MISSOULA — When the Milltown Dam was breached in March, it was hailed as a major step in the cleanup of the nation’s largest Superfund complex.

But research now indicates an enormous amount of sediment contaminated by mining wastes is moving downstream, far more than predicted when the combined flows of the Clark Fork and Blackfoot rivers were released for the first time in a century.

“Everyone involved in this project wants to see it as a great environmental success story — and hopefully it will be in the longer term but there are some short-term negatives that are being glossed over,” said Andrew Wilcox, a geomorphologist at the University of Montana.

The research data comes from scientists at the University of Montana, the U.S. Geological Survey and PPL Montana, which owns the Thompson Falls hydroelectric dam downstream on the Clark Fork.

For years, the Milltown project’s supporters and opponents argued over its potential benefits and drawbacks. In the end, the U.S. Environmental Protection Agency approved the project, saying the short-term impact — a potential fish kill and muddy waters for several years — was outweighed by the long-term benefit of restoring the aquifer, the fluvial ecosystem and fish passage.

But taken together, the latest data suggest that while the Milltown project is revitalizing one stretch of the river, it also is spreading mining pollution further across the riverine landscape with uncertain consequences and financial costs.

Below the dam, downstream metals concentrations are now as high as those in Deer Lodge, near where the mining and smelting pollution originated 100 years ago in Butte and Anaconda.

At Thompson Falls Reservoir, about 150 miles below the now-drained Milltown Reservoir, metal concentrations in the sediment have skyrocketed since the dam breaching, including a 12-fold increase in arsenic — a spike that far exceeds the EPA’s worst-case predictions.

UM’s tests on toxicity levels of the downstream metals haven’t been completed, “but I’d be cautious because these are potentially very toxic concentrations we’re seeing,” said Johnnie Moore, a geochemist and director of UM’s Center for Riverine Science and Stream Renaturalization. “I certainly don’t believe anyone is trying to cover up things, but I think people really underestimated the complexity of this river system.”

Russ Forba, the EPA’s Milltown project manager, said significantly more sediment and metals have been released than expected, which he attributed to peak spring flows being slightly higher and longer than average.

But Forba, Keith Large, Milltown project officer for the Montana Department of Environmental Quality, and Peter Nielsen, environmental health supervisor for the Missoula City-County Health Department, said the downstream metals concentrations pose no risk to human health or the environment.

“We’re not alarmed at all,” said Forba.

The project’s standards are based on concentrations of dissolved metals in the water rather than the total amount of metals in the water, suspended sediment and bedload sediment. Federal regulations allow Superfund and other construction projects to temporarily exceed water-quality standards for aquatic life, but not for drinking water.

Forba said additional test results are expected in September and a public meeting will be held in October.

He said Superfund officials are trying to figure out how to limit further releases before next spring’s runoff, and that preventative measures would be taken if their costs are reasonable.

Complicated breaching

Across the United States, a growing number of large dams have been removed in the past decade, but the $120 million Milltown project is considered one of the most complicated breachings ever attempted because of the large amount of pollution bottled up at the confluence of two rivers.

UM researchers and Superfund officials agree the contaminated sediment isn’t coming from the Milltown Reservoir cleanup site, which extends about one mile upstream of the dam and is protected by a bypass channel that diverts water around the most contaminated sediments in the lower reservoir.

Instead, the tainted material is being flushed from the upper reservoir, an area that project officials expected to remain stable, but where the riverbanks have wildly eroded as the free-flowing Clark Fork River re-establishes its natural course.

The UM scientists said the movement of mud and metals confirms that the largest sediment load ever released by a dam breaching in U.S. history is under way.

The “EPA can no longer ignore the overwhelming evidence of uncontrolled release of hazardous substances,” a lawyer for PPL Montana told the EPA in a July 31 letter. “PPLM demands that EPA undertake immediate actions to limit the risk of erosion of significant amounts of impacted sediments from the Milltown site.”

Gordon Grant, a hydrologist with the U.S. Forest Service in Oregon and a nationally known researcher on dam removals, said he wasn’t surprised the Milltown project proved more complex than many people anticipated.

“My overall impression of the removal to date is that the well-intentioned desire to ‘do something’ outstripped some very real cautions offered by knowledgeable scientists,” Grant said. “Whether there will be long-term negative consequences from the removal and its aftermath remain to be seen.”

The Milltown project includes removing 2.2 million cubic yards of the reservoir’s estimated 6.6 million cubic yards of contaminated sediment. The rest is being left in place: Some sediment in the lower reservoir is to be contained by earthen berms, while the upper reservoir’s 3 million cubic yards of sediment is unprotected.

The project’s numerical modeling predicted the breaching and peak flows last spring would release about 300,000 cubic yards of clean suspended sediment from the lower Blackfoot River.

Forba downplayed the likelihood of significant erosion in the contaminated upper reservoir, saying it could take centuries to occur and posed no hazard to human health or the environment.

But based on Geological Survey data and their own samples and observations along the Clark Fork, the UM scientists estimate that about 600,000 tons of suspended sediment were scoured this spring from both the Clark Fork and Blackfoot rivers.

The suspended sediment figure doesn’t include material moving along the riverbed, so the total sediment load sent downstream is far higher, the UM scientists said.

Since early May, university researchers have taken sediment samples from dozens of sites in the Clark Fork’s main channel and side channels for 150 miles below the dam as far downstream as Thompson Falls.

Results showed arsenic concentrations in the riverbed were five to six times higher, copper levels were three to four times higher, and cadmium, lead and zinc levels were three times higher in the Clark Fork’s main channel than before the reservoir was lowered.

Oversimplification

The project’s general contractor, Missoula-based Envirocon, and subcontractor EMC2 in Bozeman used a one-dimensional model to predict what would happen when the reservoir was drawn down and the dam breached.

One-dimensional modeling is a common approach that looks at down-cutting of a river’s bed, but not at lateral erosion of a river’s banks. Superfund officials said the modeling’s projections were established before the project started and were not updated to reflect changes in the project because it wasn’t considered necessary.

“Sediment transport modeling has a lot of uncertainty,” Wilcox said. “A relatively simple model was applied to a very complex problem. But the first time I walked that upper reservoir area, it was obvious to me that the sediment there was highly mobile and a lot of it would be scoured once the dam was breached.”

The UM researchers have focused on the Clark Fork’s main channel so far, but test results are pending from downstream side channels, where metals contamination likely will be higher than in the river itself, they said.

For several years, the EPA has contracted with the U.S. Geological Survey to monitor the groundwater, surface water and well water above and below the dam for suspended sediment and suspended metals.

Forba said arsenic levels slightly exceeded the project’s federal water-quality standards after the breaching, but dropped within 48 hours and have not exceeded the standards since.

David Schmetterling, a Montana Fish, Wildlife and Parks fisheries biologist who is studying the breaching’s impact, said there has been less of an impact on tagged and caged fish this spring than in previous years.

In part, the fish escaped harm because the uncaged ones could move upstream past the dam site and because the strong spring runoff lowered water temperatures and diluted copper contamination.

Doug Martin, an environmental scientist for the state’s Natural Resource Damage program, said another aerial survey of the upper reservoir will be completed in the next few weeks.

He said bank stabilization, revegetation and other measures starting next year should slow erosion in the upper reservoir, as will the state’s plan to remove more than 600,000 cubic yards of contaminated sediment from that area.