Montana - Mike Horse Tailings Dam - Reclamation

State: Montana
Region: Upper Blackfoot River

The Blackfoot River is one of the most storied trout streams in western Montana.  However, a massive tailings dam looms over the river’s very origins at the Continental Divide.  In the 1940s, miners used metals-laced tailings to build the Mike Horse dam.  Their idea was that a tailings dam would contain and manage their toxic mining waste. The selected dam site was across the mouth of Beartrap Creek just above its confluence with Mike Horse Creek-- where the Blackfoot River officially begins. The shallow reservoir behind the dam became the resting place for metals-laced tailings from the Mike Horse Mine as well as other gold, copper, and zinc mines scattered around the river's headwaters. Unfortunately, these metals didn’t always stay put.  In 1975, the Mike Horse tailings dam blew out, and the mine waste became a serious polluting menace to the Blackfoot.  Deadly levels of lead, copper, and zinc dumped into the upper Blackfoot.

Today, possibly the biggest threat at the Mike Horse site is the safety of the shored-up tailings dam. The base of the dam is constructed with two million cubic yards of toxic tailings, and water is seeping out, indicating that the dam could be eroding from within. Furthermore, the dam’s spillway is not up to standards, and puts the structure at risk of overtopping in a large flood. On top of these concerns, there are threats of earthquakes in the geologically active upper Blackfoot.  We need a permanent fix for the dam, and soon.  The good news is that thanks to public comments from over 8,000 citizens and hard work by conservation organizations, the Forest Service decided to implement a complete removal of the Mike Horse Tailings Dam, selecting the option of complete removal from among its alternatives in its Engineering Evaluation and Cost Analysis (EE/CA). The dam will be dismantled, mine tailings will be moved to high and dry land and stored in a safe repository, and the headwaters of the Blackfoot will be given an opportunity to heal after a quarter-century of enduring intense pollution.

Now in 2012, the long-awaited repository decision for the defunct Mike Horse mine is in: the USFS and DEQ decided to go with “Section 35” for storing contamination that's straddling the headwaters of the Blackfoot River.  With numerous sites under study, Sec. 35 looks the best from a technical and environmental protectiveness standpoint.  That's because it's big enough, it looks to be high and dry enough, it has clean fill, and it doesn't impose a slew of unresolvable public safety issues. 
A few questions about the site's groundwater linger.  But by making a decision, the agencies now can begin a detailed investigation to ensure Sec. 35 is safe.  This is the process they followed in the first go-around when “Paymaster” initially ranked out as the best location for storing the mine’s one million cubic yards of tailings. In that case, the deeper investigation raised some red flags and regulators ultimately backed away from the site. It’s possible this could happen at Sec. 35, too, but we won’t know until scientists collect and analyze the additional data.