Foster City Water Hardness & Quality Report (2026)

Foster City, in San Mateo County, California, does not operate its own water utility. The Estero Municipal Improvement District (EMID) purchases treated water from the San Francisco Water Department (SFRWS) and distributes it to Foster City and adjacent areas of San Mateo County. The SFRWS operates multiple treatment plants and sources, including the Hetch Hetchy Reservoir system in the Sierra Nevada. The treatment process includes conventional filtration and disinfection to meet all state and federal drinking water standards before distribution to residents.

The water supply originates from high-elevation snowmelt and local Bay Area watersheds that drain through granitic and metamorphic bedrock of the Sierra Nevada and Coast Ranges. These geological formations contain minimal dissolved carbonate minerals, resulting in naturally soft water. The low mineral solubility of the granite and metamorphic rock geology, combined with high-elevation reservoir sourcing, keeps calcium and magnesium concentrations low, producing Foster City's average hardness of 32 mg/L as CaCO₃.

With soft water at 32 mg/L, scale buildup in pipes and water heaters is minimal and water softeners are not necessary. Soap lathers easily and residents experience few of the spotting or scaling issues common in hard-water areas; appliance longevity is not compromised by mineral deposits. The SFRWS and EMID conduct regular water quality monitoring to ensure compliance with all state and federal standards. The 2022 Consumer Confidence Report confirms that delivered water met or exceeded all regulatory requirements; residents may contact the EMID Public Works Maintenance Division at (650) 286-8140 or the SFRWS Water Quality Bureau at (877) 737-8297 for detailed water quality data.