Drawdown also means less arsenic in wells

MISSOULA – Levels of arsenic and other toxic metals monitored in wells scattered around Milltown have dropped dramatically since the Milltown Reservoir drawdown began nine months ago, test results show.

In one monitoring well, arsenic levels fell from 339 parts per billion last June to 17.8 ppb in December.

Tests at other wells showed decreases of at least half in levels of arsenic, manganese and copper.

The federal and state safe drinking water standard for arsenic is 10 ppb.

Milltown Reservoir has been lowered by as much as 12 feet over the past nine months, in preparation for the removal of Milltown Dam and the millions of cubic yards of contaminated sediment 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 believed 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.

There is no evidence that the arsenic plume is moving downstream, said Peter Nielsen of Missoula’s city-county health department.

Besides the wells the EPA is monitoring, the county has been checking several others downstream all the way to Missoula, Nielsen said.

It also has offered residents opportunities to have their water tested. So far, there has been no sign of elevated levels of arsenic or other heavy metals, he said.

But what remains of the arsenic plume won’t disappear overnight. The EPA estimates it will take four to 10 years after the Superfund project is completed before the Milltown aquifer will meet federal drinking water standards.