Diamond Head / Kapahulu / Saint Louis Heights Water Hardness & Quality Report (2026)
Water Hardness
~0–59 mg/L
Softestimated · not lab-verified
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
$0.08
energy & soap waste
Source: See methodology section below · 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 4% efficiency due to mineral buildup.
| Appliance | In Diamond Head / Kapahulu / Saint Louis Heights | Soft Water City | Efficiency Loss |
|---|---|---|---|
| Kettle | 8.2 yrs | 8.5 yrs | -4% |
| Washing Machine | 11.5 yrs | 12 yrs | -4% |
| Water Heater | 14.4 yrs | 15 yrs | -4% |
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 | ≈ 0–59 mg/L | 1.3 ppt | 🟢 Soft | groundwater |
| Kaimuki, Hawaii | ≈ 0–60 mg/L | 1 ppt | 🟢 Soft | groundwater |
| Mo'ili'ili, Hawaii | ≈ 120–179 mg/L | 1.4 ppt | 🟠 Hard | groundwater |
| Waikiki, Hawaii | ≈ 0–60 mg/L | 0.7 ppt | 🟢 Soft | groundwater |
| McCully - Moiliili, Hawaii | ≈ 0–60 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 | ≈ 0–59 mg/L | 🟢 None |
| USA National Avg | 151 mg/L | 🟠 Moderate |
| Scarsdale Top Rated | 0.02 mg/L | 🟢 None |
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What Makes Diamond Head / Kapahulu / Saint Louis Heights's Water Unique?
Local geology and source profile
The Diamond Head–Kapahulu–St. Louis Heights area on Oahu is served by the Board of Water Supply (BWS), the primary water utility for Honolulu and surrounding regions. The BWS operates multiple groundwater sources, including wells tapping into the volcanic aquifers beneath Oahu. Water is treated at regional facilities and distributed through the municipal system serving this residential and commercial district in central Honolulu. The utility serves the broader Oahu population and provides semi-annual Consumer Confidence Reports searchable by address on its website.
The watershed and geology of Oahu are dominated by Quaternary and Tertiary basaltic volcanic formations. The primary water source is groundwater from volcanic aquifers consisting of highly porous lava flows with limited mineral dissolution. Unlike limestone or sandstone aquifers on the mainland, Hawaii's basaltic geology naturally produces water with low mineral content, contributing to the characteristically soft water supply across the islands.
At soft hardness levels, residents experience minimal scaling in appliances, reduced soap consumption, and lower maintenance demands on water heaters and plumbing fixtures. Water softening is not necessary for this supply, and residents may notice improved lather in soaps and shampoos compared to hard-water regions. The Board of Water Supply reports that Oahu's water meets or exceeds EPA and state standards; the utility emphasizes that water quality on Oahu is among the best in the world, with detailed quality data available by address via the BWS Consumer Confidence Reports.
Geology & Source: Oahu volcanic aquifers — Quaternary and Tertiary basaltic lava flows; porous basalt with limited soluble mineral contact produces characteristically soft water
Other Hawaii Water Reports
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Frequently Asked Questions
Is Diamond Head / Kapahulu / Saint Louis Heights's water safe to drink?
Do I need a water softener in Diamond Head / Kapahulu / Saint Louis Heights?
How does Diamond Head / Kapahulu / Saint Louis Heights compare to the USA average?
Data Sources & Methodology
Water quality data for Diamond Head / Kapahulu / Saint Louis Heights is derived from geographic and geological modelling of the surrounding region. No federal monitoring station data was available for this location.
Water Hardness
Modelled estimate based on state-level USGS geological survey data for this region. No direct USGS Water Quality Portal measurement was matched to this city — the value reflects a statistical range calibrated to the state's dominant rock types and typical source water characteristics.
pH
Estimated from regional geology and source water characteristics. pH is correlated with water hardness and local bedrock — values may differ from utility-reported figures.
TDS — Total Dissolved Solids
Estimated using a derived ratio from water hardness and regional conductance profiles. TDS in natural water correlates strongly with total mineral content including hardness ions.
PFAS — Perfluoroalkyl and Polyfluoroalkyl Substances
EPA UCMR5 (5th Unregulated Contaminant Monitoring Rule, 2023–2025) — sum of PFAS compounds detected at the public water system serving this city. A value of 0 indicates the system was sampled with no detection above reporting limits.
Lead
Modelled estimate based on the EPA Lead and Copper Rule 90th-percentile tap-sample methodology. No publicly available per-city lead dataset with sufficient national coverage exists. Values are a conservative baseline derived from city population tier and infrastructure age — all estimates are maintained below the EPA action level of 0.015 mg/L.
Appliance Lifespan
Calculated from water hardness using a linear degradation model. Baseline lifespans represent soft-water performance (kettle: 8.5 yrs, washing machine: 12.0 yrs, water heater: 15.0 yrs). Hard water mineral scale progressively reduces operational life in direct proportion to hardness concentration.