Private wells
Hawaii well water: who's on a well and what to test for
People on private wells
64,500
Homes on wells
4.9%
Est. above arsenic limit
Not modeled
Every Hawaii county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Hawaii County | 44,900 | 23% | n/a | n/a | n/a |
| Maui County | 10,700 | 7% | n/a | n/a | n/a |
| Honolulu County | 6,200 | 0% | n/a | n/a | n/a |
| Kauai County | 2,700 | 4% | n/a | n/a | n/a |
| Kalawao County | 30 | 39% | n/a | n/a | n/a |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
Mail-in certified lab well water test
Mid-range2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
5-micron whole-house sediment filter
Budget3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
Air-injection iron and sulfur filter
Premium4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
12 GPM whole-house UV purifier
Premium5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
Waterdrop G3P600 tankless RO
Price at Waterdrop, checked Oct 5, 2026or compare on Amazon
iSpring RCC7AK reverse osmosis
Mid-range6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Waterdrop WHF21 2-stage whole-house filter
Mid-rangeQuestions
Which Hawaii counties rely most on wells?+
By share of homes: Kalawao County (39%), Hawaii County (23%), Maui County (7%), Kauai County (4%), Honolulu County (0%). By number of people: Hawaii County (44,900), Maui County (10,700), Honolulu County (6,200).
Is arsenic a problem in Hawaii wells?+
The USGS county arsenic estimate does not cover Hawaii. Arsenic has no taste or smell, so test for it at least once.
Do I have to test my well when I sell a house in Hawaii?+
We could not confirm a statewide rule for Hawaii. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).