BONNER, Mont. (AP) -- High levels of toxic chemicals have been discovered anew at a lumber mill site near Missoula that is attracting new tenants after undergoing a cleanup last year, Montana regulators said.

The soil around the former Stimson Lumber Mill on the banks of the Blackfoot River in Bonner contains high concentrations of polychlorinated biphenyls, or PCBs, a toxic mixture of chemicals that has been linked to cancer, the state Department of Environmental Quality said Tuesday.

The levels detected by one monitoring well 50 feet from the site found PCB levels above 6,000 parts per million. The agency said an acceptable level is 0.74 parts per million.

"That's the most concentrated PCBs we've ever seen here," Keith Large, DEQ's project officer at the site, told the Missoulian ( ).

The DEQ said the contamination does not appear to be threatening the Blackfoot River.

State and federal regulators have ordered Stimson Lumber Co. to conduct additional sampling to determine the extent of the contamination, which has been completed. Results are expected in March.

Regulators will work with the company on a remediation plan.

"Thank goodness we sampled and found the problem. Our number one purpose is to protect human health and the environment," DEQ Director Richard Opper said in a statement.

Stimson, he added, "has acted responsibly in the cleanup process so far and we expect they will continue in that manner."

Stimson Lumber ran a mill at the site until 2008. A local contractor overseen by the state conducted a cleanup of the site last year, disposing of PCB contamination and reclaiming the river embankment.

The company sold the site to Bonner Property Development LLC in December, and the Missoulian reports that three businesses are currently in operation there or planning to open.

Those businesses are far away from the newly found contamination and shouldn't be affected, said Keith Large, DEQ's project officer at the site.

"I'm sure there will be more digging once it's fully understood how big the problem is," Large said. "I imagine this will be ongoing through 2012."

Copyright 2012 The AP.

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