Mercury rising

Clark Fork


When two local researchers set up at the upper Clark Fork River to study how the toxic legacy of mining impacts raptor populations, they expected to find high levels of contaminants. They did, but the contaminant they discovered was a bit confounding.

The Milltown Dam is at the western edge of the upper Clark Fork Superfund complex, a 120-mile stretch of river poisoned after a 1908 flood washed millions of cubic yards of toxic mine and smelter tailings from Butte and Anaconda into the river where they piled up behind the dam.

As work on the removal and cleanup of the Milltown Dam and reservoir continues, University of Montana research professor Heiko Langer and Rob Domenech, director of the nonprofit Raptor View Research Institute, visited eight osprey nests from Deer Lodge to Missoula to test for abnormal levels of common contaminants from mining operations.

Biologists and project engineers on the Milltown cleanup are monitoring for the five most prolific contaminants—arsenic, copper, zinc, lead and cadmium—but Langer and Domenech didn’t find abnormally high levels of any of them. Instead, they discovered extremely high levels of mercury, a toxin that no one was really looking for.

“Mercury really seems to be retained in the ecosystem,” Langer says. “And it’s a big deal.”

Mercury is a neurotoxin that can cause severe damage to humans and wildlife. In humans, it’s connected to neurological disorders, cardiac disease, attention deficit disorder and autism.

To Langer and Domenech’s surprise, the mercury levels were higher in the osprey the farther downstream the nests were from Butte.

“Contrary to initial expectations, mercury levels increased as we came down the watershed by a factor of four,” Langer says.

Langer suspects that could be because there is more biota for the mercury to collect in and move through the food chain, or there could be more mercury pollution near Missoula.

“We don’t know what effects this will have on life expectancy, Langer says, “but the levels in these 2-month-old birds are already at about a tenth of the level required to get mercury poisoning.”

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