Castro Valley Water Hardness & Quality Report (2026)

Castro Valley, California, an unincorporated community in central Alameda County in the hills above San Leandro Bay — a major Oakland eastern hills community adjacent to Hayward — draws its municipal water supply through the Alameda County Water District (ACWD), sourcing from a blend of the San Francisco SFPUC Hetch Hetchy Regional Water System (Tuolumne River, very soft Sierra Nevada source) and the South Bay Aqueduct (State Water Project–Delta Mendota Canal Sacramento–San Joaquin Delta supply). Water hardness in Castro Valley measures 159 mg/L — classified as hard.

Castro Valley's hard supply — substantially above Hetch Hetchy-only cities like South San Francisco (52 mg/L) — reflects the Alameda County Water District's significant South Bay Aqueduct and Alameda Creek blending. The South Bay Aqueduct delivers Sacramento–San Joaquin Delta water (moderately hard from the Delta's calcareous Sacramento and San Joaquin River inputs). Additionally, the Alameda Creek watershed in the Diablo Range drains the Franciscan Complex (Jurassic–Cretaceous greywacke, serpentinite, and blueschist — some calcareous serpentinite) and the Great Valley Sequence (calcareous Cretaceous turbidite sandstone). The blend of soft Hetch Hetchy and harder South Bay Aqueduct–Alameda Creek supply produces the 159 mg/L at Castro Valley.

At 159 mg/L, Castro Valley residents face regular hard water challenges. Scale deposits form on faucet aerators, showerheads, and appliances within weeks — monthly descaling with citric acid solution is standard maintenance. Alameda County Water District consistently delivers water meeting all California SWRCB and EPA Safe Drinking Water Act requirements.