Recent well water test results indicate that process may have finally begun.
Since the Milltown Reservoir drawdown began nine months ago, levels of arsenic and other toxic metals monitored in a series of wells scattered around the community have dropped dramatically.
The federal and state safe drinking water standard for arsenic is 10 ppb.
“The drop in arsenic levels in our monitoring wells was something we expected to see,” said Russ Forba, the Environmental Protection Agency's Milltown Superfund project manager. “When the river levels come up this spring, we may see arsenic levels in the wells increase some.”
Over the past nine months, the level of Milltown Reservoir was lowered by as much as 12 feet in preparation for the eventual removal of Milltown Dam and the millions of cubic yards of contaminated sediment stacked up behind it.
For the most part, the Clark Fork and Blackfoot rivers are now contained in their own separate channels. That's left most of the estimated 6 million cubic yards of arsenic-contaminated sediments collected behind Milltown Dam high and dry.
It has long been the theory that arsenic and other heavy metals from the sediments were leached into the aquifer with the help of hydraulic pressure created by standing water in the reservoir. Also, arsenic tends to bind to other material when it's exposed to oxygen. That, of course, doesn't happen underwater.
“A lack of oxygen makes it easier for arsenic to move,” said Peter Nielsen of Missoula's City-County Health Department. “It's one of those geochemical processes that we don't understand how it works, we just know it does.”
There is no indication the arsenic plume is moving downstream, Nielsen said.
Besides the wells in the Milltown area monitored by EPA, Nielsen said the county has been checking several others downstream all the way to Missoula. It has also offered residents opportunities to have their water tested and so far, Nielsen said, none have indicated any elevated levels of arsenic or other heavy metals.
“There's been nothing to indicate an expansion of the plume,” he said.
What's left of the arsenic plume won't disappear overnight. The EPA estimates it will take somewhere between four and 10 years after the Superfund project is completed before the Milltown aquifer will meet federal drinking water standards.
Still, Nielsen said that fact shouldn't be understated.
“This is one of those sites where we actually can clean up the groundwater,” Nielsen said. “There are not many Superfund sites where that actually happens.”
Arsenic levels aren't the only things dropping in the Milltown, Bonner and West Riverside areas.
The aquifer in the area also continues to drop and three water well drillers have been busy meeting the demand for new domestic wells.
Over the past couple of weeks, four wells in West Riverside and one in Milltown went dry after the aquifer dropped by as much as 6 feet, Forba said.
“It appears that people living right next to the river are being impacted more than those further away,” he said.
So far, EPA has paid for 18 new domestic water wells. The agency anticipated it might have to replace as many as 50, Forba said.
“We try to be as responsive as possible,” he said. “Water is a very personal thing. We try hard to react quickly. Everybody has my phone number.”
Reporter Perry Backus can be reached at 523-5259 or at pbackus@missoulian.com
