Monitoring of the stability of an impoundment dam at the Mike Horse Mine near Lincoln is being stepped up after recent engineering studies showed that the dam is slowly deteriorating.

Officials with the Helena National Forest said there's no imminent threat of the dam failing, and even if it did there would be little threat to human life, since there aren't any homes in the area.

Still, they want to get a grasp on how fast it's eroding and put some measures in place to limit the potential of a dam failure and its subsequent environmental damage.

"We are still in the part of the process where we are continuing our evaluation," said Jerry Meyer, public information officer for the Helena National Forest. "The Forest Service engineers use terms like degraded, but we don't know the degree of compromise.

"As we move further into this and refine the study, we'll get a clearer picture of the dam and know the kind of time frames we're talking about" in relation to how fast it's deteriorating, Meyer added. "That will dictate the sense of urgency."

He noted that the Forest Service and other groups all want to ensure that the tailings impoundment doesn't suffer a blowout similar to what happened in 1975, when water from a large storm flushed deadly levels of lead, copper and zinc into the upper Blackfoot River, killing thousands of fish.

Bob Kirkpatrick, regional environmental engineer for the Forest Service, said that since they're not sure of the rate of deterioration, they plan to install a water flow monitoring device on the dam, and also will increase the frequency of their field inspections.

"Depending on what additional information becomes available, there may be some things we need to take action on to minimize the risks," Kirkpatrick said. "But the first thing is stepping up our monitoring, to make sure debris doesn't plug any outlets.

"... As long as we maintain the structure and flow through the outlets, we should be OK. That's what folks talked about as the main risk."

The draft report of the dam's stability also notes that the dam should eventually be taken out of service.

Whether that entails removing the structure, which involves somewhere around 500,000 cubic yards of material, or just diverting water around the dam, is an issue that needs further study, Kirkpatrick said.

Conservation and environmental groups have long questioned the stability of the dam. The Clark Fork Coalition notes on its Web site that water is seeping through the base of the dam, raising the possibility that it could be eroding from within. The dam's spillway isn't up to standards, and puts the structure at risk in a large flood. In addition, it's not clear whether the dam could withstand an earthquake.

Montana Trout Unlimited also has advocated removal of the impoundment, noting that a long-term solution is needed to ensure water quality in the Blackfoot River.

The original Mike Horse dam was constructed in the 1940s with metals-laced mill tailings being disposed of in Beartrap Creek, which eventually created a large mound of railings held in place by a wooden wall. The dam runs across the mouth of Beartrap Creek, just above Mike Horse Creek, both of which flow into the Blackfoot River.

After the dam failure in 1975, Asarco -- which owns most of the land on which the Mike Horse mine sits -- rebuilt the dam, which is atop Forest Service property. Some reclamation has been done at the Blackfoot's headwaters in a piecemeal fashion, but Asarco and the Forest Service, along with the state Department of Environmental Quality, in November released a "Draft Alternative Technical Memorandum" that they hope will help create a plan to put an end to the pollution by removing, diluting or capping in place most of the upstream contaminants.

That plan is to be paired with this engineering study on how to deal with the dam impoundment. Kirkpatrick said the draft report is currently undergoing peer review, and a final copy should be available to the public by the end of the month.

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