San Bruno Water Hardness & Quality Report (2026)

The City of San Bruno Public Works/Water Division serves approximately 40,945 residents in San Bruno, California, within San Mateo County. Water is sourced primarily from the San Francisco Public Utilities Commission's (SFPUC) Regional Water System, including surface water from Hetch Hetchy Reservoir in Yosemite, Crystal Springs and San Andreas Reservoirs in San Mateo County, and Calaveras Reservoir in Alameda County. Locally, the City operates four groundwater wells tapping a deep aquifer in northern San Mateo County, contributing 29% of the 2023 supply (306 million gallons). Blending occurs at distribution points with basic disinfection and corrosion control.

The watershed spans the Tuolumne River basin in the Sierra Nevada, feeding Hetch Hetchy, and local peninsula reservoirs capturing runoff. Surface water contacts granitic batholiths and metavolcanic rocks of the Franciscan formation, producing very soft water with minimal mineral dissolution. Groundwater from the San Mateo aquifer filters through Pleistocene alluvial deposits and older sedimentary layers rich in carbonates, imparting a hard character through leaching of alkaline earth metals. This geological contrast — soft mountain runoff versus mineralized local groundwater — defines the blended supply's hard profile, varying by seasonal pumping ratios; hardness averaged 215.6 ppm as CaCO3 (range 158–324 ppm).

Hard water at 215.6 mg/L promotes significant limescale buildup in pipes, water heaters, dishwashers, and washing machines, reducing efficiency and lifespan. Faucets and showerheads clog quickly, laundry feels stiff, and soaps lather poorly. Installing a water softener is recommended to prevent scaling and extend appliance life; regular vinegar descaling helps with ongoing maintenance. The 2023 Consumer Confidence Report shows average pH of 7.27 (range 6.86–7.55); the system complies with lead/copper rules via corrosion inhibitors. Third-party analyses note PFAS and chromium-6 above health guidelines, though the utility reports all contaminants below MCLs. Treatment includes chloramination, fluoridation, and blending.