The spill of heavy fuel oil covered 500 square metres, authorities say. (CBC)

Public security officials say they believe they have contained most of the bunker oil spill that has blocked a section of the St. Lawrence Seaway since Monday night.

The fuel tank of the Canada Steamship Lines vessel Richelieu was punctured when it ran aground at around 7:30 p.m. ET Monday after losing power near the Côte-Sainte-Catherine lock, south of Montreal.

CSL keeps emergency booms aboard all its ships, and deployed them immediately after the Richelieu ran aground, said Andrew Bogora, a spokesman for the St. Lawrence Seaway Management Corp.

The breach was fixed just after 11 p.m., but an unknown amount of fuel escaped, possibly as much as 200 tonnes, according to SIMEC, a company handling the containment and cleanup. The spill initially covered approximately 500 square metres, forcing emergency crews to shut the seaway and lock as a precaution.

On Tuesday morning, the South Shore Canal and the lock remained closed while containment efforts continued around the ship, tied up on the lock's approach wall.

The cleanup was expected to take about two days, and the affected section of the seaway will remain closed during that time.

The lock acted as a funnel for the spilled fuel, explained Quebec public security services spokesman Yvan Tremblay. "And if you close the funnel, we have a very nice element that will limit any containment spread," he told CBC's French-language service.

Canada Steamship Lines said Tuesday that it will take full responsibility for the incident and the cleanup.

Cause unclear

The company cannot yet pinpoint the cause of the accident, Claude Dumais, vice-president of technical operations, told a news conference. "It's under evaluation right now, but it is a mechanical failure at this point in time, more than human error."

A warning about the spill was issued to nearby towns, including Candiac, Sainte-Catherine and La Prairie, where public health authorities will monitor the water filtration systems for any sign of fuel.

Divers examining the ship's reservoir found a hole the size of a basketball.

A helicopter surveyed the area to measure the spill of the heavy fuel oil.

The CSL vessel was carrying wheat. It's not clear how many people were on board, but the Richelieu usually operates with a crew of 23. No one was hurt.

About a dozen other ships will be affected by the canal closure.

Federal Transport Minister John Baird said he's monitoring the situation closely.

"We have our officials on hand. We'll do everything we can to assist local officials," said Baird. "Obviously there'll be an investigation. The first priority, obviously, is to contain the spill."

'Tough regulations'

Baird defended Canada's standards and regulations for maritime transportation, in light of the spill. "We have tough regulations in this country, and we'll do our level best to enforce them."

An official from the Transport Ministry emphasized the department's "polluter-pay principle."

"If any company breaks the rules, we will hold them to account," James Kusie, director of issues management and parliamentary affairs at Transport Canada, said in an email to the CBC.

"The freighter will not be authorized to navigate until further notice. A Transport Canada inspection will have to be conducted before the freighter receives proper clearance to resume its operations."

Canada Steamship Lines is the country's largest maritime company. Former prime minister Paul Martin bought CSL in 1981, and handed operations over to his sons in 2003.