Ontario Water Hardness & Quality Report (2026)

Ontario, California receives its municipal water supply through the Ontario Municipal Utilities Company (OMUC), blending imported State Water Project (SWP) water delivered via the Metropolitan Water District of Southern California (MWD) Inland Feeder pipeline, locally pumped groundwater from the Chino Groundwater Basin, and rights to San Antonio Creek water from the San Gabriel Mountains in San Bernardino County. OMUC operates its own water treatment and distribution infrastructure in the western Inland Empire. Despite Ontario's location in the Inland Empire — where many cities face hard water — OMUC's supply achieves a hardness of just 41.5 mg/L — classified as soft, one of the softest readings for any San Bernardino County city.

Ontario's unusually soft supply for an Inland Empire city reflects the high fraction of State Water Project imports in its water blend. SWP water originates from Sacramento River snowmelt — inherently low in hardness from draining the Sierra Nevada granite batholiths — and when delivered with a high MWD SWP blend ratio through Ontario's system, the soft imported water dominates over the harder local groundwater fraction. The Chino Basin groundwater, though contributing some additional calcium from alluvial plain sediments, is diluted substantially by the SWP component, producing a final supply far softer than most Inland Empire cities that rely more heavily on groundwater.

With hardness at just 41.5 mg/L, Ontario residents enjoy very soft water with negligible scale challenges. Faucets, showerheads, and kettles remain largely scale-free with only occasional light cleaning. Soap and shampoo lather exceptionally well, and dishwashers produce spot-free glassware. Hot water systems accumulate minimal scale over their full operational life. OMUC consistently delivers water meeting all California SWRCB and EPA Safe Drinking Water Act requirements throughout the Ontario distribution area.