Superfund Site

MILLTOWN RESERVOIR SEDIMENTS

MT | EPA ID MTD980717565

About this site’s exposure pathways: As of September 2025, the Milltown Reservoir Sediments Superfund site is considered “Current Human Exposure Not Under Control” (HENC). While arsenic concentrations in groundwater within Operable Unit (OU) 2 have significantly decreased since the remedy construction was completed (2012), the arsenic concentrations in many of the compliance monitoring wells are still not meeting the remedy performance standards. This exposure pathway is considered unacceptable based on EPA risk-based criteria because arsenic concentrations in groundwater within OU2 are in exceedance of the maximum contaminant level (MCL) of 10 micrograms per liter (µg/l).

Remedial design and remedial action activities for the remediation of the Clark Fork River streambanks is not complete and activities are ongoing within OU3. There are public exposure risks to contaminated soil and sediment at a recreational park within OU3 (Arrowstone Park) and exposure to recreators utilizing the Clark Fork River for recreation where cleanup has not yet occurred. This exposure pathway is considered unacceptable based on EPA risk-based criteria because concentrations of arsenic in soil and sediment exceed the recreational cleanup standard for arsenic in soil identified in the record of decision.

Currently, the planned activities to address the exposure pathway of OU2 groundwater will evaluate the technical impracticability of achieving the arsenic MCL. Additionally, it is likely that a controlled groundwater area will be required to address the unacceptable exposure within OU2. Currently, the planned activities to address the exposure pathway of OU3 include implementation of the remedial actions identified in the record of decision, to remove tailings and sediment from the streambanks and the floodplain. If you are impacted by risks inherent at this site, you can minimize your contact with soil and sediment along the streambank of the Clark Fork River, especially in areas void of vegetation, and wash hands before touching your face and eating. Montana Department of Environmental Quality is currently in the remedial design phase of the project for the recreational park within OU3. Prior to construction beginning in the Park, Montana Department of Environmental Quality, in consultation with the Department of Health, Powell County, and EPA, has placed temporary warning signs within Arrowstone Park to ensure that recreators are aware of the risks due to arsenic. Montana Department of Environmental Quality currently anticipates that human exposure will be under control by 2027 because remediation of the tailings and impacted soil within the streambank of the Clark Fork River will be substantially complete at that time.