Cornwall Water Hardness & Quality Report (2026)

Cornwall's drinking water is managed by the City of Cornwall, drawing from the St. Lawrence River via the Cornwall Water Treatment Plant on the river's north bank. Water undergoes coagulation, sedimentation, filtration, UV disinfection, and chloramination, fully meeting the Guidelines for Canadian Drinking Water Quality (GCDWQ). Hardness measures 187 mg/L (10.9 gpg) — classified as very hard by Health Canada, reflecting the St. Lawrence River's significant calcium load from the Ordovician limestone Eastern Ontario and St. Lawrence Lowlands geology.

Cornwall's St. Lawrence River intake draws water that has accumulated dissolved calcium throughout its passage through the Great Lakes and St. Lawrence Lowlands — a geological corridor dominated by Ordovician and Silurian carbonate formations including the Trenton Group, Black River Group, and Queenston Formation limestone and dolostone from eastern Ontario and western Quebec. By Cornwall's position on the river, the combined carbonate dissolution from the upstream Lowlands bedrock and calcareous shoreline geology produces the 187 mg/L hardness and significant TDS (474 mg/L) measured at the tap.

At 187 mg/L, Cornwall residents experience regular scale deposits on kettle elements and inside hot water tanks — monthly descaling is advisable. Hot water tank elements accumulate scale within two years; annual inspection and flushing are recommended. As an eastern Ontario heritage city with significant pre-1960 residential and commercial building stock, Cornwall residents should follow Health Canada precautionary lead guidance and enquire about lead service line status with the City of Cornwall, which provides water quality reporting at cornwall.ca.