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Submerged cables carrying power and fluid between Michigan’s two peninsulas were shut down after leaking nearly 600 gallons of coolant fluid into the Straits of Mackinac.

It remains uncertain whether the spill — first suspected on Sunday and detected on Monday — has caused environmental damage in the waterway that connects Lake Huron and Lake Michigan, said Joe Haas, district supervisor for the Michigan Department of Environmental Quality.

But department said in a statement that the spill of mineral-based synthetic oil used for insulation poses “no immediate public health threat.” Michigan’s environmental agency advised nearby towns to monitor their drinking water.

The leak was discovered after the two cables tripped offline Sunday evening, according to the American Transmission Co., the Pewaukee, Wisconsin-based utility that operates the cables. The company said it shut off power to all six submerged power lines under the Straits of Mackinac on Tuesday after reducing pressure on the system Monday to keep the loss of fuel to a minimum as personnel traced the source of the leak.

No warnings were issued for people to avoid the area, where shoreline areas remain blanketed with ice, said Lt. Rachel Wellman of the U.S. Coast Guard.

“We’ve confirmed the risk to human health is very low at the greatest,” Wellman said.

“It was an extraordinary set of circumstances, but ultimately, the decision to shut down the cables had to be made,” American Transmission Chief Operating Officer Mark Davis said in a statement. “We will continue to investigate the cause of the incident, determine any necessary remediation efforts and continue communicating with the appropriate regulatory agencies.”

The cause was unknown and it wasn’t immediately clear whether both cables were leaking, although both had sustained damage, American Transmission spokeswoman Jackie Olson said.

Crews were attempting to use hoses and a vacuum truck to pump out whatever fluid remained in the cables, Wellman said. The Coast Guard had two vessels standing by, with four mechanical skimmers and several thousand feet of boom, in case any of the material rose to the surface.

Any of the liquid that isn’t recovered or does not wash onto shoreline ice is likely to break up within a few days, Wellman said.

The leaked coolant is known to be highly toxic to at least one type of aquatic insect in the Great Lakes but is considered less of a threat to fish, Haas said.

“That’s encouraging, but it doesn’t mean we aren’t taking this very seriously,” he said. “This is obviously something we don’t want in these public trust waters.”

The American Transmission Co. owns and operates most of the electric transmission grid in Michigan’s Upper Peninsula as well as parts of Illinois, Minnesota and Wisconsin.

Olson said one of the Straits of Mackinac cables was installed in 1975 and the other in 1990. She wasn’t exactly sure how deep the cables were but said she read they could be as deep as 200 feet.

Icing in the channel and on shore hampered the company’s investigation of the leak and influenced its decision Tuesday to shut down the cables.

No customers lost electric service because power was routed from other sources, she said. Although the cables were damaged beyond repair, the company is looking into restoring an electrical connection between Michigan’s Lower Peninsula and Upper Peninsula.

“We will continue to investigate the cause of the incident, determine any necessary remediation efforts and continue communicating with the appropriate regulatory agencies,” said Mark Davis, chief operating officer.

lfleming@detroitnews.com

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