Pierrefonds Water Hardness & Quality Report (2026)

Pierrefonds (a western sector of the Pierrefonds-Roxboro borough of Ville de Montréal, historically an independent city) receives its drinking water through Ville de Montréal — Direction de l'eau potable, drawing from the St. Lawrence River primarily via the Atwater Water Treatment Plant. Treatment includes ozonation, biofiltration, UV disinfection, and chloramination, meeting all Guidelines for Canadian Drinking Water Quality (GCDWQ) standards. Hardness in Pierrefonds' distribution sub-zone is 128.5 mg/L (7.5 gpg) — classified as hard by Health Canada, notably higher than the Roxboro portion of the same borough (73.5 mg/L), reflecting a proportionally higher share of harder St. Lawrence mainstream supply in this sub-zone.

Pierrefonds' elevated hardness within the Montréal distribution network reflects the distribution zone's primary connection to the Atwater plant's direct St. Lawrence mainstream intake, where the harder Ordovician limestone-influenced river water dominates over the softer Rivière des Prairies Shield tributary inflow that moderates hardness in northern borough sub-zones. Historical infrastructure from the former municipality of Pierrefonds may channel supply predominantly from the western Atwater corridor, producing the 128.5 mg/L recorded here.

At 128.5 mg/L, Pierrefonds residents experience more noticeable scale deposits than typical Montréal boroughs — kettle descaling every six to eight weeks is advisable. Hot water tanks benefit from annual flushing. For Pierrefonds' extensive post-war and 1970s–1980s suburban housing stock, Montréal's Direction de l'eau potable provides Health Canada-compliant guidance on water quality and recommends that owners of older properties check lead service line status and access the city's lead replacement programme.