Millbrae Water Hardness & Quality Report (2026)

The City of Millbrae Public Works Department serves 22,795 residents in San Mateo County, California. The utility sources water primarily from the San Francisco Public Utilities Commission's Hetch Hetchy system — a surface water supply from the Sierra Nevada — supplemented by local groundwater wells. The Hetch Hetchy reservoir captures snowmelt from the Tuolumne River watershed in the Sierra Nevada mountains, while local wells access Bay Area aquifers beneath the city, creating a mixed-source supply for residential and commercial customers throughout the service area.

Millbrae's water supply reflects two distinct geological sources. The Hetch Hetchy component originates in granitic Sierra Nevada bedrock, which naturally produces soft water with low mineral content. The supplemental groundwater wells tap into Bay Area aquifers formed in Quaternary alluvial and marine deposits overlying older bedrock. The blending of soft Sierra Nevada surface water with moderately mineralized local groundwater results in a moderately hard combined supply affecting household appliances and plumbing throughout the service area.

At moderately hard levels, Millbrae residents experience noticeable scale buildup in water heaters, kettles, and shower heads, though the impact is less severe than in hard-water regions. Dishwashers and washing machines show gradual mineral accumulation; regular descaling maintenance is recommended. According to the City's 2022 Consumer Confidence Report, all samples tested negative for coliforms and the system recorded zero MCL violations. However, independent testing identified bromodichloromethane above EPA health-based guidelines (MCLGs). The utility collects over forty samples monthly through a state-certified laboratory, ensuring ongoing Safe Drinking Water Act compliance.