MONTREAL — The City of Montreal is standing by its plan to dump eight billion litres of untreated wastewater into the St. Lawrence River while it does construction work, saying it's the best alternative.

The one-week sewage dump is to relocate a snow chute on a tunnel that needs to be empty for workers to access it. The project is part of an ongoing plan to raze the downtown Bonaventure Expressway, which runs above it.

Mayor Denis Coderre told reporters Monday he's confident that city officials have done their due diligence, made sure that drinking water for communities downstream will not be affected and selected the best option in terms of time and cost.

Coderre suspended the plan last week, but experts maintained the temporary diversion of wastewater was the best option.

"After the re-evaluation, I decided I was satisfied with the answer," Coderre said.

However, the plan has its detractors.

More than 58,000 people had signed an online petition as of Monday afternoon against the proposed dumping.

The issue also caught the attention of some federal election candidates.

The Conservatives' Denis Lebel said he is "concerned" about the discharge of sewage into the river, adding Environment Canada is working with the city to assess the impact.

Coderre said the same type of work was done twice before — in 2003 and 2007 — without issue.

An Environment Canada spokesperson said discussions with city officials are ongoing to gather information about the potential impact, but the agency was unable to provide with The Canadian Press with an interview.

"The Fisheries Act prohibits unauthorized deposits of deleterious substances into water frequented by fish," a spokeswoman said in an email. "Under the Wastewater Systems Effluent Regulations, Environment Canada cannot authorize this type of wastewater deposit."

The provincial Environment Department has approved the plan, which is expected to start Oct. 18.

"We evaluated the situation regarding the impacts on the environment, the impact on the wildlife, the impact on people ... making sure people had clean water," Environment Minister David Heurtel said in Quebec City.

Federal Green party candidates in Quebec called the decision "ill-advised," suggesting the city could have done better.

Daniel Green says parsing the work over a longer period instead of proceeding with a one-shot deal in October could have lessened the impact, as would having the work done in February, when frigid temperatures would kill off bacteria.

"That's a lot of waste water to be discharged, in October, in the St. Lawrence River, when you have fishermen, duck hunters, surfers, kayakers using the water," Green said.

The Green party is asking Environment Canada to stop the proposed work and Coderre said if that's the case, the city would come up with a response after Oct. 18.

The mayor also dismissed concerns raised by a U.S. state senator from New York, Patty Ritchie, who wrote a letter to the International Joint Commission — whose mandate it is to protect the St. Lawrence and Great Lakes — to probe Montreal's plan.

Ritchie said Montreal's plan to dump "the equivalent of 2,600 Olympic-sized swimming pools filled with wastewater" from homes and businesses needs further study.

"While I realize that the dumping will occur in Canadian waters, downstream from any U.S. communities, I am very concerned by the precedent Montreal is setting for other communities along the St. Lawrence and the lakes," she wrote.

Coderre insists the plan is the right one.

"When you're factual, when you look at the experts, at the end of the day that was the decision to be taken because it was the only one," Coderre said.

Sidhartha Banerjee, The Canadian Press

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