Milltown Sediments Fail to Grow Plants at the BP-ARCO Repository

December 13, 2010
Contaminated sediments from the Milltown Reservoir Superfund site have yet to grow the much hoped for vegetation at the BP-ARCO Waste Repository at the Anaconda Smelter Superfund site --  and may end up capped themselves.

CFRTAC will hold a public meeting at the Anaconda Courthouse at 7 p.m. on Tuesday December 14 to discuss the Milltown sediments issue. For details call 846-1628.

Milltown sediments were once believed to be a solution to a problem at the 4000-acre repository, which for decades has been the toxic home for hundreds of millions of tons of heavy metal contaminated waste from the Anaconda Smelter. The site needed borrow material for a cap to grow grasses over the older and much more toxic smelter wastes at the repository. Vegetative cover, according to ARCO planners, would capture most the precipitation, thus preventing infiltration and contamination of groundwater. And a grassy cap would also help reduce the chronic dust storms at the site.

The Milltown sediments, some three million tons, were excavated from the reservoir between 2007 and 2009 and spread over 800 acres of smelter waste at the BP-ARCO site. (See the Other End of the Dam Project). Although Milltown's sediments were contaminated with arsenic, copper and zinc, they were also rich in organic matter and thought to be capable of growing plants. But after three growing seasons, vegetation is sparse to nonexistent, and EPA and ARCO are trying to figure out why.

"Research has been conducted showing successful growth on the sediments using various treatments like lime, organic matter and mulch," says Charlie Coleman, EPA Project Manager for the site. "Lab conditions don't always work in the field though."

Coleman says researchers have put a good deal of work into studying various conditions including salinity, nitrogen content, and texture. ARCO has used the knowledge gained from these studies to build demonstration sites at the repository to see what soil amendments will work. ARCO continues to investigate the mystery and is designing a new "treatability study," which is due to EPA in March of 2011.

The time for study seems to be drawing to a close as growth fails to achieve the levels required under the Record of Decision (ROD) for the Smelter site. Coleman says EPA will require a soil cap for the area regardless of the success of the treatability study. If the ARCO's study is deemed successful, EPA will require a 6-inch cap placed over the sediments. If the study is not successful, an 18-inch cap will be required. Capping the sediments with soil will be an expensive action either way. Coleman estimates the 18-inch soil cap would cost about $20 million and a 6-inch soil cap would run about $10 million. "But either way it gets a cover," he says.

ARCO is searching for an area that can provide the soil needed for such a cover.