San Mateo Water Hardness & Quality Report (2026)

California Water Service (Cal Water) - South San Francisco Division supplies San Mateo, serving approximately 137,000 residents on the Peninsula. Primary sources include surface water from the San Francisco Regional Water System (SFRWS) — including the Hetch Hetchy system and reservoirs in Alameda County delivered to the Sunol Valley Water Treatment Plant and in San Mateo County to the Harry Tracy Water Treatment Plant — supplemented by groundwater from a deep aquifer in northern San Mateo County. Treatment involves filtration, disinfection, fluoridation, taste and odor removal, and corrosion control. In 2023, Cal Water conducted 9,055 tests on 1,724 samples for 83 constituents, meeting all primary and secondary federal and state standards.

The SFRWS watershed draws from Sierra Nevada reservoirs underlain by Mesozoic granitic and metavolcanic rocks, as well as Alameda Watershed ponds and San Mateo County surface storage, blended with local groundwater. The Sierra Nevada's granitic bedrock yields soft water through rapid snowmelt transit with limited rock interaction, while Cenozoic sedimentary limestone and calcium-rich aquifers in the Santa Clara Valley moderately mineralize groundwater. The dominant surface dilution from low-interaction granitic sources overrides local hardness influences, producing a characteristically soft overall supply.

Soft water minimizes scale buildup, sparing water heaters, pipes, and fixtures from mineral deposits. Appliances like dishwashers and washing machines perform efficiently, with good soap lathering and no crusty residues on shower doors or faucets. Maintenance is straightforward — occasional aerator cleaning suffices. A water softener is unnecessary and not recommended, as it could add sodium without functional benefit. Notable trace detections include arsenic, hexavalent chromium, and disinfection byproducts, all below MCLs; full lead and copper rule compliance is maintained via corrosion control.