Makakilo City Water Hardness & Quality Report (2026)

Makakilo City residents receive their water from the Board of Water Supply (BWS), the main utility serving Oahu. The supply is sourced from groundwater aquifers, primarily the Waianae Aquifer and Waipahu Valley Aquifer. These basal aquifers are replenished by abundant rainfall across the island, infiltrating through volcanic terrains. While Lake Wilson in the Waikele Watershed can supplement the supply, the system relies heavily on numerous wells tapping into the island's underground water reserves. Water undergoes treatment at facilities like the Halawa Wells Station and Aiea Wells Station, where it is disinfected, filtered, and treated for corrosion control.

The groundwater consumed in Makakilo City originates from the Waianae and Koolau volcanic series, specifically Pleistocene-era tholeiitic basalts. These volcanic rocks, characteristic of Hawaii's shield volcanoes, are largely impermeable and lack the significant deposits of limestone or dolomite found in continental regions. As water percolates through fractured lava flows and ash layers, it dissolves very few minerals. This geological makeup, combined with the islands' recent volcanic age and isolated Pacific location, results in groundwater that is naturally very soft, with a low concentration of dissolved minerals like calcium and magnesium.

Because the water is so soft, Makakilo City's supply doesn't typically cause significant scale buildup in household plumbing, water heaters, or appliances such as dishwashers and washing machines. You'll likely find that soap lathers easily, requiring less product, and you won't notice mineral spotting on dishes or fixtures. Routine descaling of appliances is usually unnecessary, and installing a water softener isn't recommended as it could potentially lead to over-softening and accelerate pipe corrosion. The BWS reports a pH typically between 7.5 and 8.5, which aids in water stability.