Milltown Reservoir Site History

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More than 100 years of mining, milling and smelting produced contaminated sediments that ended up in the Milltown Reservoir. These sediments, containing heavy metals and arsenic, have contaminated the underlying aquifer. They also may be released into the lower Clark Fork River during flooding and ice scouring events.

The site covers 820 acres in Milltown. Missoula, a major population center, is directly downstream of the dam and reservoir. Community concern comes largely from the Missoula area.

BACKGROUND
In 1906 a hydroelectric dam was constructed, forming a reservoir that trapped sediments from mining, milling and smelting operations in the Upper Clark Fork Valley. During the years since the construction, the reservoir storage has been nearly filled with approximately 120 million cubic feet of sediments.

Approximately 91 people live within one half mile of the site. The nearest house is 100 meters away. The site is adjacent to the Milltown Dam, where the Big Blackfoot River joins the Clark Fork River. The rivers are used for recreational activities.

RESIDENTS USED BOTTLED WATER
In 1981, Milltown's four community water supply wells, serving 33 residences, were found to be contaminated with arsenic and other heavy metals. Residents were advised not to use this water for drinking or cooking and to use alternative supplies of water. The Environmental Protection Agency (EPA) and the State of Montana obtained a different domestic water supply for the residents affected by contaminated drinking water.

Ground water and sediment contaminants include arsenic and manganese. The Clark Fork River and Milltown Reservoir contain elevated levels of copper, arsenic, zinc and cadmium. An alternative water supply has been provided, and contaminated wells have been taken out of service. Residents, therefore, have little chance of exposure to contaminants by drinking ground water. Metals laden sediments released from the reservoir continue to pose a risk to a downstream fishery.

EPA added the site to its National Priorities List (NPL) in September 1983. The site is being addressed through the combined actions of federal and state agencies and the Potentially Responsible Parties (PRPs).

CLEANUP CONDUCTED IN STAGES
The site is being addressed in four stages: an initial action and three long-term cleanup phases. The latter phases focus on the water supply, sediment source control, and the Clark Fork River.

In 1983, volunteers using National Guard equipment supplied residences with door-to-door water service on a biweekly basis for three months.

In 1984, EPA selected a remedy to cleanup the Milltown water supply by:

Digging a new well from a separate aquifer;
Constructing a new distribution system;
Flushing the plumbing system of each house to remove contaminants from the water system and plumbing; and
Testing the water quality to ensure standards have been met.

In 1985, EPA added a supplemental remedy, which included replacement of household water supply equipment that remained a source of contamination, as well as continued sampling of individual residences to ensure the sources of contamination had been removed. The state completed the new water supply system and the installation of household water equipment, also in 1985.

In 1989, EPA completed a study on the effects of the contamination downstream from the reservoir. Preliminary results show no environmental damage. However, the sediment investigation will continue to ensure that there is no human health or environmental threat.

CONTAMINATION STUDY
EPA and Atlantic Richfield Company (Arco), a PRP, signed an Administrative Order on Consent (and subsequent amendments), under which Arco agreed to study the extent of site contamination. The study was completed in 1995. EPA also conducted a separate Human Health and Ecological Risk Assessment in consultation with an advisory committee, which included representatives from the public and the Potentially Responsible Parties.

EPA and Arco initiated the Feasibility Study in 1995 to determine groundwater cleanup. In early 1996, an unusually wet winter and early thaw led to an ice scouring event, which churned up river and reservoir sediments. The draft Feasibility Study for the contaminated ground water was placed on hold pending further study of the impacts of the ice scouring. After public discussion, the local government asked EPA to delay choosing a final remedy for the impacted ground water and instead to combine it with a decision addressing the impact of "continued releases" of sediments on the downstream fishery. EPA produced an Ecological Risk Assessment Addendum for the issue of continued releases over the Mill town Dam. The addendum was released to the public and Arco on April 15, 2000, and a public meeting was held in May to discuss the findings and receive public reaction.

ENVIRONMENTAL PROGRESS
The construction of new water supply wells and the replacement of household water supply equipment have provided a safe drinking water supply to affected residents, significantly reducing the potential health threats from contaminated ground water while investigations leading to cleanup of the sources of contamination continue at the site.

RIVER OPERABLE UNIT PROGRESSES
In 1992, EPA designated the Clark Fork River, from the outlet of Warm Springs Ponds (see Silver Bow Creek/Butte Area site) to upstream of the Milltown Reservoir, as part of the site.

An investigation into the nature and extent of contamination of the Clark Fork River began in 1995. A Remedial Investigation report is complete and available for review. EPA released a human health risk assessment in March 1998 that concluded risks to human health were minimal, except in specific areas of the flood plain and old irrigation channels that for the most part are uninhabited. The ecological risk assessment was released in December 1999, and public meetings were held in early 2000. EPA is currently responding to comments received from the general public and Arco on the risk assessment. The feasibility study is ongoing, and a ROD is scheduled for late 2001.

An extensive study of the geomorphology of the river resulted in eleven documents which are available online or from EPA.

Through its Technical Assistance Grants (TAG) program, EPA funds the Clark Fork River Technical Assistance Committee (formerly the Milltown TAC) to review the results of EPA studies and to relay these findings to the community.