Rainwater collects many potentially toxic products from city streets before being discharged into waterways. The phenomenon deserves more attention, according to researchers at McGill University.
Posted at 6:00 a.m.
It was with the COVID-19 pandemic that Mathieu Lapointe began to take an interest in stormwater contamination. “I was walking on Sainte-Catherine Street and I saw all these degrading intervention masks lying on the edge of the sidewalk,” says the postdoctoral researcher in water treatment at McGill University. I started to think about all that these masks were throwing back into the environment. »
During showers, water runs down the streets, carrying waste and other pollutants to the sewers. “In the past, we considered that this runoff water was relatively clean,” explains the researcher. Intuitively, it’s just rainwater, after all. But we realized that this is not always true. »
PHOTO PHILIPPE BOIVIN, THE PRESS
Mathieu Lapointe, postdoctoral researcher from McGill University studying water treatment
Masks are just the tip of the iceberg. Mathieu Lapointe and his colleagues became aware of the extent of the chemical cocktail transported by runoff water: hydrocarbons in oil stains left by vehicles, microplastics in masks and waste, phosphorus, tire residue, bacteria, cigarette butts, etc.
Modern sewage systems usually transport this contaminated water into waterways without prior treatment. In a recent article in Nature Sustainabilityresearchers are sounding the alarm.
A chemical compound such as 6PPD-Q, used in the manufacture of tires, can exceed up to 20 times the threshold tolerated in waterways in urban areas. PAHs, molecules released during the combustion of automobile fuel, can have a concentration twice as high as the lethal dose after precipitation.
Eventually, aquatic organisms will absorb and metabolize these contaminants.
Mathieu Lapointe, postdoctoral researcher in water treatment at McGill University
In some aquatic species such as salmon, there is even a mortality syndrome caused by stormwater runoff.
“It’s a problem that we already knew in part, remarks Mathieu Lapointe. But it has been exacerbated with population density and climate change which sometimes generates more rain. »
Areas particularly at risk are those that are densely populated, with small surrounding streams. In large bodies of water, such as the St. Lawrence, pollutants are more diluted.
PHOTO PHILIPPE BOIVIN, THE PRESS
Runoff water carries many contaminants to waterways: hydrocarbons, microplastics, phosphorus, bacteria, etc.
The contamination of runoff water in Quebec should still be taken seriously, according to the postdoctoral researcher. “The problem is that we lack data on these contaminants. Moreover, if we look just north of Montreal, in the Rivière des Prairies, the flow is lower and the dilution factor is lower compared to the St. Lawrence River. »
No perfect system
In the combined sewer systems, which cover approximately two-thirds of the island of Montreal, stormwater is mixed with domestic and industrial wastewater. They are treated together in a treatment plant. In the other third of the island, rainwater is not treated before it is discharged into waterways.
However, single networks bring their share of problems, notes environmental chemistry professor Sébastien Sauvé. “If it rains a lot, the treatment plant is not able to manage everything. So it overflows everywhere in the network. The toxic broth of sewage and runoff is then released into the environment.
Solutions ?
For Sébastien Sauvé, the problem must be solved at the source. “The best thing is to have these considerations upstream in urban development plans,” he says.
PHOTO EDOUARD PLANTE-FRÉCHETTE, LA PRESSE ARCHIVES
Green roofs are one of the means of retaining rainwater and limiting its runoff.
Solutions exist. Retention basins, for example, make it possible to unclog sewer networks. In these vast underground or open reservoirs, rainwater accumulates. Suspended particles settle naturally in the bottom of the basins, which treats the water, explains Mathieu Lapointe.
More broadly, green spaces are real sponges in cities. They allow rainfall to nourish vegetation rather than run off onto the streets.
“There are many passive and simple solutions to implement: green roofs, green pavers or any greenery that can replace concrete,” insists Nathalie Tufenkji, professor of chemical engineering at McGill University.
The latter collaborates with Mathieu Lapointe and Professor Chelsea Rochman of the University of Toronto for the study of runoff water. They are now working to better quantify the contaminants released via these waters.
“The first message that we must understand is that this is a real problem that must be studied, concludes Nathalie Tufenkji. Quebec could be a leader in green solutions for water management. »