Quebec study finds 'cocktail of different pesticides' in treated tap water

CBC

13-05-2025

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A recent Quebec study shows that a multitude of pesticides can be found in tap water, even after being treated at a water treatment plant.
Sébastien Sauvé, an environmental chemistry professor at Université de Montréal, supervised the study that was published in the June issue of the scientific journal Water Research.
For more than two years, from 2021 to 2023, researchers collected water samples twice a week from the Châteauguay River in Quebec's Montérégie region.
The samples were collected from the river as water was being pumped into a water treatment plant and were compared to treated water samples leaving the plant for distribution as drinking water.
The river was chosen because it runs through areas of intensive agriculture activity, where pesticides used by farmers eventually make their way from the fields into waterways.
The objective, Sauvé said, was to look at how pesticide concentrations in the water varied over time, as well as before and after processing at a water treatment plant.
Highest concentrations of pesticides occurred in June and July at the beginning of growing season, and while none of the samples collected exceeded Quebec guidelines for safe drinking water, researchers discovered that the treatment process did not effectively filter out pesticides.
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Scientists are warning that they have found traces of pesticides in the drinking water of municipalities along the Châteauguay river. While the levels found complied with Quebec regulations, the study's authors recommend authorities make changes.
Traces of at least 50 different kinds of pesticides and metabolites were found in tap water, in some cases in higher concentrations than the source water. Metabolites are molecules that result from the degradation or breakdown of pesticides.
"There is quite a bit," Sauvé said, adding some pesticides were found in almost all the samples, while others weren't detected as often.
"One of the main ones, or the highest concentration, was for glyphosate," Sauvé said, which is the main active ingredient used in many herbicide formulations.
It is used to kill weeds and is commonly sold under the brand name Roundup.
In Quebec, many municipalities, including Montreal, have banned its sale and use for domestic purposes over concerns for the environment and possible impact on human health.
In 2015, the World Health Organization's International Agency for Research on Cancer (IARC) concluded that glyphosate was probably carcinogenic to humans.
Health Canada, however, says it does not cause any harmful effects if used properly.
Cocktail effect a concern
More than the impact of any specific pesticide, however, Sauvé said he's worried about their combined effect.
"We're getting a cocktail of different pesticides and all together, they add up," he said.
How the compounds react together can affect toxicity, he said, explaining they can add up, partially neutralize each other or even multiply.
"In a situation like this, with about 50 different pesticides and metabolites that we've measured, we cannot know what the actual combination effect of the toxicity of all these compounds are, so there's a lot of unknown," Sauvé said.
Maryse Bouchard, a professor of environmental health at the Institut national de la recherche scientifique, told Radio-Canada that the study highlights the need to better assess the risks of the "cocktail effect."
"It's disturbing to see the quantity of different pesticide molecules, many with proven toxicity," she said.
Possible solutions
While Sauvé said there was no reason for consumers of tap water to panic, the findings do raise some concerns.
"It's definitely not a case for panic. It's a case for asking questions, asking the local authorities, 'can we improve our system?'" he said.
What he'd like to see are stricter guidelines similar to those used in Europe. Specifically, he'd like to see an upper limit on the total concentration of combined pesticides allowed in tap water.
"And then, the cities that are exposed to those levels would simply have to improve their water treatment," Sauvé said, adding it should be on a case-by-case basis.
Sarah Dorner is a professor in the department of civil, geological and mining engineering at Polytechnique Montréal and industrial co-chair on drinking water at Polytechnique.
She said having stricter standards for drinking water could help, but said it's important to note that 20 per cent of exposure to pesticides is assumed to come from drinking water, and 80 per cent through other sources such as food.
In her view, upgrading water treatment plants can be expensive and would only solve part of the problem.
"It'll solve the problem for drinking water," she said, but not other problems linked to biodiversity and ecosystem health.
A better approach, she said, might be to reduce the application of pesticides at the source and work with agricultural communities who use them to get a better understanding of how they are used and what can be done.
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In a written statement to CBC, Mélina Jalbert, a spokesperson for Quebec's Environment Ministry, said it collaborates with several government partners, including Health Canada and the Institut national de santé publique du Québec, to ensure the quality of drinking water in Quebec.
Furthermore, Jalbert said the ministry checks pesticide levels in the water around agricultural lands every year.
"It should be noted that the latest pesticide assessment shows a decrease in the agricultural sector and that we are continuing our efforts to reduce their use," she wrote.
Sauvé acknowledged the government does annual checks, but said they might not be taking measurements when levels of pesticides peak.