Heavy runoff from a spring storm in 1975 tore through the tailings pond dam at the Mike Horse Mine on the headwaters of the Blackfoot River 15 miles northeast of Lincoln.
Mine tailings contaminated with heavy metals, including high concentrations of zinc and iron, were flushed into the upper Blackfoot.
The result, says Ron Pierce, a fisheries biologist for Montana Fish, Wildlife and Parks, "was an acute toxic event," which caused a major fish kill in the upper Blackfoot. Native westslope cutthroat trout were particularly hard hit, he says.
FWP fish surveys in the early 1970s, before the tailings dam failed, showed healthy cutthroat populations between 68 and 100 adult fish per 1,000 feet in two stretches of the upper Blackfoot directly downstream of the breached dam, according to Pierce. After the dam blew out, he says, cutthroat numbers declined drastically. And the impacts of the contamination linger today.
In FWP surveys as recent as 1999, Pierce says, cutthroat numbers in the same stretches of the upper Blackfoot ranged from about 15 fish per 1,000 feet, to zero.
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After the Mike Horse Mine tailings dam failed in 1975, a new earthen dam was constructed to contain the remaining tailings.
But a draft report released in January by U.S. Forest Service engineers indicates that the Mike Horse dam is slowly deteriorating and again could pose a threat to the Blackfoot River. The Forest Service report recommends that the dam should "eventually be taken out of service."
The report notes that "taken out of service" does not necessarily mean removal of the dam. There may be other options that meet the objective of protecting water quality, according to Jerry Meyer, information officer for the Helena National Forest.
One alternative, says Meyer, could be to divert water around the dam and remove the contaminated mine tailings after the reservoir is drained. Another possibility, he adds, could be to construct an overflow structure to prevent dam failure.
The engineering report on the stability of the dam is part of a larger effort at the site by the mine's owner, Asarco, Inc., the Forest Service, and the state of Montana to clean up historic mine wastes and water discharges, according to Meyer. Asarco is paying for the cleanup.
The Mike Horse Mine and tailings dam are part of an area of extensive historic mining activity identified by the Forest Service as the Upper Blackfoot Mining Complex. Mining began in the area with the discovery of silver, lead and zinc ores in the late 1880s. The most significant mining activity at Mike Horse, primarily for zinc, occurred in the late 1930s and early 1940s.
As part of processing ores, mill tailings were deposited in Beartrap Creek, a tributary of the Blackfoot. Eventually, a large tailings pile was developed, held in place by an impoundment. The 1975 breach of that dam washed the tailings downstream into the flood plains of Beartrap Creek and the upper Blackfoot River on national forest land.
In 1991, the Upper Blackfoot Mining Complex was identified by the state as a superfund site and slated for reclamation. In 1993, Asarco entered an agreement with the Montana Department of Environmental Quality to begin voluntary cleanup efforts at the Mike Horse site. Over the next five years, Asarco worked to reclaim mine wastes, install waste repositories, and construct a water treatment system for mine discharge waters.
The Forest Service draft report on the Mike Horse tailings dam indicates that gradual deterioration is making it less resistant to seepage. Voids were also detected in the embankment structure, suggesting internal erosion of the dam core.
The report concludes that the dam could fail as a result of an earthquake or, more likely, a flood.
Forest Service officials expect a final version of the report to be completed this winter, following peer and Montana DEQ reviews. Alternatives for addressing the dam and impounded tailings will be developed by Asarco and the Forest Service this year, according to Forest Service officials.
The report on the Mike Horse tailings dam, "is actually really encouraging," says Bruce Farling, executive director of Montana Trout Unlimited. "But there's good news and bad news. The good news is there's going to be some momentum to remove the dam and remove the (contaminated) material. The bad news is that it confirms the fact that this thing is not very stable, that it could blow out in an earthquake or a flood, and contaminate the Blackfoot again.
"The Forest Service and Asarco are coming close to an agreement that something needs to be done. Trout Unlimited has been telling them since the late '80s that that thing's got to go. This is a signal that they largely agree. It seems the most permanent, and long-term the most cost-effective thing is to get the dam out, and remove the material, just like at Milltown Dam."
Analysis of alternatives for the Mike Horse dam will examine costs, effectiveness and feasibility, as well as an analysis of risk to human health and the environment, according to Meyer. And opportunities for public involvement will be provided.
Pending a decision on the long-term disposition of the dam, Jane Kollmeyer, acting forest supervisor for the Helena National Forest, plans to step up monitoring efforts on the facility.
"Our first priority is to provide for public safety," Kollmeyer says. "The overall objective is to ensure that the tailings currently held in place by the dam do not enter downstream waters. We plan to install a monitoring system for the facility that will ensure we are receiving timely data on the status of the reservoir. We'll also be increasing the frequency of our field inspections."