Milpitas Water Hardness & Quality Report (2026)

Milpitas, California, in northern Santa Clara County adjacent to San Jose and Fremont at the south San Francisco Bay — known as 'The Hub of the Silicon Valley' for its semiconductor and technology industry density — draws its municipal water supply through the City of Milpitas Water Division, receiving a blend of SFPUC Hetch Hetchy Aqueduct water (very soft Tuolumne River source) and Santa Clara Valley Water District (SCVWD) local groundwater from the Santa Clara Valley Basin alluvial aquifer. Water hardness in Milpitas measures 140 mg/L — classified as moderately hard.

Milpitas' moderately hard supply (140 mg/L) — substantially higher than San Francisco (75 mg/L) on the same SFPUC Hetch Hetchy source — reflects a larger local groundwater fraction in the Milpitas distribution blend. The Santa Clara Valley Basin alluvial aquifer underlying Milpitas is recharged by: the Guadalupe and Coyote Rivers draining the Diablo Range (Franciscan Complex — serpentinite and greenstone with calcium-bearing ophiolitic minerals), spreading operations along the Guadalupe River, and SCVWD percolation ponds throughout the valley floor. The Diablo Range and serpentinite component contributes elevated magnesium and calcium to local groundwater, raising the blended hardness well above the SFPUC aqueduct baseline. The north San Jose–Milpitas zone consistently shows higher hardness than South Bay cities closer to the Pacific slope.

At 140 mg/L, Milpitas residents encounter moderate scale accumulation. Faucet aerators and showerheads develop deposits after several months — monthly cleaning with citric acid solution is practical maintenance. City of Milpitas Water Division and SCVWD consistently deliver water meeting all California SWRCB and EPA Safe Drinking Water Act requirements.