Mililani Town Water Hardness & Quality Report (2026)

The Board of Water Supply (BWS) of Honolulu serves Mililani Town in Honolulu County, Oahu, Hawaii, delivering water to over 1 million residents across central and windward Oahu. Water is drawn from groundwater sources including the Schofield Plateau Basal Aquifer and Waipahu Valley Basal Aquifer. Key production and treatment facilities include the Waiawa Shaft, Halawa Shaft, and 'Aiea Wells, with treatment limited to disinfection via chloramination at plants such as the Waiawa Treatment Plant.

Recharge occurs via the Ko'olau Range watershed, where high annual rainfall exceeding 100 inches percolates through the Pleistocene Ko'olau basalt formation into basal aquifers beneath the Schofield Plateau and Waipahu Valley. These aquifers are unconfined freshwater lenses in porous volcanic rock, capped by low-permeability caprock of tuffaceous sediments. The young basaltic geology dissolves sparingly, contributing negligible calcium or magnesium; rainwater chemistry dominates, producing a very soft, low-mineralised supply.

With soft water, scale buildup is minimal, sparing water heaters, dishwashers, and washing machines from calcification, and soap and detergent efficiency is high. No water softener is needed or recommended. However, soft water carries a higher corrosion risk — monitor for metallic tastes and use anode rods in heaters, with copper or PEX pipework to prevent pinhole leaks. BWS water maintains pH 7.5–8.5, compliant with EPA standards; no PFAS detections above lab limits appear in recent reports, and disinfection byproducts such as TTHMs are managed below MCLs, with low arsenic and radiological levels monitored from volcanic sources.