Terrebonne Water Hardness & Quality Report (2026)

Terrebonne's drinking water is managed by Ville de Terrebonne, drawing from the Rivière des Mille-Îles and supplementary intake from the Rivière des Prairies corridor north of Montréal. Water is treated at the Terrebonne Water Treatment Plant using coagulation, sedimentation, filtration, UV disinfection, and chloramination, fully meeting the Guidelines for Canadian Drinking Water Quality (GCDWQ). Hardness measures 107 mg/L (6.2 gpg) — classified as moderately hard by Health Canada, notably harder than the city's Saint-Louis-de-Terrebonne sector (79 mg/L), reflecting greater influence of the Ordovician carbonate bedrock in Terrebonne's main supply zone.

Terrebonne's urban water supply draws from the Rivière des Mille-Îles — a distributary of the Ottawa River flowing through the St. Lawrence Lowlands plain north of Laval island. At this location, the lowland river traverses Ordovician dolostone and limestone (Trenton and Black River groups) exposed across the Laurentian lowland plain, picking up higher dissolved calcium than upstream Shield-influenced zones. The resulting 107 mg/L reflects a moderate balance of harder Ordovician carbonate groundwater and softer Shield tributary inflow.

At 107 mg/L, Terrebonne residents notice moderate scale deposits in kettles and on tap aerators — descaling every two months is typical. Hot water tanks benefit from periodic inspection; electric heating elements accumulate scale at this hardness over several years. A basic scale inhibitor on the cold water inlet to the hot water tank provides cost-effective protection for high-efficiency appliances. Ville de Terrebonne provides annual water quality reports to residents in compliance with Ministère de l'Environnement provincial regulations.