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Montreal says if sewage isn’t dumped into the St. Lawrence River, the treatment plant can be damaged and more waste will end up in the river.Paul Chiasson/The Canadian Press

Before Canada's new Environment Minister attends a high-profile climate change conference in Paris with global mucky-mucks at the end of the month, she will have to muck out a more mundane problem: What to do about Montreal's raw sewage.

Environment and Climate Change Minister Catherine McKenna faces a Monday deadline to deal with what has become known in Montreal as Flushgate: the city's plan to dump eight billion litres of raw sewage into the St. Lawrence River so it can conduct urgent repairs on about 30 kilometres of the sewer main that rings the city.

The problem was plopped into Ms. McKenna's lap by the Conservative government, which put a temporary halt to the plan after it became an issue in the recent election campaign. The Conservatives commissioned an independent scientific review, pushing back work beyond last month's election and into Ms. McKenna's fresh mandate.

Environment Canada unveiled the study's findings on Friday. The scientists largely reinforced the position of the city and other expert panels: Damage to the river and fish will be limited if the spill is conducted before winter but there is a "significant risk of a breakage" that could cause an even worse sewer system discharge if the work is put off.

The three scientists noted that little hard data exist on Montreal's occasional sewage spills and recommended the city boost water-quality monitoring during and after the spill so the impact can be assessed for future discharges. (The city's aging system will need work down the road that will require dumping more untreated sewage.)

"There is an information gap," said Caroline Blais, the director with the Environmental Stewardship Branch of Environment Canada who released the report. "The studies on what the impacts of untreated effluent are on fish do not exist."

Hard data would allow the city to look at mitigation measures such as using mobile treatment units at particularly polluting points in the system, such as hospitals. The city gave cursory consideration to using a tanker or mobile units to prevent the dump but they were dismissed as impractical on a large scale.

"They should be seriously considered for longer-term improvement in mitigating future discharges," said the report signed by academics Daniel Cyr, Robert Hausler and Viviane Yargeau. But "it is essential that the current monitoring plan be improved."