MILLTOWN RESERVOIR SEDIMENTS

As of September 2026, the Milltown Reservoir Sediments Superfund site is considered “Current Human Exposure Not Under Control” (HENC). While arsenic concentrations in groundwater within Operable Unit 2 (OU2) have decreased since remedy construction was completed in 2012, concentrations in many compliance monitoring wells still do not meet 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 Clark Fork River streambanks are not complete, and work is ongoing in 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 arsenic concentrations in soil and sediment exceed the recreational cleanup standard identified in the Record of Decision.

Currently, EPA plans to evaluate whether achieving the arsenic MCL in OU2 groundwater is technically impracticable. 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 2040 because remediation of the tailings and impacted soil within the streambank of the Clark Fork River will be substantially complete at that time.