Longueuil Water Hardness & Quality Report (2026)

Longueuil's drinking water is managed by Ville de Longueuil, drawing from the St. Lawrence River via the Longueuil Water Treatment Plant on the South Shore. Water is treated with conventional coagulation, sedimentation, filtration, ozonation, and chloramination, fully complying with the Guidelines for Canadian Drinking Water Quality (GCDWQ). Hardness measures 115 mg/L (6.7 gpg) — classified as moderately hard by Health Canada, consistent with the South Shore Montérégie water zone supplied from the St. Lawrence River corridor.

Longueuil sits on the South Shore of the St. Lawrence, directly across from Montréal on the St. Lawrence Lowlands — a region underlain by Ordovician limestone and dolostone formations that dissolve moderate calcium into the river system. The St. Lawrence at this point has accumulated dissolved minerals from upstream carbonate geology and multiple tributaries, including the Richelieu River and Ottawa River inflows. Longueuil's 115 mg/L hardness is slightly lower than the central Montréal average due to blending with upstream Precambrian Shield tributary inflows that contribute softer water to the main river channel.

At 115 mg/L, Longueuil residents notice moderate scale build-up in kettles, on tap aerators, and around hot water tank elements — descaling every six to eight weeks is advisable. Hot water tanks benefit from periodic inspection; electric heating elements can accumulate significant scale deposits over two to three years without maintenance. A basic scale inhibitor installed on the cold water inlet to the hot water tank provides cost-effective protection for high-efficiency appliances at this moderately hard water level.