A panel of scientists asked to review Montreal's controversial plan to dump eight billion litres of raw sewage into the St. Lawrence River has concluded that the city should only proceed with the release once steps are taken to better understand and mitigate the environmental impacts.

The panel has raised a number of questions about the plan, pointing out gaps in information about the environmental effects and the need to address them in the public interest.

But the scientists said the city's plan is preferable to an accidental release of waste water caused by Montreal's decaying sewer system.

The release, should it proceed, would allow Montreal to work on a sewer interceptor that has deteriorated.

The panel noted that the planned release would be carried out at a time of year when fish aren't spawning, thereby minimizing a key environmental concern.

It agreed that the fall is the best time for such a move.

The report recommended improvements to the city's plan, including better monitoring of the discharge plume beyond the Island of Montreal, the implementation of an emergency plan for cleaning affected areas, particularly near the Sorel Islands northeast of Montreal, and a mobile treatment unit for use at sites such as hospitals or industries where potentially toxic discharges could enter the sewer system.

In a news release, the City of Montreal said it would study the panel's recommendations to determine which are priorities and which can be acted on at a later date.

The decision to approve or deny the plan now rests with federal Environment Minister Catherine McKenna.

Montreal's plan has already been approved by Quebec's Environment Ministry, and the city says there are no alternatives that would allow it to carry out the required maintenance.

Conclusions questioned

Sarah Dorner, an associate professor in the department of civil, geological and mining engineering at École Polytechnique, said she agrees with the report's assertion that a planned dump is the preferable option.

From a civil engineer's perspective, the solutions they're proposing just don't make technical sense. – Sarah Dorner, École Polytechnique

She questioned, however, whether the panel had the right mix of experts to address certain critical questions.

"They didn't have any construction engineers, any civil engineers — these were chemical engineers," she said. "We had chemical engineers asked to solve civil engineering problems.

"From a civil engineer's perspective, the solutions they're proposing just don't make technical sense. For a lot of these mitigation measures, you need people who know about pipes and pumps, because that's what this is really about."

Plan on hold until Nov. 9

Montreal announced its plan to dump the raw sewage in September, saying it was necessary in order to divert effluent temporarily from a major sewer interceptor as part of the Bonaventure Expressway rebuilding project.

The plan called for the sewage to be dumped at the end of October to minimize possible effects on the river's ecosystem.

The announcement sparked a public outcry, prompting former Conservative environment minister Leona Aglukkaq to intervene and halt the plan while an independent scientific assessment of the sewage dump was conducted.

A ministerial order that Aglukkaq issued forbids Montreal from proceeding with the sewage dump until Nov. 9.