Kaimuki Water Hardness & Quality Report (2026)

Kaimuki, Hawaii, in Honolulu County — a historic Honolulu neighborhood on the slopes of the extinct Diamond Head and Kaimuki ridge above Waikiki, known for its charming early 20th-century Craftsman bungalows and its vibrant restaurant and coffee shop scene centered on Waialae Avenue — receives its municipal water from the Honolulu Board of Water Supply (BWS), which draws from the Koolau Range groundwater system — the ancient eroded shield volcano forming the eastern spine of Oahu. BWS draws from dike-impounded tunnel systems and deep basal lens wells in the Koolau volcanic aquifer.

The very soft 43.5 mg/L hardness and TDS of 119.8 mg/L reflect Oahu's Koolau Basalt groundwater character. The Koolau Range is composed of the Pliocene-Pleistocene Koolau Basalt — tholeiitic shield basalt lava flows forming the eroded remnants of the Koolau volcano (the original Oahu shield). Hawaiian tholeiitic basalt is a calcium-rich mafic rock (plagioclase feldspar — anorthite component), but the basalt's low solubility in cool groundwater and the volcanic dike-tunnel system's rapid transit of rainfall produces relatively soft water. The Honolulu BWS supply combines dike-tunnel water from the high Koolau ridges (very soft) with some basal lens water (slightly harder from longer contact), producing the moderately soft supply characteristic of east Honolulu communities.

At 43.5 mg/L, Kaimuki's water is very soft — excellent for household use. Scale essentially never forms, soap lathers richly, and appliances operate at peak efficiency. Annual descaling is more than sufficient. The PFAS level of 1.0 ppt is among the lowest in the dataset — reflecting Oahu's island aquifer's natural isolation, the Koolau mountain watershed's limited industrial development, and the absence of direct PFAS manufacturing in the Honolulu urban supply zone.