Montana Earth Science Picture of the Week

Dam Controversy Near Missoula

Photo courtesy of the Clark Fork Coalition

Everybody lives downstream . . .
The Milltown Dam, located near the confluence of the Blackfoot and Clark Fork Rivers, has been a focal point of controversy in the Missoula area. Since the dam was completed in 1907, over 6.5 million cubic yards of sediment were deposited on the bottom of the reservoir behind the dam. Unfortunately these sediments contained an abundance of arsenic and other heavy metals from mining activity in the Butte-Anaconda area. In some places the sediments were 25 feet thick beneath the reservoir. Starting in the 1800s, mining and smelting in the Upper Clark Fork Basin by the Anaconda Copper Company produced tremendous amounts of loose rock material (called tailings) that contained arsenic and heavy metals. Flooding caused by heavy rains in 1908 is blamed for transporting huge amounts of these materials to the reservoir.

Poisoned wells . . .
Pressure caused by the weight of the water forced water into the contaminated sediments, pushing arsenic into the groundwater. As a result several private wells were contaminated. The Environmental Protection Agency (EPA) determined the only way to remove the weight was to remove the reservoir, which meant removing the dam. In the early part of this century the EPA announced plans to remove the dam and restore the confluence to its natural state. Plans also call for removal of about half of the sediment and storage of these in a permanent repository (storage area where they will not threaten natural water systems). The cost of this project was estimated to be $90 million.

A bad investment . . .
As a result of a 1977 merger with the Anaconda Company, Atlantic Richfield Company (ARCO) is responsible for paying to clean up the reservoir. For several years ARCO contended that it would be better to remove the contaminated sediments while keeping the dam in place. While the "dam-in-place alternative" scored higher in terms of its cost, its ease of implementation, and its lack of short-term impacts, the EPA believed that the only permanent solution was to remove both the dam and the sediments.

Update . . .
The photo below shows how far the clean-up had progressed by October of 2008 . The toxic sediment was/is hauled by train to a repository near Anaconda where it will be kept (capped by) a field of grass. In March of 2008 the bypass channel was used to divert the stream around the dam so that the dam and powerhouse could be removed. To watch a 4-minute video that shows how they diverted the stream, CLICK HERE. The Milltown Superfund Site clean-up project is scheduled to be finished by 2011.

Photo courtesy of Kris Cook of Envirocon

Term: repository


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By Rod Benson
Earth Science Teacher at Helena High School

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