In response to requests from the Idaho Conservation League, the federal and state agencies monitoring the Thompson Creek Mine near Clayton say the tailings pond dam there is reasonably safe from the effects of a large earthquake.

The conservation organization sent a letter to the agencies after a 6.5 magnitude quake rocked the area on March 31, noting that the sand dam at the mine site is engineered to withstand a quake of just that magnitude, and that at magnitude 6.9, the Mount Borah earthquake in 1983 was even stronger.

The ICL told the agencies that if the sand dam were to liquefy and breach, it could release tens of millions of tons of toxic sediment into the nearby creeks, which empty into the Salmon River 13 miles downstream from Sunbeam.

An April 23 letter drafted by the Idaho Department of Water Resources, the BLM’s Idaho Falls District and the Salmon-Challis National Forest addressed the ICL’s concerns.

The letter stated that “liquefaction is associated with loose, saturated, well sorted silt and fine grain-size soils.”

“Although any embankment may exhibit some damage if subjected to strong lateral forces and/or long duration shaking,” the letter said, the coarse, dry sand of the Thompson Creek Mine’s tailings dam won’t liquify like silt or fine sand when shaken.

The letter noted that an inspection of the mine site was carried out on the morning after the March 31 earthquake.

“As reported, the results of that inspection revealed no discernible evidence of the earthquake and no discernible change in discharge rate at the toe of the dam,” the letter stated.

The letter also stated that because the quake occurred about the same time as a heavy snowstorm, “[a] complete independent inspection of the tailings dam will be conducted as soon as the winter snow cover has receded,” adding that the report will be a matter of public record.

The ICL had also advocated review of an existing emergency action plan for the facility and updating it if needed. The agencies’ letter stated that the mine operator, Toronto-based Centerra Gold, has added a comprehensive review of the plan to the agenda for the next meeting of the Interagency Task Force, a body created to coordinate mining plan reviews and oversight.

According to the agencies’ letter, the Idaho Department of Water Resources has asked the mine operator to consider a review of the design of the tailings pond dam to ensure that it is valid for a quake of similar magnitude as the recent one whose epicenter is as close as 10 kilometers from the dam.

“Instead of this being a discretionary action, we believe that such a design review would be a reasonable requirement coming from the state and federal agencies to the mine owner given the changed circumstance of a recent earthquake of that magnitude roughly 30 miles from the mine site,” the ICL stated in a letter dated April 27.

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