Quebec

Longueuil water hardness

The Longueuil agglomeration treats water from the St. Lawrence at three plants. The Louise-Gravel plant serves Vieux-Longueuil, Saint-Hubert, Boucherville and Saint-Bruno-de-Montarville. The Le Royer plant serves Saint-Lambert, Brossard and part of Longueuil, including Greenfield Park and Le Moyne. In 2025, total hardness averaged 113 mg/L in water from Louise-Gravel, from 84 to 130 mg/L, and 117 mg/L in water from Le Royer, from 98 to 136 mg/L. That is moderately hard water, close to the 120 mg/L line where hard water starts, and about twice as hard as Laval's. The smaller Mario-Petrone plant has no separate hardness figure in the City's 2025 reports, so this page does not list it.

Data last reviewed October 17, 2026.

At a glance

What this means for your appliances

Moderately hard

Some scale builds up over time, mostly where water gets heated: kettles, coffee makers and washers. A monthly clean keeps it in check.

Recommended for your water

Dishwasher Cleaner

5.0(15)

Dissolves grease, detergent residue and limescale buildup, and removes odour-causing buildup.

Clears grease, limescale and odour buildup

$23.95

Washing Machine Cleaner

4.9(14)

Breaks down residue and odour-causing buildup inside your washer.

Removes odour and buildup from the drum and hoses

$24.95

Coffee Maker Cleaner

4.8(14)

Removes limescale, mineral buildup and coffee oils inside your coffee maker.

Descales for better-tasting coffee

$24.95

Guides for hard water

What counts as hard water?

We use the U.S. Geological Survey scale, in mg/L as CaCO₃.

  • Soft 0 to 60 mg/L
  • Moderately hard 61 to 120 mg/L
  • Hard 121 to 180 mg/L
  • Very hard Over 180 mg/L

How to read the units

Utilities usually report hardness in mg/L as calcium carbonate (CaCO₃), which is the same as ppm. Dishwasher and softener manuals often use grains per gallon or German degrees instead. Here is how they convert:

  • 1 US grain per gallon = 17.1 mg/L
  • 1 imperial grain per gallon = 14.25 mg/L
  • 1 °dH = 17.85 mg/L
  • 1 mmol/L = 100.09 mg/L

Questions and answers

How hard is the water in Longueuil?

Moderately hard. In 2025, the City of Longueuil measured an average of 113 mg/L in water from the Louise-Gravel plant and 117 mg/L in water from the Le Royer plant. Water counts as hard above 120 mg/L, and the highest samples, 130 and 136 mg/L, crossed that line.

Which plant serves Brossard, Boucherville or Saint-Lambert?

According to the City of Longueuil, the Louise-Gravel plant serves Vieux-Longueuil, Saint-Hubert, Boucherville and Saint-Bruno-de-Montarville, and the Le Royer plant serves Saint-Lambert, Brossard and part of Longueuil. The figures on this page were measured on the City of Longueuil's own networks. Your water comes from the same plant, but it reaches you through your city's local pipes.

Is Longueuil's water harder than Montréal's?

About the same. Both cities treat St. Lawrence water. Longueuil's plants averaged 113 and 117 mg/L in 2025, and Montréal's Atwater and Des Baillets network averaged 117 mg/L in 2024.

Do I need to descale appliances in Longueuil?

Yes, now and then. Moderately hard water slowly leaves scale in kettles, coffee makers, dishwashers and washers. Descaling the coffee maker regularly and cleaning the dishwasher and the washer about every two weeks keeps it in check. Your appliance manual may also have a water hardness setting.

What is Longueuil's water hardness in grains per gallon?

Louise-Gravel plant: 6.6 grains per US gallon, or 7.9 grains per imperial gallon. Le Royer plant: 6.8 grains per US gallon, or 8.2 grains per imperial gallon.

Where do these numbers come from?

Ville de Longueuil, 2025 water quality report, Louise-Gravel network (PDF, in French). Data for 2025, distributed water, annual average. Ville de Longueuil, 2025 water quality report, Greenfield Park and Le Moyne network (PDF, in French). Data for 2025, distributed water, annual average. We update this page when a new report comes out.

Sources

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