Even before Milltown Dam's heavy radial gate lifted Thursday morning to start the permanent drawdown of the reservoir, officials from a variety of agencies were afield putting together the system needed to monitor the effects of the $100 million project to rebuild the confluence of the Clark Fork and Blackfoot rivers.
A broad surface and well water monitoring program was initiated before the drawdown began and will continue through the life of the project, said Peter Nielsen of the Missoula City-County Health Department.
Turbidity samples in the river below the dam will be taken three times a day during the drawdown. If those samples exceed a specific threshold, then officials will begin taking samples for heavy metals and arsenic.
The sediment thresholds are conservative, Nielsen said. During previous drawdowns, the threshold was 25 units. This time around it's 12, he said.
“We wanted to keep a lid on it,” he said. “When those thresholds are hit, it triggers our contingency plans. During this first drawdown, we have the luxury of being able to control the amount of water being released at the dam.”
Eventually, the drawdown will drain Milltown Reservoir, allowing workers to excavate millions of cubic yards of sediment contaminated by mine and smelter tailings washed down the Clark Fork River over the past 100 years.
Once the sediment is removed, so too will Milltown Dam go, followed by the reconstruction and rehabilitation of the confluence of the Clark Fork and Blackfoot rivers.
As the reservoir level drops in the weeks ahead, water in both the Blackfoot and Clark Fork will scour sediments from behind the dam. It's estimated that upward of 100,000 cubic yards might be uprooted during this drawdown alone.
If monitoring finds too much sediment in the river, officials can slow it down by simply closing the dam's gates and backing up water in both rivers.
“We have the luxury of the fact there won't be any work actually started until later on this year,” Nielsen said. “So there's no rush to drop the reservoir. We'll be watching this closely.”
In past drawdowns, turbidity levels remained relatively low until the reservoir dropped by 6 feet.
“We can't even get to that point until the flow in the rivers drop,” he said.
Officials also will continue to monitor local domestic water wells for both arsenic contamination and water levels.
Various agencies are tracking the plume of arsenic-contaminated water in the aquifer near the reservoir using both wells drilled specifically for monitoring and a series of domestic wells. The network starts near Piltzville and continues downstream to Missoula.
Nielsen said those wells are being sampled up to four times a year. Should arsenic levels in the river reach 8 parts per billion, sampling would be bumped up to once every two weeks.
Testing for arsenic in domestic wells started last fall.
So far, 175 domestic well owners from Milltown and Bonner all the way down to the mouth of the Bitterroot River have taken advantage of the free offer to test their water for arsenic.
The arsenic tests are paid for by the Atlantic Richfield Co.
Nielsen said the kits, which include bottles and a prepaid mailer, are still available at the health department, the Piltzville Fire Department and Bonner School.
“I'm not sure that we could make it any more convenient,” he said. “People just have to fill up the bottles, complete a form and put the box in the mail. There's no cost for the arsenic test.”
For an additional $10, people can also have their well water tested for nitrates. A bacteria test can also be included.
“We recommend that people periodically test their private wells at a minimum for bacteria and nitrates,” he said.
The recent arsenic testing did turn up a couple of surprises.
A well in Bonner Pines, on the upstream end of the reservoir, and a second well in Milltown were found to have arsenic levels higher than the new federal standard of 10 parts per billion. The standard was 50 ppb before January.
Arco responded by digging a new, deeper well for the residents in Bonner Pines and providing bottled water to the Milltown home.
The company is responsible for paying for new wells needed because of arsenic contamination. The Environmental Protection Agency picks up the cost for any work needed on wells affected by a drop in the aquifer tied to the drawdown of Milltown Reservoir.
Last year, when the reservoir was dropped to facilitate the removal of Bonner Dam, upstream on the Blackfoot, a number of people in the nearby area experienced some water shortages.
The EPA stepped up and paid to resolve those problems.
“I really think the EPA is being responsive on their front,” said Nielsen. “They are listening to our recommendations. ... Their track record looks pretty good right now.”
This year, when the reservoir begins to drop, anyone interested will be able to keep an eye on what's happening in the aquifer by simply going online to either ftp://milltown.envirocon.com or www.cfrtac.org.
The county has installed 20 recording devices in local wells as part of a new water level monitoring network in the Milltown area. The devices will provide real-time water level measurements for all to see.
“These will be minute-to-minute measurements,” Nielsen said. “We installed them before the spring runoff began at a time of historical lows. We should really be able to see what's happening out there.”
In addition, the county will measure another 25 or so wells to add to their database.
“We'll use the information we gather to help determine which wells need to be fixed or replaced,” Nielsen said.
The county completed a domestic well inventory for the area Thursday. The inventory includes a variety of information, such as well depth, location and drilling dates.
“We're there to try to ensure the public will be taken care of through this process,” Nielsen said.
Reporter Perry Backus can be reached at 523-5259 or at pbackus@missoulian.com
Keeping watch
For a minute-by-minute accounting of water levels in 20 wells in the Milltown-Bonner area, go online to either ftp://milltown.envirocon.com or www.cfrtac.org.
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