$100 million settlement for Milltown
About 100 years ago, William Clark, a US senator and copper magnate, constructed a dam in Riverside, Montana — now known as Milltown — just below the confluence of the Clark Fork and Blackfoot Rivers to generate electricity. Just five months after it went into operation, the dam was destroyed. A great flood scoured the Clark Fork for more than 160 km upstream. That torrent, along with later, lesser floods, brought with them some 4.6 million m3 of sediment containing thousands of tonnes of waste metals including arsenic, copper and cadmium. The floods dumped this right behind the dam and the resulting groundwater arsenic plume impacted local drinking water and fishing.
In August, the US Department of Justice (DOJ) and EPA announced that Atlantic Richfield and NorthWestern Corp had agreed to cleanup and restore the Milltown Reservoir Superfund site. The settlement is valued at over $100 million. The negotiations involved the State of Montana and the Confederated Salish and Kootenai Tribes, as well as EPA, DOJ, and the two corporations. The planned cleanup will restore local drinking water, improve conditions in the Clark Fork and Blackfoot Rivers, and improve local fishing.
The Milltown Reservoir remediation and restoration is expected to take place over the next six to seven years. The settlers have agreed to remove nearly 2 million m3 of contaminated sediments from the Clark Fork and Blackfoot Rivers and remove the Milltown Dam to prevent recontamination. These funds will also allow addtional work, including historic preservation and the removal of the nearby Stimson Dam, and will reimburse past and future federal response costs. After remediation is complete, the State of Montana will implement the restoration plan for the Clark Fork and Blackfoot Rivers, including reconfiguring and revegetating the channel and floodplain to return the rivers to their original free-flowing conditions. Once the remediation and restoration are complete, a local community group, the Milltown Redevelopment Working Group, will oversee the implementation of the redevelopment plan it created for the area.
The Milltown Reservoir site was added to the National Priorities List in September 1983 after Missoula County health officials identified arsenic contamination in the drinking water supply. Residents were provided with an alternative water supply. Multiple environmental investigations were performed in the Milltown Reservoir between 1982 and 1992 to determine the source and scope of the contamination. EPA and the Montana Department of Environmental Quality issued their cleanup plans for the site in a December 2004 Record of Decision.
This project is complicated because all the sediment is part of the largest Superfund site in the US, stretching from the headwaters of the Clark Fork in Butte to the Milltown dam. It is also a technically challenging project calling for removing the sediment, leaving the rest in place above the water line, and removing a 150-m-wide dam without releasing toxic contamination downstream.
EPA officials who oversee the Superfund programme have estimated the cleanup and restoration of the Blackfoot from Butte to the dam will reach nearly $1 billion.
Envirocon is the environmental remediation company contracted to remove the dam and the contaminated sediment at Milltown. The first step in the project – lowering the water level in the reservoir by 3 m – will be a gradual one. It will be monitored carefully to ensure a minimum amount of contaminated sediment is washed out.
Most of the contamination is in a V-shaped area of nearly 40 ha, in depths of 4.6 to 7.6 m, directly behind the dam. Lowering the water level will allow much of the sediment to dry out, so that it can be removed quickly and safely. Next spring, Envirocon plans to begin building roads into the contaminated site and sinking supports for a bridge that will be built over a channel that is to be dug to redirect the Clark Fork.
Excavating the 100-m-wide, 1,070-m bypass channel will require removing 570,000 m3 of sediment and stockpiling it for later removal and treatment. An abandoned rail spur will be rehabilitated to allow waste to be rail hauled away from Milltown. The channel will be 5.2 m deep in the centre and berms will be built to contain the river as it is directed around the contaminated area.