Pesticides and by-products in Montréal's tap water

What the City measures at the tap

Montréal's water is drawn from the St. Lawrence and Lake Saint-Louis, treated at the Atwater and Charles-J.-Des Baillets plants and distributed to Montréal, Montréal-Ouest, Mont-Royal and, since 2025, Montréal-Est. Every spring the water department publishes an annual summary as a PDF: for each parameter, the year's minimum, mean and maximum, or the highest value measured in the network for organic compounds. There is no open dataset; MTL Stats read the figures from the ten reports, 2016 to 2025, and plots them here as one value per year.

Fifteen pesticides are tested in the network: glyphosate, atrazine, cyanazine, metribuzin, simazine, 2,4-D, dicamba, metolachlor, chlorpyrifos, diazinon, diuron, malathion, bromoxynil, diquat and paraquat. The five that matter most for the region, and are also tracked in the farm rivers, get their own curve; the counter gives the number of pesticides detected in the year, all included. Trihalomethanes and haloacetic acids, formed by chlorination, and nitrates complete the page.

What the independent 2026 test found

In June 2026, Vigilance OGM had the tap water of thirty cities analysed by the environmental analysis laboratory of the Université de Montréal (Sébastien Sauvé's team), with a method a thousand to a hundred thousand times finer than the municipal laboratory's and some forty substances screened, metabolites included. In Montréal, eighteen pesticides and breakdown products were detected in a single sample, adding up to 0.363 µg/l. The first is metolachlor-ESA, a breakdown product of S-metolachlor, at 0.153 µg/l, one and a half times the European per-pesticide limit; then metolachlor-OA (0.089 µg/l), atrazine (0.042 µg/l) and its metabolites, then S-metolachlor itself (0.026 µg/l). Glyphosate, on the other hand, was not detected in Montréal even by this method (limit 0.005 µg/l), while it was in Québec City, Ottawa or Sherbrooke at 0.008 to 0.035 µg/l. Longueuil, fed by the same river, shows an almost identical profile (0.364 µg/l); Laval, which draws from the Mille Îles and des Prairies rivers, is sixteen times lower (0.022 µg/l).

Those figures appear on the page under the Montréal, Longueuil and Laval, UdeM test 2026 buttons, next to the City's: same quantities, same limits, but a single sample, one dot on each curve. Québec's regulation requires no metabolite analysis in drinking water, so metolachlor-ESA, the main pesticide in Montréal's tap water, appears in none of the City's reports. The environment ministry told Radio-Canada that its data show no health issue and that the metolachlor standard (35 µg/l) could be reviewed; Health Canada is reassessing metolachlor and its metabolites.

Reading the curves

On the City's side, in ten years a single report shows pesticides at the tap: in 2017, atrazine and 2,4-D reached 0.3 µg/l, ten times below Québec's atrazine standard but three times the European limit of 0.1 µg/l per pesticide. Every other year reads "not detected". That verdict depends on how finely the laboratory looks: the reported detection limit is 0.1 to 0.7 µg/l for most pesticides, but 10 µg/l for glyphosate, a hundred times the European limit. Atrazine, "not detected" by the City in 2025 below 0.3 µg/l, is measured by UdeM at 0.042 µg/l in 2026: both results are compatible, and show what "not detected" means.

For trihalomethanes, Québec's 80 µg/l standard applies to the annual mean, plotted first; the year's highest measurement, plotted next, exceeded it in 2017, 2020 and 2025 while the mean never did. Nitrates stay around 0.3 mg/l as nitrogen, forty times below the standard.

Three limits on every curve

In blue, Québec's standard (Regulation respecting the quality of drinking water, Q-2, r. 40); in red, Health Canada's maximum acceptable concentration, where one exists (the metolachlor guideline was withdrawn in 2022); in amber, the European Union's value (Directive 2020/2184): 0.1 µg/l per pesticide, 100 µg/l of trihalomethanes, 60 µg/l of haloacetic acids and 50 mg/l of nitrate, i.e. 11.3 mg/l as nitrogen. Lead at the tap, measured house by house, has its own page.

Frequently asked questions

Why only one value per year?

The City publishes only the annual summary: the year's minimum, mean and maximum, or the highest measurement. Individual results are not released.

Is glyphosate really absent from the tap water?

The City has never detected it, but with a 10 µg/l limit. The independent June 2026 test, a hundred times finer (0.005 µg/l), did not find it either in Montréal, Longueuil or Laval, whereas in 2024 ten Montérégie towns fed by the Yamaska or Richelieu rivers had 2.0 to 2.5 µg/l. The St. Lawrence dilutes a lot; presence varies strongly with the spraying season and the rain.

What is metolachlor-ESA?

A breakdown product of S-metolachlor, a corn and soybean herbicide, very soluble and persistent. It has no standard in Québec or Canada; in Europe the 0.1 µg/l per-pesticide limit applies to metabolites deemed relevant. In Montréal it reached 0.153 µg/l in June 2026.

Where do trihalomethanes come from?

From the reaction of disinfection chlorine with the organic matter of the raw water. They rise in summer, when the water is warm and rich in organic matter, hence a yearly maximum well above the mean.