All that's left now is to see if it's all going to work.
Over the next couple of weeks, Envirocon crews will install the features they hope will drain thousands of cubic yards of waterlogged dirt - enough to allow excavators to move in and start digging a small part of the bypass channel.
Plans call for digging the bypass channel over the winter, but to stay on schedule Envirocon needs to know it can efficiently move an estimated 600,000 cubic yards of sediment.
“If we can't get the water out of the sediment, then we're going to have to look at a different way to go about it,” said Matt Fein, Envirocon's senior project director. “Engineering and planning says it will work, but I wouldn't want to bet the farm on it until we try it.”
Right now the sediment along the edge of the interstate where the bypass channel will be built is soggy.
“If you were to go out today and try to dig a hole, you'd get down about 6 feet and hit water,” Fein said. “Once that happens, the sides of the hole begin to collapse. You can't dig anymore. The hole just gets wider. The sediments are just too soupy, too soft.”
The challenge facing Envirocon is the sediment behind the reservoir has been deposited over the years. Each layer is somewhat different - some layers have more clay and others are more sandy. As a result, water in the sediment tends to drain horizontally rather than downward.
“It would take a long time for it to drain naturally,” Fein said.
On top of that, water in the alluvium - the gravel, cobbles and boulders found between the sediment and bedrock - is under pressure.
“It's called an artesian,” Fein said. “You put a straw into it and the water would rise above the top of the alluvium.”
To relieve pressure in the alluvium, the company is working to install six large wells - each about twice the size of an average domestic well.
In addition, Envirocon is also putting in “wick drains” that will run vertically through the sediment and into the alluvium. The drains are simply fabric-covered pieces of hollow plastic which allow water caught in the sediment to drop down into the aquifer.
The company is also installing a series of “eductor wells,” which essentially send a column of water through a perforated pipe in the sediment, which creates back pressure to suck out additional water.
“They use water to pick up more water,” Fein said. “It's a physics thing.”
The water pumped out of the aquifer will end up in the Blackfoot River just above its confluence with the Clark Fork River. The discharge will be somewhere around 3.6 cubic feet per second, or about 1,600 gallons a minute.
The Clark Fork River on Thursday was running at about 1,180 cfs at the Deer Creek Bridge, 2.8 miles downstream from the reservoir.
The water pumped from the aquifer will have elevated levels of arsenic and other metals, but it's not expected that arsenic levels downstream will increase dramatically.
While there likely will be some variability during construction, Peter Nielsen of the Missoula City-County Health Department said computer models predict that arsenic levels will rise from about 3.48 parts per billion to about 5.12 ppb. The safe drinking water federal standard is 10 ppb.
“The impact will be noticeable, but we should stay well below limits,” Nielsen said. “Our monitoring plan should help ensure that we do that.”
Arsenic levels in the Clark Fork River at Turah are often already higher than what's expected downstream.
“Turah was up to 8 on Sunday. Š The ice jam in 1996 brought up arsenic levels downstream of the dam to 90 ppb,” Nielsen said.
There is some concern that a portion of the eastbound lane of the interstate could settle as water drains from the sediment upon which it is built. That will also be monitored closely, Nielsen said.
With a project this large and complex, there's always the potential for setbacks.
“We are dealing with a lot of variables,” said Russ Forba, the U.S. Environmental Protection Agency's Milltown project manager. “We could have high flows next spring that could delay us. There are a number of things that could occur that could change the schedule. We're hopeful that won't happen.”
On Thursday, Milltown Reservoir had been drawn down 11.5 feet - a level not seen for more than a quarter century, Nielsen said.
“The radial gate is wide open right now,” Nielsen said. “The dam isn't controlling the water levels. The rivers have reached equilibrium.”
Over last weekend, there was a significant spike in turbidity levels below the dam.
“There was a lot of sediment right in front of the dam that scoured out last weekend,” Nielsen said.
Turbidity levels below the dam jumped up to 14.35 turbidity units on Sunday. By Wednesday, those levels had dropped to 5.5, Nielsen said.
The jump in turbidity didn't impact fish in monitoring cages just downstream, probably in part because of cool water temperatures, he said.
During summer, fish in monitoring cages died from bacterial infections likely caused by a combination of high water temperatures and turbid water. The Environmental Protection Agency ordered the reservoir level raised after the fish kill in an effort to clear up the water.
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