San Carlos Water Hardness & Quality Report (2026)

Cal Water (California Water Service) operates the San Carlos System in San Mateo County, California, serving the City of San Carlos and surrounding areas through approximately 112 miles of mains, 18 storage tanks, and 24 booster pumps. Primary water sources are purchased from the San Francisco Public Utilities Commission (SFPUC), drawn from Hetch Hetchy Reservoir in Yosemite via the Bay Division Pipeline, and local reservoirs including Crystal Springs, San Andreas, and Pilarcitos. The supply is augmented by groundwater from the West Bay Aquifer; treatment involves disinfection with chloramine at SFPUC facilities, with additional monitoring and blending by Cal Water.

The water originates from the Tuolumne River watershed via Hetch Hetchy, flowing through the Peninsula Watershed encompassing rugged coastal mountains. Key geological features include the Franciscan Complex melange — Jurassic-Cretaceous submarine accretionary rocks of greywacke, chert, shale, and volcanic materials — overlain by thin soils and alluvial fans. Groundwater from the West Bay Aquifer taps a shallow unconfined system in Quaternary sediments overlying this low-carbonate, siliceous bedrock, which contributes minimal dissolved hardness ions, producing a characteristically soft supply at 60 mg/L.

Soft water at 60 mg/L means scale buildup in water heaters and dishwashers is minimal, reducing maintenance needs. Soap lathers effectively with less detergent, lowering cleaning costs. However, low mineral content can slightly increase corrosion risk in pipes — watch for blue staining from copper fixtures. A water softener is not recommended and could over-demineralize the supply; focus on corrosion inhibitors if needed. Water quality is excellent — zero violations since 2023; pH typically 8.5–10.5; lead and copper rule compliance is strong; PFAS non-detects; treatment via chloramination, filtration at source reservoirs, and fluoride addition.