With the dam and the majority of the sediments removed, remediation at the Milltown Superfund site steadily winds down, while restoration work continues to gear up along the Clark Fork and Blackfoot Rivers.
The EPA has been in charge of remediation - defined here as the removal of the dam and more than two million cubic yards of contaminated reservoir sediment. But the EPA will soon hand off the lead role to the Montana Natural Resource Damage Program to carry out its restoration plan to return the Clark Fork and Blackfoot Rivers to a more naturally functioning condition.
Here's a recap of Milltown remediation: After first breach of the Milltown Dam on March 28, 2008, the river flowed free through the former powerhouse area for the first time in 100 years. Workers then walled off the spillway with an upstream earthen berm and gradually demolished the remaining part of the Milltown Dam. The concrete rubble and unsalvageable timbers were trucked a short distance to the adjacent tunnel pond until nothing was left of the dam but a large hole under the bluff with a couple of knobs of bedrock sticking up.
On March 27, 2009, nearly a full year after the initial breach, the last of the spillway cofferdam was scooped aside, and the combined waters of the Clark Fork and Blackfoot Rivers began to flow into the channel they ran through before the dam was built. While this second breach wasn't nearly as dramatic as the first one, the fact that it was returning the river to its original channel made it just as historic.
In the week leading up to the breach, Envirocon excavators narrowed the earthen cofferdam that had kept the spillway work area dry over the past year. In the meantime, the new channel area under the bluff filled with groundwater until it became a large pool. On the day of the breach, it took the excavator only a few minutes to scrape aside the remaining berm between the present river channel and the historic river channel under the bluff.
Initially, about a fifth of the river's flow spilled into the new channel, but within a few days about 70 percent of the river had moved to its new home. The river is currently flowing through both channels, though with the rising runoff roughly 85 percent has been captured by the new channel. The temporary powerhouse channel where the river flowed for the past year will accumulate sediment as runoff recedes and will eventually become part of the flood plain.
The final breach was less dramatic than the first in terms of downstream impacts as well. The water level dropped only two feet, compared with 12 feet last year. A week after the breach, suspended sediment and arsenic were back to normal levels after being slightly elevated two days after the breach - when Envirocon returned to remove more of the cofferdam. Monitoring of the river, groundwater, fish and aquatic insects continues in force this spring and summer, so check the CFRTAC website for those results as they become available.
Elsewhere at the Milltown site, Envirocon is on track to finish excavating and shipping sediments by late summer. It has already removed most of the haul roads and began pulling the rail spur in May. Later this summer, Envirocon will begin to re-grade the flood plain, including roughing in the future channel of the Clark Fork.
Envirocon also plans to armor (e.g. rip-rap) remaining sediments in the old bed of the Clark Fork River if they're not removed as proposed. It will also reclaim on-site repositories filled with rubble from the dam. One repository is on the right bank near the confluence and another at the former Tunnel Pond. The abutment along I-90 will also be buttressed.
Much of this wrap-up remediation work is scheduled to happen by the end of this year. Envirocon will demobilize but return in the summer of 2011 to backfill and re-grade the bypass channel.