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“Vessel activity” may have caused a mineral oil leak from utility lines in the Straits of Mackinac — a spill that has prompted renewed calls for alternatives to the transmission and pipe lines running beneath the straits.

The U.S. Coast Guard has opened a marine casualty investigation related to the spill that occurred early this week, said agency spokeswoman Pamela Manns, who declined to release any additional information.

“We have some evidence that it might have been a cause, and we’re going to conduct an investigation,” Manns said.

The Coast Guard has received no reports of spills from other companies, but the agency is asking all entities with lines in the straits between Lakes Michigan and Huron to check for potential damage to their infrastructure, she said.

Submerged cables belonging to the Wisconsin-based American Transmission Co. had leaked nearly 600 gallons of coolant fluid into the straits by the time the leak was detected early this week. The cables carried power and fluid between the state’s two peninsulas.

Macomb County Public Works Commissioner Candice Miller said the combination of the coolant leak and a state-approved increase in Nestlé’s groundwater withdrawals made for a “terrible week for our Great Lakes.”

Officials need a plan to deliver energy in a safer manner than “throwing a pipe under the water,” said Miller, who is also a member of the Great Lakes Commission.

“We just keep fingers crossed that nothing happens,” Miller said in a statement. “That is not a plan. This week’s leak from the transmission lines under the Straits of Mackinac was as predictable as it was disheartening.”

For James Clift, policy director at the Michigan Environmental Council, the spill re-emphasized the need for alternatives to submerged transmission lines.

“I am guessing that in some way, shape or form there is a feasible and prudent alternative and they need to explore it,” Clift said.

The American Transmission Co. discovered the leak after the two cables tripped offline Sunday evening, according to the Pewaukee, Wisconsin-based utility. 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.

American Transmission spokeswoman Jackie Olson said the company has not yet been able to see the extent of the damage to the lines because of icy weather conditions.

The Upper Peninsula electric transmission grid, mostly owned and operated by American Transmission, continues to operate normally. Olson said the company is exploring ways to get at least some of the circuits beneath the Straits back online.

“They’ve been testing the undamaged lines and have been looking at ways they can reconfigure the three operable cables to make a circuit that would create a power line between the Upper Peninsula and Lower Michigan,” Olson said.

Officials with the Coast Guard and Michigan Department of Environmental Quality have yet to see any sheen on the water or evidence of pollution resulting from the leak, Manns said. The leak has not resulted in any noticeable effects on wildlife in the area.

Vacuum trucks on land will continue to remove the remaining fluid from the line. The company isolated an estimated 800 gallons and had removed about 200 of those by Saturday afternoon.

“Given the amount that has spilled and the dilution that does occur naturally…we haven’t seen any indications of pollution,” she said. “But it doesn’t mean we don’t take it seriously.”

Attorney General Bill Schuette on Wednesday applauded the DEQ’s decision to require American Transmission to address the remaining coolant in the cables.

“The remaining American Transmission Co. cables should not be re-activated until there is a thorough and complete DEQ review of what caused the leakage of coolant into the Straits of Mackinac,” Schuette said in a statement.

The Coast Guard and ATC will continue to assess the situation through the weekend via monitoring vessels and flyovers, Manns said.

The Michigan Department of Environmental Quality has said the spill of mineral oil used for insulation posed “no immediate public health threat,” but nearby community leaders were notified of the issue.

The mineral leak occurs amid concerns from environmentalists regarding Enbridge Energy’s Line 5, the pipeline that carries about 23 million gallons of oil and liquid natural gas along the bottom of the Straits of Mackinac.

Analysts contracted by the State of Michigan in 2017 found that the most dominant threat to the Line 5 pipeline is “anchor hooking” from ships that inadvertently deployed their anchors while passing through the straits.

Researchers estimated a Line 5 spill could affect roughly 20 miles of Great Lakes shoreline, primarily affecting Cheboygan, Emmet and Mackinac counties.

In November, Gov. Rick Snyder reached an agreement with Enbridge requiring the replacement of a section of Line 5 under the St. Clair River and a study exploring the possibility of replacing the entire pipeline with a new one in a tunnel. Enbridge is supposed to have evaluated three options for eventually replacing Line 5 by June.

eleblanc@detroitnews.com

(517) 371-3661

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