Peterborough Water Hardness & Quality Report (2026)

Peterborough's drinking water is managed by the City of Peterborough, drawing from the Otonabee River (part of the Trent-Severn Waterway) via the Peterborough Water Treatment Plant on the Otonabee riverbank. Water undergoes conventional coagulation, sedimentation, filtration, UV disinfection, and chloramination, fully meeting the Guidelines for Canadian Drinking Water Quality (GCDWQ). Hardness measures 95 mg/L (5.5 gpg) — classified as moderately hard by Health Canada, reflecting the Otonabee River's position at the transition between the soft Precambrian Shield and the harder Paleozoic limestone plains of southern Ontario.

Peterborough sits at the southern edge of the Precambrian Canadian Shield, where the Otonabee River flows south from the lake-studded Shield country through the Kawartha Lakes system and across the Cambrian and Ordovician limestone plain before reaching the city. The Otonabee drains a mix of soft Shield granite and harder limestone terrain, producing a moderately hard supply. The Kawartha Lakes system acts as a settling and dilution reservoir, blending Shield and limestone-influenced water before delivery to Peterborough's intake at the Otonabee.

At 95 mg/L, Peterborough residents see moderate scale deposits in kettles and on tap aerators — descaling every six to eight weeks is typically sufficient. Hot water tanks accumulate scale gradually and benefit from annual inspection and periodic flushing. The city's moderately hard supply is manageable without a home water softener for most households; a basic inline scale inhibitor cartridge provides adequate protection for high-efficiency appliances. The City of Peterborough provides annual water quality reports and follows all Health Canada guidelines for safe drinking water supply.