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Detroit — The Marathon oil refinery in southwest Detroit acknowledged it was the source of a "rotten egg" odor reported over the weekend.

Refinery officials said it had not detected any dangerous levels of substances. The Michigan Department of Environmental Quality identified the source of the odor the combustion of hydrogen sulfide and mercaptan compounds.

"Both of these compounds have extremely low odor thresholds, meaning they are detected when very small amounts are present," said Mary Ann Dolehanty, division director of the Air Quality Division of the MDEQ. 

Michigan State Police began receiving reports of the odors around 4 a.m. 

Marathon Petroleum Corp., owner of site since 1959, said in a statement that the source of the odor was a nonfunctioning flare at the refinery.

Flares "are safety devices that allow (Marathon) to safely combust excess materials at the refinery," the company said.  It is working to deactivate the flare "as quickly and safely as possible."

Marathon said it was monitoring air in affected areas and hoped to complete repairs by Monday night. 

"Although there is an odor, we have not detected dangerous levels of any substances," the company said, adding that it was making air-monitoring data available to regulatory agencies and emergency responders.

"We apologize to the community for the odor and the inconvenience," Marathon said. The company said an investigation was ongoing.

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Steve Considine, a National Weather Service meteorologist, said that winds were blowing from the west on Sunday, about five to 15 mph hour. Odors would travel north and northeast of the source.

The Michigan Department of Environmental Quality had air-quality staff at the site Saturday and Sunday, said Chelsea Lewis, a spokeswoman for the agency.

MDEQ said there were "no health risks to residents" identified and that it will continue to advise Marathon in its efforts to resolve the odor issue.

"Marathon has informed the DEQ that they are taking steps to mitigate the odor caused by hydrogen sulfide and mercaptan compounds being combusted by the flare," said Dolehanty in a statement Sundayy.

Marathon is looking at short-term options to neutralize the odor-causing compounds in the gas being flared and is trying to safely take the flare system out of service, which could take days, Dolehanty said. 

"Both of these compounds have a strong rotten-egg smell that can be noticed at extremely low levels," she said. "EPA conducted ambient monitoring in the downwind neighborhood, including Oakwood Heights, and did not detect elevated levels of concern."

The state is coordinating with the Environment Protection Agency and the city of Detroit on air monitoring.

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