A shipping accident caused up to 200 tonnes of bunker oil to gush into the St. Lawrence Seaway and had cleanup crews working Tuesday to contain the mess.
A freighter carrying wheat ran aground the previous evening when it suffered engine failure near Montreal, said the federal St. Lawrence Seaway Management Corp.
The ship - which belongs to Canada Steamship Lines, the family company of former prime minister Paul Martin - spilled between 50 and 200 tonnes from its punctured fuel tank.
The vessel's crew deployed oil booms to limit the damage. The federal seaway management corporation sealed the lock at Cote Ste-Catherine and summoned emergency crews to add more booms.
The environmental emergency response team said a breach was fixed, but some of the bunker oil had leaked into part of the waterway.
"Given the rapid response, the spill was contained within the canal above the lock, with no oil entering the lock or moving downstream," the corporation said in a statement.
"No impact has been observed on municipal water intakes within the surrounding municipalities."
Yvan Tremblay, the emergency team's regional director, said it was unclear how much fuel poured out of the ship, the 200-metre-long Richelieu.
Two to three kilometres of shoreline were affected by the slick.
"We can't take it lightly," said Tremblay, head of the regional branch of Quebec's Urgence-Environnement. "We have to stay alert and follow through with the right operation," he said Tuesday.
Navigation was suspended in the seaway's South Shore Canal immediately after the spill, but was expected to reopen within two days.
The rest of the seaway - which extends from Montreal to the Great Lakes - remains open.
Neighbouring towns were alerted to monitor their potable water supply.
"While we regret spills of any nature, we are pleased to note our emergency response plans came together smoothly to ensure this incident was quickly dealt with, sharply limiting the impact," said Richard Corfe, head of the federal seaway corporation.