Remediation Alternatives for Milltown Reservoir

Milltown Reservoir is located at the confluence of the Clark Fork and Blackfoot Rivers, approximately seven miles upstream of the city of Missoula. It was created by the construction of Milltown Dam in 1907 and has collected contaminated sediments over the years from mining, milling, and smelting activities upstream in Butte and Anaconda. Approximately 6.6 million cubic yards of sediment has accumulated in the reservoir, containing thousands of tons of arsenic, copper, zinc, iron, and manganese. Periodic scouring events mobilize metal-contaminated sediments, causing water quality standards to be exceeded in the Clark Fork River below the dam. A plume of contaminated groundwater covering approximately 110 acres has developed below and downgradient of the reservoir, with arsenic concentrations exceeding 20 times the drinking water standard. The dam is a barrier to migrating fish, including endangered Bull Trout. Milltown Reservoir was declared a Superfund site in 1982, and the Environmental Protection Agency is currently evaluating alternatives for remedial action at the site. The Missoula Water Quality Advisory Council has developed a list of remediation outcomes essential to the success of any remedial action program for Milltown Reservoir: 1. Surface and groundwater standards should be met downgradient of the site. Surface water should meet Montana WQB-7 standards for total recoverable metals. 2. Scouring incidents that mobilize metal-contaminated sediments should be prevented. 3. There should be passage for all fish species during their migration season. EPA’s alternatives for remedial action fall into four categories: 1. No action beyond institutional controls and monitoring. 2. Dam modification. 3. Dam modification and partial sediment removal. 4. Dam and sediment removal. The Council has concluded that, of these alternatives, only the last has potential to achieve their desired outcomes. The poster explains this view