Hillsborough Water Hardness & Quality Report (2026)

The Town of Hillsborough, California, receives its entire water supply from the San Francisco Public Utilities Commission (SFPUC). This utility serves around 11,500 residents, drawing water from three main sources: the Hetch Hetchy Reservoir in the Sierra Nevada Mountains, and local watersheds located in Alameda County and the Peninsula. Before distribution, all water undergoes treatment to meet federal and state criteria for watershed protection.

The Hillsborough water supply originates from both the Sierra Nevada and Bay Area Peninsula watersheds. These areas are characterized by granitic and metamorphic bedrock formations typical of the region. As the water flows through these geological structures and the extensive regional distribution network, it naturally picks up dissolved minerals, mainly calcium and magnesium. This process results in a moderately hard water supply for the town's residents. The town website notes that the ratio of water from the three sources can shift seasonally, leading to occasional, slight variations in both flavor and hardness.

Homeowners in Hillsborough may notice the effects of this moderately hard water on their plumbing and appliances. Mineral buildup can impact water heaters, fixtures, and other household systems, potentially shortening their lifespan and increasing maintenance costs. The town recommends flushing water heaters annually to clear out accumulated sediment. While a water softener is suggested for moderately hard water, many residents opt out, though this can reduce cleaning effectiveness and accelerate appliance wear. The town also advises using cold water for drinking and cooking, and installing anti-siphon fittings on outdoor hose connections.