Duncan Water Hardness & Quality Report (2026)

Duncan's drinking water is managed by the City of Duncan, drawing from the Koksilah River watershed in the Cowichan Valley on southern Vancouver Island — sourced from the mixed forest and agricultural terrain of the Vancouver Island Ranges watershed above the Cowichan agricultural plain. Water undergoes coagulation, sedimentation, filtration, UV disinfection, and chloramination, meeting all Guidelines for Canadian Drinking Water Quality (GCDWQ) requirements. Hardness measures 68.5 mg/L (4.0 gpg) — classified as moderately hard by Health Canada, notably harder than the adjacent North Cowichan municipality (36.5 mg/L from batch 13) despite sharing the same regional Cowichan Valley geography.

The difference between Duncan City (68.5 mg/L) and North Cowichan (36.5 mg/L) reflects distinct water source systems: North Cowichan draws from the very soft Chemainus River watershed on Vancouver Island's basement granites and Wrangellia volcanic terrane, while Duncan's Koksilah River drains a longer valley corridor with greater contact with Cretaceous intrusive and metasedimentary terrain, releasing more dissolved calcium and producing the moderately harder 68.5 mg/L supply. Both communities share the Cowichan Valley's generally coastal soft-to-moderately-hard character.

At 68.5 mg/L, Duncan residents experience light-to-moderate scale deposits — kettle cleaning every two to three months is adequate. Hot water tanks have a good operational lifespan. The City of Duncan provides water quality information at duncan.ca. Duncan is the regional centre for the Cowichan Valley; Health Canada lead precautionary guidance applies to residents in the city's older pre-1975 downtown core housing stock.