McCully - Moiliili Water Hardness & Quality Report (2026)

The Board of Water Supply (BWS) serves McCully-Moiliili in Honolulu, Oahu, as part of its municipal water system covering urban Honolulu. Water sources are basal groundwater aquifers on Oahu, drawn from wells tapping the island's volcanic aquifer system. There are no surface reservoirs or rivers used; treatment occurs at plants including the Halawa Wells Station and Aiea Wells Station, with chlorination and fluoridation applied before distribution. BWS does not report water hardness in its semi-annual Consumer Confidence Reports, as hardness is not a regulated parameter in Hawaii.

Oahu's mountainous recharge zones in the Ko'olau and Waianae Ranges channel rainfall through volcanic soils into basal aquifers confined beneath the ocean-influenced freshwater lens. The geology features tholeiitic basalt from shield volcanoes of the Hawaiian hotspot, with dike impoundments creating perched groundwater above the basal zone. This volcanic terrain produces very soft water with low mineralisation, as basalt weathers to clays without releasing substantial hardness ions — unlike sedimentary carbonate basins found elsewhere on the mainland.

With very soft water, scale buildup is negligible, sparing water heaters, dishwashers, and washing machines from calcium deposits. Soap lathers easily, and plumbing fixtures resist spotting. Water softeners are unnecessary and not recommended, as they could introduce sodium to an already low-mineral supply. BWS water meets all federal and state standards per Consumer Confidence Reports, with pH typically 7.5–8.5; lead and copper rule compliance is achieved through corrosion control, with 90th percentile copper below 1.3 mg/L; no PFAS detections above limits have been reported. Primary treatment consists of disinfection at wells and blending from multiple aquifer sources for reliability.