October 4, 2006
From Peter Nielsen of the Missoula Valley Water Quality District comes this update on progress at the Milltown Superfund Site:
Construction work has begun at the Milltown Reservoir Superfund Site. The Reservoir is drawn down 11.25 feet below full pool elevation. Turbidity has increased noticeably in the river downstream, but water quality remains below construction limits and water quality standards.
The photo above, courtesy Gary and Judy Matson, shows the progress of work to fortify the embankments of the Interstate 90 bridges over the Blackfoot River. This work is under the direction of the U.S. Environmental Protection Agency and Army Corps of Engineers, and is being performed by Spaulding Construction and GeoCon. Concrete is being injected underground along the base of each embankment, through a process called jet grouting. The concrete is injected in interlocking columns that go down 44 feet to bedrock. Additional work will be performed on the embankments, abutments and center piers of the Interstate and Highway 200 bridges over the coming year. This will strengthen the bridges so they can withstand the forces of a free-flowing Blackfoot River following dam removal in early 2008. The access road on the west side of the river (left) is constructed in the location where the Milltown Redevelopment Working Group has proposed a pedestrian trail be constructed following cleanup and restoration, linking the Bonner and Milltown communities with the confluence and a proposed interpretive center to be built near the current dam site.
The image above shows the confluence of the Blackfoot River (left) and Clark Fork River (right) from the bluff overlooking the dam (lower left hand corner). Note the shallow submerged sediments just upstream of the dam in the river channel. These are the sediments that are eroding and causing increased turbidity downstream. The sediments in the triangle shaped area between the two rivers are the most heavily contaminated in the reservoir, and were washed downstream from the Butte and Anaconda area during flood events, mainly the 1908 flood. These sediments have become exposed since the drawdown began in June. Vegetation, including some grasses and willows, have started to re-vegetate these sediments in the past few months. In the background of this photo, near the Interstate, construction activity conducted by Envirocon is now visible.
This image is a close-up of the Envirocon construction area. This work is known as the dewatering and bypass channel constructability test. Envirocon is installing wells and wick drains to draw water out the sediments in the area where the bypass channel will be constructed. Over the next two months Envirocon will drain and pump water from the sediments, and construct a section of the bypass channel. Pumping of water from the site is expected to begin October 16. Excavation of the bypass channel will begin by about November 1, and will remove contaminated sediments down to the native river gravels, about 20 feet below the current ground surface. Water will be discharged to the Clark Fork River, and monitored closely to ensure that water quality remains within construction limits and standards. This work will allow the company to refine the design of the bypass channel, which is scheduled to be completed within the next year. The Clark Fork River is scheduled to be diverted into the bypass channel in the fall of 2007, prior to the second reservoir drawdown which will lower the water surface an additional 6 feet. The purpose of the bypass channel is to protect the most contaminated reservoir sediments from erosion when the reservoir is drawn down and the dam removed.
About 300 to 400 logs in the Blackfoot River upstream of the former Bonner Dam will be removed from the river banks by the Montana Department of Natural Resources and Conservation. These logs were floated down the river to the mill before roads and the railroad extended up the Blackfoot, and were collected behind the Bonner Dam, which was removed last fall. Timber crib piers in the Blackfoot River behind the mill will be removed by the Army Corps of Engineers in November and December.
EPA has replaced 13 domestic wells that were affected by the lower water table in the aquifer caused by the reservoir drawdown. The water table has dropped about 4-6 feet in Milltown and West Riverside, near the Reservoir, as compared to historical water levels when the reservoir was full.