March 28, 2008, excavators breached the Milltown Dam near Missoula, MT, allowing the waters of the Blackfoot and Clark Fork rivers to run freely for the first time in a century. It was a historic moment in the long effort to clean up the 820-acre Milltown Reservoir Sediments Superfund Site, but there is still a lot more work ahead for Envirocon Inc., the Missoula-based contractor for the $120-million project.
The Milltown Reservoir Sediments Superfund Site is an operable unit of a larger multiple site complex generally known as the "Clark Fork Basin Superfund Complex," which stretches upstream 120 miles from the reservoir to Butte and Anaconda. The Milltown Site is at the confluence of the Blackfoot and Clark Fork Rivers a few miles upstream of Missoula.
Behind the 660-foot-long dam, approximately 6.6 million cubic yards of contaminated sediments had built up over the years. Arsenic in the sediments polluted the local drinking water aquifer, and release of copper in the sediments threatened downstream fish and other aquatic life. These sediments were the result of mining operations upstream in Butte, MT.
According to U.S. Environmental Protection Agency, the Milltown cleanup integrates remediation, restoration and redevelopment, and will:
- Remove the Milltown Dam and Powerhouse;
- Excavate approximately 2.2 million cubic yards of the most highly contaminated sediments in the Milltown Reservoir;
- Restore the Milltown community's drinking water supply in as little as a decade;
- Allow unrestricted fish passage; and
- Return the Clark Fork and Blackfoot Rivers to a more natural and free-flowing state.
Russ Forba, project manager for the EPA, has been overseeing this work for the past 11 years and described the breaching as yet another step in completing the cleanup at Milltown.
"Every day we move a little bit closer to cleaning up the drinking water supply and restoring the rivers," Forba said. "Today, however, is a giant step forward."
More than a year of work preceded the breaching of the dam. One of Envirocon's first tasks was to build a bypass channel to divert the course of the Clark Fork just upstream of the dam. EPA required construction of the bypass to isolate the Cark Fork from the contaminated sediments in the Milltown Reservoir. This reduced the sediment scouring during reservoir drawdown and minimized impacts to the downstream ecology.
The channel work involved excavation of about 600,000 cubic yards of earth, using excavators and articulated haulers, Forba said. Berms were left at each end of the channel, and in mid-March the outlet berm was opened to allow about 44 million gallons water from the Blackfoot to flow upstream and fill the channel. Once that happened, the inlet of the bypass channel was opened, allowing the Clark Fork to flow through the channel.
Demolition of the historic Milltown Powerhouse started in late January. The work began with removal of the north wall, and then the 100-year old structure was taken down in sections over about three weeks. During that time, workers from Envirocon were constructing a coffer dam just upstream of the powerhouse to completely divert the water through the dam's radial gate and allow for removal of the powerhouse foundation.
"We spent three months working on the powerhouse in preparation for the first breach," said Brian Vibbert, Envirocon's superintendent for the dam demolition. "We had five excavators working on it."
In order to keep the work area dry, a temporary earthen cofferdam was built just upstream of the powerhouse, and a work pad was constructed downstream. The cofferdam was reinforced with sheet piling driven into the bed of the river.
After workers finished demolishing the powerhouse foundation, they removed the sheet piling between the earthen coffer dam and the rivers. Then they excavated fill from the coffer dam and created a pilot channel to direct the rivers toward the area to be breached. Workers are also created a 15-foot-wide lip that would serve as the initial spillway in the coffer dam.
Until then, the rivers had been flowing over the radial gate of the dam. Raising the radial gate caused the river levels to rise behind the dam. As the river levels rose, they began to slowly overtop the coffer dam. As more water flowed through the ever-increasing opening in the coffer dam, the Milltown Dam was officially breached.
At this point, the radial gate and the south half of the dam remained in place. This spring and summer, Envirocon built a temporary diversion dike and a downstream cofferdam to isolate the remaining dam section and the spillway from the river. As with the previous demolition work, extensive dewatering was required.
This section of the dam, built in 1907, was constructed of timber, earth and fill. By September, crews were removing old sheet piling that was part of the spillway, and they expected to finish removing the dam by this fall.
The cleanup is removing polluted soil from 167 acres of the former Milltown reservoir immediately behind the dam.
"One of the initial tasks was dewatering the site so we could build the diversion channel," Forba said.
Clearing and grubbing for the test pits began in October 2006, and the first tests were done two months later. Test holes were drilled on 100-foot centers to find the contamination, and then several types dewatering tests followed, including inductors and dewatering wells.
The testing found that the water table in the area was at 3 feet, and that up to 20 feet of soil needed to be removed from the wedge-shaped area.
"When we started this job, if you followed an excavator through the brush, the ground felt like Jello," Envirocon's Vibbert recalled.
Fortunately, the testing showed that the area could be excavated by conventional means, rather than using hydraulic dredging.
"Maybe the biggest steps in the project were the dewatering tests," Forba observed. About 1 billion gallons of groundwater was pumped from the site last year to allow the soil removal to begin, he added.
Flood control berms protect two sides of the excavation area, and an armored embankment fronts the river at the west end. Inside them, the excavation area is divided into six cells, with haul roads in between them.
A mile-long rail spur was built atop the south side berm, providing access to the site for the trains that remove the contaminated sediment daily. The 45-car trains carry the material to the Anaconda Smelter Superfund site for use in site reclamation.
When PB&E visited the project in late August, the excavation was about half finished. Excavators dug out the soil, using GPS rather than soil chemical sampling to control the depth of the digging, and loaded articulated haulers that would carry the materials up to a dumping berm next to the rail cars. There, a dozer was shaping the berm on one side while two front-end loaders were filling the 100-ton railroad cars from the other side. A scale on the 10-cubic-yard bucket of each loader kept track of the day's production.
If all goes according to plan, removal of the reservoir sediments should be completed next year, Forba said.
The final step in the restoration process will be to build a new, meandering course for the Clark Fork through the remediation triangle after the sediment removal is complete. That work probably will continue into 2011, according to the project timeline, and then the river will be shifted from the diversion channel into its permanent location. The state of Montana's Natural Resource Damage Program will design the new river channel and revegetation of the site, Forba said.
Through the end of August, project personnel had worked 201,802 hours without time lost to an injury, the EPA reported.
"It's been a cooperative effort," Forba said. "We have a great relationship with Envirocon. Right from the beginning we spoke the same language. We want to keep the project going."
With completion of the project, the Clark Fork and Blackfoot rivers will be restored to a more natural state, and western Montana will no longer face the potential for a disastrous failure of the Milltown Dam.