South San Francisco Water Hardness & Quality Report (2026)

California Water Service Company operates the South San Francisco water system, serving the city and surrounding areas in San Mateo County. The utility sources water from a combination of surface supplies — including reservoirs fed by Sierra Nevada snowmelt — and local groundwater aquifers. Treatment plants process water to meet state and federal standards before distribution to residential, commercial, and industrial customers throughout the service area.

The South San Francisco water supply originates from the broader San Francisco Bay Area watershed, fed primarily by snowmelt from the Sierra Nevada mountains and supplemented by local groundwater from Quaternary alluvial aquifers. The geology is dominated by Franciscan Formation rocks (Jurassic–Cretaceous metamorphic and sedimentary units) and younger alluvial deposits. The granitic composition of the Sierra Nevada source region, combined with limited carbonate rock exposure, results in low dissolved mineral content and the supply's soft character.

At 32 mg/L, customers experience minimal scale buildup on fixtures and appliances, excellent soap lathering, and reduced maintenance demands on water heaters and plumbing systems. Soft water is gentle on skin and hair; most households do not require water softening, and a water softener is not recommended for this supply. The 2023 South San Francisco water quality report confirms compliance with all federal and state drinking water standards. Treatment includes coagulation, sedimentation, filtration, and chlorination; pH remains within the acceptable range, lead and copper levels are maintained below action levels through corrosion control, and no PFAS contamination has been reported.