November 16, 2006
Missoula County’s Peter Nielsen offers this update of work at the Milltown Reservoir Superfund site:
Construction work continues on the sediment dewatering and bypass channel test at the Milltown Reservoir Superfund Site. Construction at the Interstate 90 bridges has temporarily stopped due to instability of the construction pad used to support heavy equipment near the bridges.
The photo above shows the point of the contaminated sediments known as Area 1, at the confluence of the Blackfoot and Clark Fork Rivers. This photo is taken from the south side of the reservoir near the dam on November 2, 2006. The Blackfoot River is flowing under the Interstate 90 bridges toward the Clark Fork River in the foreground.
On this date, the reservoir was drawn down 11.5 feet from full pool, the deepest level that the reservoir has been drawn down since about 1980 when a 20 foot drawdown was carried out by the Montana Power Company for maintenance. Note the vegetation established on the reservoir sediments since the drawdown began in June, and the sediments exposed on the edge as the reservoir reached this level over the past several weeks.
This photo, also taken on November 2, shows the upstream face of the dam from the south shore. Note the accumulation of sediments exposed at the upstream face while the reservoir was drawn down 11.5 feet. Just above the exposed sediments are the 8 foot wooden flashboards at the crest of the dam’s spillway, which are used to maintain the normal full pool elevation in the reservoir. Above the flashboards is the bridge on which people can walk across the spillway.
The two photos above were taken one week later, on November 9, 2006 after a heavy rain event occurred, particularly in the Blackfoot River drainage. Note the turbid, brown color of the water in the Blackfoot, which increased in flow from about 600 cubic feet per second (cfs) to more than 3,000 cfs in 48 hours. Flows in the Clark Fork did not increase as dramatically following the storm. The reservoir level had been entirely controlled by the radial gate, which is visible between the brick powerhouse and the spillway. But as river flows increased, the reservoir level increased by 4 feet from the photos taken the previous week (7.5 foot drawdown), and water began flowing again over the spillway. As flows have dropped over the weekend, the reservoir level is gradually dropping, and the reservoir is drawn down by about 8.9 feet today. Despite the dramatic, visible increase in turbidity, levels of arsenic and copper downstream of the dam actually decreased during this event due to dilution with Blackfoot River water.
This photo shows the discharge pipe from the sediment dewatering test in the foreground. Envirocon has installed six, 12-inch wells to pump water from saturated sediments in an area of the reservoir where a bypass channel is planned to be constructed over the next year. Envirocon has also installed a network of wick drains and smaller wells called eductor wells to help remove water from the saturated reservoir sediments. The test has pumped about 200 gallons per minute of water into the river. Water quality is monitored daily. Although the discharge contains contaminants such as arsenic, the volume of the discharge is small compared to the flow of the river and increased concentrations downstream in the river have not been detected.
The sediments must be dewatered in order to allow them to be handled efficiently by excavators during construction of the bypass channel. To understand the reasoning behind this, imagine that the reservoir is drawn down about 9 feet today, but the bypass channel must be dug down about 25 feet through the contaminated sediments to the clean gravels below, and it must be large enough to accommodate the 100-year flood flow of the Clark Fork River when it is complete. When excavators try to dig deeper than 10 feet, the sediments are below the water level of the river and they are too wet and soupy to handle. The sides of the excavation simply cave or flow into the hole as they dig. But, if enough water can be drained from the sediments, the excavators can dig them and the bypass channel can be completed. The schedule calls for completion of the bypass channel by this time next year. The dewatering test will provide information needed to finalize the design of the bypass channel and keep the project on schedule. The test is set to be complete in December. A section of the bypass channel will also be constructed.
This final picture, also taken on November 9, shows a pile of rock riprap at the left side of the photo, delivered to the site for the Interstate bridge mitigation work coordinated by the EPA and Army Corps of Engineers. Under the bridge is a temporary construction pad created for heavy equipment to complete the bridge work. The pad is built out from the edge of the river bank into the reservoir. The pad has become unstable in recent weeks, in part because it is built on top of reservoir mud. In an effort to shore it up, an excavator placing riprap actually slid off the pad and into the reservoir in about ten foot deep water. No one was injured, and no fuel was released to the river. But the incident has slowed work on the bridges. Contractors had been injecting concrete into the ground at the base of the embankment below the bridge, but this work has now been stopped until spring when warmer weather returns. Additional riprap will be placed on the edge of the pads at both ends of the bridge in coming weeks.
Additional work to be completed by the EPA and Corps this fall includes the removal of an abandoned bridge pier at the west end of the Highway 200 bridge, and removal of timber crib piers in the river behind the Stimson lumber mill on the Blackfoot River. Envirocon plans to begin construction of a flood control berm on the Clark Fork River side of the Area 1 sediments in December.