The St. Lawrence River could reek like sewage for a week, starting on Oct. 18.

That's when the City of Montreal plans to start dumping a colossal amount of untreated wastewater directly into the river from a major sewer interceptor for a seven-day stretch.

The amount dumped will be 13 cubic metres per second over seven days: that's eight million cubic metres in all, or eight billion litres — the equivalent of 2,600 Olympic-sized pools of raw sewage, from homes, industries and hospitals.

Up until the 1980s, it was common practice to clear the sewers this way, although it is no longer considered acceptable.

Tied to Bonaventure demolition work

It's been six years since the city last had to dump untreated waste directly into the river, City of Montreal spokesman Philippe Sabourin said.

"There is no other possible choice," Sabourin said. He said the work is tied into the demolition of the Bonaventure Expressway.

The river has a huge dilution capacity. It isn't a major environmental concern. - Philippe Sabourin , City of Montreal

A major snow-dumping area under the expressway must be moved. The existing snow dump feeds into a major sewer interceptor, and in order to modify that interceptor, it – and other sewer lines flowing into it – must be cleared.

The interceptor, located under Mill St. between Riverside and Bridge streets, is one of two on the island of Montreal. It collects the wastewater for the Southwest borough, LaSalle and Lachine, as well as parts of Verdun and Notre-Dame-de-Grâce.

River flows quickly, diluting pollution

"The river has a huge dilution capacity, with a flow rate of 6,000 to 7,000 cubic metres per second," said Sabourin. "It isn't a major environmental concern."

However, Sabourin did acknowledge the plan does pose a problem for sports fishermen, kayakers and surfers, especially those who carry out their recreational activities behind Habitat 67.

He said the city is to post information about the planned dump along the shoreline and conduct "targeted communications."

The City of Montreal said it has obtained the authorization of the provincial environment ministry to clear the sewers.

Find another solution, demands opposition

Wouldn't it be possible to pump the wastewater into another sewer line temporarily? - Sylvain Ouellet , Projet Montréal

"It's hard to believe there is no other mitigation measure that could be put in place to avoid untreated wastewater from being dumped directly into the river for a week," said Sylvain Ouellet, Projet Montréal's spokesman on environmental matters.

"Wouldn't it be possible to pump the wastewater into another sewer line temporarily, to avoid polluting the river?" Ouellet asked.

"Isn't it possible to come up with a portable water treatment method, some way of screening?" he asked. "Is it not possible to ask industries in the area to temporarily stop putting noxious materials into the sewer system?"

We cannot ask residents to stop using their bathrooms, said Sabourin. There would have to be portable toilets all over the place.