Following a recent strong earthquake in the mountains northwest of Stanley, the Idaho Conservation League is asking federal and state land-management agencies to reassess the potential for collapse of a 676-foot-tall tailings pond dam at the Thompson Creek Mine, which is just north of the Salmon River near the small town of Clayton. Thompson Creek enters the Salmon River about 13 miles downstream from Sunbeam.
According to the BLM, the sand dam is engineered to withstand an earthquake of about magnitude 6.5—just the power of the March 31 quake—as close as 10 kilometers away.
Though the epicenter of that earthquake was about 32 miles west of the mine site, the facility sits near the center of the Central Idaho Seismic Zone, a seismically active area that includes the site of the 6.9-magnitude Mount Borah earthquake in 1983.
A BLM analysis determined that the probability of a quake exceeding magnitude 6.5 is 0.02 percent in any given year.
“Even if the likelihood of a catastrophic failure may be relatively low, the human and environmental costs of a collapse would be extremely high,” the ICL stated in a letter dated April 8. “If the sand dam were to liquefy and breach during a large-scale seismic event, it could release tens of millions of tons of toxic sediment into Bruno Creek, Squaw Creek and the Salmon River (6 miles away), putting human life at risk.”
The ICL contended that tailings could smother the creeks and the critical habitat they provide for endangered salmon, steelhead and bull trout.
The Thompson Creek Mine is one of the largest molybdenum mines in the world, spanning 3,000 acres on mostly private land, though it includes some BLM and national forest land. Mine production began in November 1983, and since then, in addition to the ore produced, over 100 million tons of processed mine waste has been generated at the site. The sand-sized waste materials after the ore is extracted are called tailings. Tailings are piped in a slurry from the mill to the tailings impoundment.
According to the ICL, while Thompson Creek is not currently actively mining on site, the mill continues to process ore from other mines. The mining company did not return a call from the Idaho Mountain Express seeking information on its current operations by press deadline.
An Interagency Task Force has been created to coordinate plan reviews and oversight of the mine. The ICL’s letter was sent to the BLM’s Idaho Falls District, the Salmon-Challis National Forest, the Idaho Department of Lands and the Idaho Department of Water Resources.
The organization suggested that the agencies, in concert with mine owner Centerra Gold, a company headquartered in Toronto, inspect the dam to assess damage from the earthquake as well review its stability, and make those results public in a report. It also advocated review of an existing emergency action plan for the facility and updating it if needed.
In an April 8 response to the ICL, Mick Thomas, administrator of the Idaho Department of Lands’ Minerals, Public Trust, Oil & Gas Division, said the Interagency Task Force was “taking steps to coordinate a response to your letter while complying with current health protocols.” Thomas said the Task Force members would schedule a telephonic meeting in the near future and would share the outcome of that meeting as soon as possible.
“Safety continues to be the primary focus of the Interagency Task Force and we appreciate the points you discussed,” Thomas said.