Sun-Canadian Pipe Line shut its ultra low sulphur diesel (ULSD) pipeline located one kilometre from river shore on Tuesday afternoon.
At the time, Sarnia police said no diesel fuel had got into the river. However, Sun-Canadian spokeswoman Melanie Paradis confirmed to CBC News on Wednesday that it had.
"There has been a limited release … into the river," Paradis said.
She didn't know how much of the fuel made its way into the water, nor did Sarnia Mayor Mike Bradley.
According to the Sarnia Observer, Ontario's Technical Standards and Safety Authority said "the leak buckled the roadway, and the diesel surfaced on the road and entered a sewer drain."
Fellow company spokesman Sam Vorstein said the eight-inch pipe has "a pinhole-sized" leak and that the line was closed three minutes after it was discovered by the pipeline's electronic monitoring system.
He wasn't sure how much fuel is in the water but said "even one barrel is too much."
Bradley told CBC News that Sun-Canadian told city officials and first responders it thought the diesel was contained in a drain, but it did end up in the water.
Bradley said there was a sheen on the river in the area of Stag Island on Tuesday night. That's eight kilometres downriver from the leak.
The River and its watersheds cover over 335,000 hectares and are home to more than 170 000 people, many of whom rely on the river for drinking water.
Paradis said Sun-Canadian is working with Chemical Valley Emergency Control Organization to clean up the spill.
A Sun-Canadian media release said the company had deployed booms in the St. Clair River.
"To ensure the most effective mitigation of a limited release of ULSD into the river, Sun-Canadian has initiated comprehensive water sampling to determine the extent of the release and to measure the effectiveness of the cleanup efforts," the release said.
Bradley said the Ministry of Environment is also on hand.
According to the Wallaceburg Courier Press, Wallaceburg, which is downriver of the leak, closed its water intake as a precautionary measure.
Sun-Canadian Pipe Line has been transporting refined petroleum products from Sarnia to marketing plants in London, Hamilton and the Toronto region for 60 years.