The Mike Horse Dam is coming out.

A decision by Regional Forester Tom Tidwell, which was announced Tuesday, means that at least 370,000 cubic yards of mill tailings containing lead, zinc, copper, cadmium, aluminum, manganese and arsenic in and around the unstable dam — formally known as an impoundment — must be removed and encapsulated.

The earthen impoundment, which reportedly includes a car chassis in it, is about 25 feet wide, 500 feet long, and 45 to 60 feet tall. The materials from in and around the dam will be placed in the existing Paymaster repository, about 1.5 miles northwest of the dam on private property owned by Asarco, LLC.

The impoundment and nearby mining sites are part of the Upper Blackfoot Mining Complex, 16 miles east of Lincoln and about 50 miles northwest of Helena.

Originally constructed in 1941, the impoundment is poised at the headwaters of the Blackfoot River, made famous in the Norman MacLean novel “A River Runs Through It.” The dam blew out in 1975 and the contaminated water and sediments ended up killing all aquatic wildlife in a 10-mile stretch of the Blackfoot River.

The dam was rebuilt using nearby materials that included mine tailings. Two years ago questions arose regarding its stability, and a recent report noted large holes, or “voids” in the structure. The Forest Service reported that the dam was a compromised structure that should be removed from service.

News of its upcoming removal was heralded as a critical victory by both of Montana’s senators, as well as conservation groups.

“It’s the right decision,” said Montana’s senior Sen. Max Baucus. “This is a big win for the Blackfoot River and for all those who work and recreate in the area.”

Matt Clifford, conservation director and staff attorney for the Clark Fork Coalition, added that the decision for full removal comes on the heels of 8,000 comments sent to the National Forest.

“A lot of options were floating around, from leaving part of it in place to full removal, and this is what we wanted, so it no longer poses a threat to the Blackfoot River,” Clifford said. “This is happening because over 8,000 citizens wrote to the Forest Service and asked to get rid of this threat to the Blackfoot River.”

Stabilizing the dam in place was not an option, according to Forest Service officials, because it would have to be monitored and maintained for at least 30 years, and that wouldn’t have addressed the metal contamination problems in water flowing at up to 500 gallons per minute through the impoundment. In addition, an emergency response plan would have to be in place and functioning for the next three decades.

The Forest Service is now pursuing funding for the estimated $26.7 million removal action from the responsible parties. In the past, efforts have focused on Asarco footing the bulk of the bill, but the Atlantic Richfield Company also has been named as a potentially responsible party.

Asarco owns the Mike Horse Mine, which is slightly southwest of the dam and allegedly contributed to the contamination in and around the impoundment. The land on which the Mike Horse Dam sits is managed by the Helena National Forest’s Lincoln Ranger District.

After the original Mike Horse Dam failed, it was repaired by the Anaconda Mining Co., which was bought by ARCO in 1977.

ARCO officials couldn’t be reached for comment Tuesday.

Asarco filed for Chapter 11 bankruptcy protection in 2005. Tom Aldrich, vice president of environmental affairs for Asarco, said on Tuesday that the issue of financial liability is part of the bankruptcy litigation.

“As such, the company is not allowed to comment on the Forest Service’s decision,” Aldrich said.

But in documents filed with the bankruptcy court, Asarco disputed its burden at the Mike Horse impoundment.

“Asarco contends that the alleged harms addressed by these costs are divisible and/or should be estimated on an allocable share basis. The harms allegedly associated with the Mike Horse tailings dam and impoundment resulted primarily from a breach of the tailings dam in 1975 at a time when Asarco did not own or operate the dam,” the company wrote, adding that the federal government also is a potentially responsible party since part of it is on public lands.

The company adds that complete removal of the dam “would not be cost effective and would present substantial practical difficulties associated with removal and off-site disposal of a large volume of tailings.”

Clifford scoffs at the argument, saying it’s just Asarco’s lawyers “throwing everything they can” at the plan to find a way out for their client.

“At the end of the day, Asarco and its predecessors put the tailings right across the river because it was the most convenient thing to do,” Clifford said. “Clearly, they’re liable.”

The Upper Blackfoot Mining Complex includes more than 50 acres that was mined intermittently for silver, lead and zinc from 1890 to the 1950s.

“This is a critically important, once-and-for-all decision for Montana,” said Sen. Jon Tester. “It’s the right thing to do for the Blackfoot, and its residents, and for all Montanans who value hunting and fishing Montana’s wild places the way they’re meant to be.”

Bob Kirkpatrick, group leader in the Forest Service’s regional headquarters in Missoula, said they’re getting estimates for rerouting Beartrap Creek, which will allow the pool of water to be drained from behind the Mike Horse dam. He hopes that work will be completed this fall.

Removing the impoundment and associated contaminated soils hinges on getting through bankruptcy court proceedings and bringing ARCO to the table, Kirkpatrick added. He expects the entire project will take years to complete.

“We’re negotiating through that and hope to do it this winter,” he said. “By the time spring snow melts … we should be in a position to start initial actions at the impoundment.”

Reporter Eve Byron can be reached at 447-4076 or at eve.byron@helenair.com.