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Missoula Valley Water Quality District
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Missoula Valley Water Quality District

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Photograph showing the Milltown Dam Power House.

Milltown Dam: Ground Water Contamination

In 1981, the Missoula City-County Health Department found domestic water supply wells to be contaminated with arsenic. The arsenic plume in Milltown’s groundwater covered approximately 350 acres and affected more than 200 parcels of land. This posed a potential health risk for Missoula County residents, and affected the economic value of lands owned by citizens in the vicinity of the reservoir.

The Milltown to Hellgate aquifer has been found to be hydrologically connected to the Missoula Valley Sole Source Aquifer; in fact it is an extension of the Missoula Valley Aquifer. The Missoula Valley aquifer in the Missoula urban area receives approximately 22% of its flow from the Milltown to Hellgate Aquifer. The reservoir sediments contained an estimated 2,100 tons of arsenic, and released 105 pound of arsenic per day, or 19 tons per year into the Missoula Aquifer. Approximately 5.5 trillion gallons of arsenic contaminated groundwater flowed downstream into the Missoula aquifer each year. Concentration of arsenic in community wells in 1981 ranged from 220 to 510 parts per billion. Concentrations of arsenic in monitoring wells at the site were as high as 1,100 parts per billion. The revised federal drinking water standard for arsenic, effective January 2006, is 10 parts per billion.

The State of Montana concluded that “without effective source removal, the discharge of tens of thousands to hundreds of thousands of acre feet of contaminated groundwater per year will persist from the Milltown Reservoir for hundreds of years.” And that, “Without remediation, the groundwater system will remain contaminated for hundreds to thousands of years to come.

Contaminated wells in Milltown have been replaced with clean sources of drinking water. No domestic supply wells currently exceed the arsenic drinking water standard.

The State of Montana has estimated that sediment removal will clean up the polluted aquifer in three years, Arco estimated four years and EPA estimated 4-10 years, after removal. Restoration of the aquifer is a primary goal of the Superfund remediation project. Since the remediation project began in 2006, arsenic concentrations have dropped significantly in most monitoring wells near the former reservoir. Map of DataMonitoring will continue for domestic and monitoring wells near the site and downstream into Missoula.

Smaller version of an image showing the arsenic plume in the Milltown Dam area.
Click on the image above to view a larger image.

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