About 250,000 litres of highly volatile chemicals leaked into the St. Clair River south of Sarnia, Ont., provincial Ministry of Environment officials said Sunday.
Downstream communities were advised to shut down their water intake systems because the chemicals could make their way into the water supply.
The Ministry said communities on the U.S. side of the river, which were closer to the spill, would see traces of the chemicals Sunday afternoon but Canadian municipalities likely wouldn't see the effects until Sunday night.
"Notification came in this morning to us and we were able to notify the authorities to shut down their systems," said MOE spokesman John Steele.
Pat Spitzly, a spokeswoman with the Michigan Department of Environmental Quality, said the state began notifying area communities around 11:30 a.m. that they should close drinking water intakes.
"We were notified promptly," said Ms. Spitzly.
Methyl-ethyl ketone (MEK) and methyl-isobutyl ketone (MIBK), the chemicals released into the river, are "low-toxicity solvents," said Imperial Oil spokeswoman Kim Fox.
They are used in the manufacture of lubricating oil, but can also be found in nail polish and paint.
Ms. Fox said the company had plugged the leak and despite the seemingly high volume of chemicals in the water, Imperial considers the leak to be "small."
"On a relative scale and due to the significant dilution, it would be considered a small amount in terms of concentration, because of the size of the river itself," Ms. Fox said, adding she did not know the source of the leak.
By contrast, the Exxon Valdez tanker spewed about 41.6 million litres of oil into the Prince William Sound in 1989, killing an estimated 250,000 seabirds and thousands of marine mammals along 1,900 kilometres of shoreline.
Ms. Fox said the source of the spill was under investigation and she wasn't sure if the plant had shut down or was still running on Sunday.
Mr. Steele said that while the plume was flowing downriver, the water supplies for communities along the St. Clair were not in immediate danger.
"We are of the view that the intakes have been shut down," said Steele.
"The large systems have a minimum of two days of reservoir time, so there shouldn't be a problem.
"The material itself is flammable, etc., but the temperatures we are experiencing today shouldn't be a problem."
Accidental releases of toxic chemicals into the environment have long been an issue throughout Chemical Valley - the largest grouping of chemical and petrochemical plants in Canada.