CLARK FORK — Mining contaminants have turned up in soil samples gathered from the site of Clark Fork Delta restoration project.
Ducks Unlimited, which is partnering with the Idaho Department of Fish & Game on the restoration effort, commissioned a sediment assessment due to historic mining activity upstream from the project.
More than a hundred samples were collected from 33 hand-auger explorations and tested for cadmium, copper, lead, mercury and zinc. Those metals are standard chemicals of concern in a regional sediment evaluation framework.
Analytic results indicated that 13 sediment samples contained one or more metals exceeding acceptable screening levels, according to GeoEngineers’ report. Those samples were collected from 10 exploration sites ranging from 1 1/2 feet to 6 1/2 feet below ground, the report said.
Kathy Cousins, a Fish & Game biologist, believes the contaminants came from upstream mining projects which predated the installation of the Cabinet Gorge and Noxon Rapids dams.
“Cabinet Gorge and Noxon Rapids are essentially sediment traps and stopped a lot of the contaminants from going further downstream,” Cousins said.
The Idaho Department of Environmental Quality does not consider the contaminants a public health threat mainly because the soils are not submerged due this winter’s deep winter drawdown of Lake Pend Oreille.
The contaminated soils pose a greater threat if they were being disturbed while submerged, which can suspend the material and cause it to move downstream, said June Bergquist, DEQ’s regional water quality compliance officer.
“This work will largely be done in the dry, so much of that concern will be eliminated,” said Bergquist.
The discovery throws a wrinkle into the $11 million restoration project, but it’s not an insurmountable one, Cousins said.
“We’re going to have to do more sampling and that’s expensive, so we’re going to have to find more money,” said Cousins.
GeoEngineers is recommending that project planners limit excavations in areas where excessive levels of metals were not detected and to place metals-impacted sediments in locations to minimize exposure risks and cover them with a cap of non-impacted sediments.
Officials from the U.S. Environmental Protection Agency and the Idaho DEQ are being invited to the restoration project’s design team meeting next month to take up the issue.