AU GRES, Mich. (AP) — Small levels of toxic flourochemicals have been found in a drinking-water supply system linked to Lake Huron that serves more than 260,000 people in dozens of Michigan communities.

The Michigan Department of Environmental Quality disclosed the discovery of the substances on Wednesday in the Saginaw-Midland Municipal Water Supply Corporation system, MLive.com reported . The system draws water from two points near Au Gres.

Total toxic chemicals found in the water tested at 9.7 parts per trillion, with per- and polyfluoroalkyl substances accounting for 4 parts per trillion, according to a December letter by Sue Leeming, the department's external relations director.

The concentrations are below the Environmental Protection Agency's health advisory level of 70 parts per trillion of per- and polyfluoroalkyl substances in drinking water. Exposure to the chemicals has been linked to certain cancers, thyroid disorders, elevated cholesterol and other diseases.

Officials have found higher concentrations in other parts of the state, said Michael Quinnell, the water system's general manager.

"We're testing at parts per trillion," he said. "That's 0.000004 parts per million."

More than 50 communities in seven counties take water from the system, including Midland, Saginaw and Bay City, according to the department.

Kim Mason, the water director in Saginaw, said the discovery was concerning.

"When we read the reports in other communities, it wasn't something we thought we'd have a problem with," she said. "I can't say it's completely shocking, but it isn't something I thought would be a concern looking at the source of water we're using."

The department didn't identify a source for the chemicals. But it recommended that local officials monitor affected water systems, sample water, minimize public exposure and begin considering treatment options.

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Information from: The Grand Rapids Press:MLive.com, http://www.mlive.com