Pomona Water Hardness & Quality Report (2026)

Pomona, California receives its municipal water supply through the City of Pomona Water Division, blending locally pumped groundwater from the Pomona Basin — part of the greater San Gabriel Valley Groundwater Basin managed by the Upper San Gabriel Valley Municipal Water District — with imported water delivered by the Metropolitan Water District of Southern California (MWD) via the Inland Feeder infrastructure in Los Angeles County. The Pomona Basin is recharged by the San Gabriel River and its tributaries, supplemented by artificial injection of imported water. Water hardness in Pomona measures 178 mg/L — classified as hard.

Pomona's hard supply reflects the combined character of its San Gabriel Valley basin groundwater and Colorado River-dominated MWD imports. The Pomona Basin draws from Quaternary alluvial sediments — sand, gravel, and silt deposited by the San Gabriel River from the San Gabriel Mountains (Precambrian gneiss, Jurassic quartz diorite, and Cretaceous granodiorite). While these granitic source rocks are calcium-poor, long aquifer residence times and calcareous cement within alluvial fill contribute moderate dissolved calcium. Colorado River aqueduct imports, carrying substantial mineral loads from Permian and Triassic carbonate canyon geology, add significantly to the hardness of the blended supply — producing a finished product harder than pure SWP or Los Angeles Aqueduct blends.

At 178 mg/L, Pomona residents encounter regular limescale challenges. White calcium deposits build up on faucet aerators, showerheads, and inside kettles within a few weeks — monthly descaling with citric acid solution is a standard maintenance practice. Dishwashers produce noticeably better glassware results with rinse-aid, and water heaters benefit from annual inspection for element scale build-up. Pomona Water Division consistently delivers water meeting all California SWRCB and EPA Safe Drinking Water Act requirements.