Overview
The Clark Fork River Operable Unit (OU) is the principal component of the Milltown Reservoir Superfund Site located in Missoula County, Montana. The OU encompasses a 17mile stretch of the ClarkFork River, the associated floodplain, and the abandoned Milltown Dam site. Designated a National Priorities List (NPL) site in 1998, the OU has been the focus of extensive investigation and remediation to address historic miningrelated contamination.
The Clark Fork River flowing past the former Milltown Reservoir.
Historical Background
From the late 1800s through the mid20th century, the Silver Bow and Butte mining districts discharged massive amounts of acid mine drainage (AMD) and metalladen tailings into the ClarkFork River system. These wastes traveled downstream to what eventually became the Milltown Reservoir, a bargeandhydroelectric dam constructed in the 1900s to supply power to the city of Missoula.
By the 1970s, the reservoir was filling with contaminated sediments, causing water quality impairments, fish kills, and public health concerns. In 1975 the City of Missoula began studying the problem, and in 1981 the reservoir was dewatered and the dam was removed. The removal exposed a 40foot thick layer of contaminated sediment, confirming the need for a comprehensive remedial approach.
Nature of Contamination
The primary contaminants identified in the Clark Fork River OU are:
- Heavy metals arsenic, lead, copper, zinc, cadmium, and manganese.
- Acidic mine drainage low pH water that mobilizes metals.
- Organic compounds petroleum hydrocarbons from historic industrial activities.
These substances are present in both the riverbed sediments and the floodplain soils. Elevated concentrations exceed both Montana Department of Environmental Quality (MDEQ) and EPA waterquality criteria, posing risks to:
- Human health direct contact with contaminated water or soil, and consumption of fish.
- Aquatic ecosystems toxic effects on macroinvertebrates, fish, and riparian flora.
- Recreational uses swimming, boating, and fishing are limited in affected reaches.
Key finding: The highest concentrations of arsenic and lead occur in the former reservoir basin, where sediments were historically trapped.
Cleanup Actions and Progress
Remedial Investigation/Feasibility Study (RI/FS)
Between 1998 and 2004 the EPA, in partnership with the state of Montana and the City of Missoula, conducted a detailed RI/FS to characterize the extent of contamination and evaluate remedial alternatives. The preferred remedy, selected in 2005, combined three main elements:
- Excavation and removal of highly contaminated sediment from the former reservoir basin.
- Stabilization of the floodplain through capping and vegetative cover to reduce erosion and exposure.
- Longterm waterquality treatment using passive treatment systems (e.g., constructed wetlands, anoxic limestone drains) to neutralize AMD upstream.
Implementation Highlights
- 20082012 Removal of ~3.5 million cubic yards of sediment from the reservoir area. The material was transported to an approved landfill in Anaconda, Montana.
- 20132016 Installation of two largescale passive treatment wetlands at the headwaters of the former mineimpacted streams. These systems have reduced acidity by an average of 85%.
- 20172020 Construction of an engineered riverbank capping system using a mix of clean fill, geotextile membranes, and native vegetation. The cap minimizes direct contact with residual metals and prevents downstream migration.
Community Involvement
The EPA established a Community Advisory Group (CAG) that meets quarterly to review progress, discuss concerns, and provide input on remedial actions. Outreach activities have included public open houses, school field trips, and an interactive website that tracks realtime waterquality data.
Future Monitoring and Site Management
Even after the primary remedial work is complete, the Clark Fork River OU requires ongoing stewardship:
- LongTerm Monitoring Quarterly sampling of water, sediment, and biological indicators to ensure contaminant levels remain below healthbased thresholds.
- Maintenance of Passive Treatment Systems Inspection and periodic removal of accumulated sediments in wetlands to sustain performance.
- Adaptive Management Use of monitoring data to adjust the caps vegetative cover or implement supplemental treatments if needed.
The City of Missoula, the EPA Region8 Superfund Program, and the MDEQ have signed a LongTerm Stewardship Agreement that outlines responsibilities, funding mechanisms, and reporting schedules through at least 2035.
Recreation and Economic Benefits
Restoration of the Clark Fork River has opened new opportunities for outdoor activities. The river corridor now supports:
- Whitewater kayaking and rafting
- Angling for native trout species
- Expanded trail networks linking Missoulas urban core with adjacent natural areas
Local businesses have reported increased tourism, and property values along the river have risen, illustrating the broader socioeconomic gains from a successful Superfund cleanup.
Further Reading & Resources
- EPA Superfund Site Profile Milltown Reservoir, Clark Fork River (Site ID: 0600840)
- Montana Department of Environmental Quality Clark Fork River Water Quality Dashboard
- City of Missoula Clark Fork River Recreation Guide
- Community Advisory Group Meeting Minutes EPA Region8
