Diamond Head / Kapahulu / Saint Louis Heights Water Hardness & Quality Report (2026)
Water Hardness
3.7 grains per gallon
Source
groundwater
pH Level
7.6
neutral = 7.0
Lead
0.007 mg/L
✓ Below action level
TDS
199.9 mg/L
Est. Daily Cost
$0.17
energy & soap waste
Source: USGS Water Quality Portal · Updated 2026
0–60
mg/L
Soft
61–120
mg/L
Moderately Hard
121–180
mg/L
Hard
180+
mg/L
Very Hard
Appliance Damage Report
In Diamond Head / Kapahulu / Saint Louis Heights, your appliances are currently losing 9% efficiency due to mineral buildup.
| Appliance | In Diamond Head / Kapahulu / Saint Louis Heights | Soft Water City | Efficiency Loss |
|---|---|---|---|
| Kettle | 7.5 yrs | 8.5 yrs | -12% |
| Washing Machine | 11.2 yrs | 12 yrs | -7% |
| Water Heater | 13 yrs | 15 yrs | -13% |
Regional Water Comparison
How Diamond Head / Kapahulu / Saint Louis Heights compares to its nearest neighbours
| City | Hardness | PFAS (ppt) | Risk | Source |
|---|---|---|---|---|
| ▶ Diamond Head / Kapahulu / Saint Louis Heights, Hawaii | 64 mg/L | 1.3 ppt | 🟡 Moderately Hard | groundwater |
| Kaimuki, Hawaii | 43.5 mg/L | 1 ppt | 🟢 Soft | groundwater |
| Mo'ili'ili, Hawaii | 69.5 mg/L | 1.4 ppt | 🟡 Moderately Hard | groundwater |
| Waikiki, Hawaii | 20 mg/L | 0.7 ppt | 🟢 Soft | groundwater |
| McCully - Moiliili, Hawaii | 20.5 mg/L | 0.7 ppt | 🟢 Soft | groundwater |
National Benchmark
How Diamond Head / Kapahulu / Saint Louis Heights compares to the USA average
| Benchmark | Hardness | Appliance Risk |
|---|---|---|
| ▶ Diamond Head / Kapahulu / Saint Louis Heights | 64 mg/L | 🟡 Low |
| USA National Avg | 150 mg/L | 🟠 Moderate |
| Badger Top Rated | 8.5 mg/L | 🟢 None |
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What Makes Diamond Head / Kapahulu / Saint Louis Heights's Water Unique?
Local geology and source profile
Diamond Head / Kapahulu / Saint Louis Heights, Hawaii, in Honolulu County — the iconic Diamond Head crater district, Kapahulu (an urban corridor adjacent to Waikiki), and the Saint Louis Heights residential hill community above Manoa, collectively a densely populated Honolulu urban district — receives its municipal water from the Honolulu Board of Water Supply, drawing from the East Honolulu Aquifer and Manoa-Palolo groundwater system in the central Honolulu corridor.
The soft 64 mg/L hardness and TDS of 199.9 mg/L — notably harder and higher TDS than Waikiki (20 mg/L, TDS 47) — reflect the East Honolulu Aquifer's different groundwater characteristics. The Diamond Head / Kapahulu district draws from wells in the Honolulu Series volcanic lava flows — Pleistocene basalt, Diamond Head Tuff (palagonite tuff from the Diamond Head eruption cycle), and the Mamala Bay volcanic vents. These younger Honolulu Volcanics are compositionally different from the main Ko'olau Basalt (slightly higher alkali content — more calcium-sodium-rich), and the groundwater residence times in the Honolulu Series tuff and basalt allow slightly more mineral dissolution than the fresh Ko'olau Basal Aquifer.
At 64 mg/L, this Honolulu district's water is moderately soft — comfortable for household use. Scale forms slowly, soap lathers well, and appliances operate efficiently. Semi-annual descaling is adequate. The PFAS level of 1.3 ppt is excellent — consistent with Hawaii's exceptional isolation from continental military-industrial PFAS sources, and among the very lowest in the continental US or US territories.
Geology & Source: Diamond Head / Kapahulu / Saint Louis Heights in Honolulu County is served by the Honolulu Board of Water Supply on the East Honolulu Aquifer (Pleistocene Honolulu Volcanics — Honolulu Series basalt and Diamond Head Tuff) — the Honolulu Volcanics tuff aquifer has longer residence time than the Ko'olau basal lens — Hawaiian volcanic basalt-tuff aquifer produces soft water at 64 mg/L with TDS 200 mg/L in this Honolulu Hawaii community.