San Jose Water Hardness & Quality Report (2026)

San Jose's water supply is managed by the San Jose Water Company (SJWC) and the Santa Clara Valley Water District (Valley Water), blending water from three distinct sources. A significant fraction comes from the Hetch Hetchy Regional Water System — owned by the San Francisco Public Utilities Commission (SFPUC) — which delivers ultra-soft Sierra Nevada snowmelt from the Tuolumne River watershed via the Hetch Hetchy Aqueduct. This is supplemented by local groundwater pumped from the Santa Clara Valley Groundwater Basin, a deep alluvial aquifer underlying the Silicon Valley floor, and by State Water Project imports via the South Bay Aqueduct during high-demand summer periods.

The relatively high hardness of 189.5 mg/L is largely attributable to the local Santa Clara Valley groundwater component. The valley floor sits atop a thick sequence of Quaternary alluvial fan and floodplain deposits derived from erosion of the surrounding Diablo Range to the east — a complex of Cretaceous Great Valley Sequence sedimentary rocks and Franciscan mélange assemblage, both of which contain calcareous and serpentinite-derived material. As groundwater circulates through these sediment-filled basin deposits over long residence times, it dissolves calcium and magnesium from calcareous matrix material, contributing the dominant mineral load in San Jose's blended supply.

San Jose residents in areas with a high groundwater blending ratio experience noticeably hard water effects: white scale deposits on shower fixtures and inside appliances, reduced soap and shampoo lather, and scale buildup on dishwasher spray arms. Hardness varies somewhat by neighborhood depending on the local blending ratio with Hetch Hetchy imports. Descaling dishwashers and water heaters every 3–4 months is practical maintenance, and households in the highest-hardness service areas may benefit from a point-of-entry softener or salt-free water conditioner system.