Private wells
Arizona well water: who's on a well and what to test for
People on private wells
651,400
Homes on wells
8.9%
Est. above arsenic limit
17,000
Every Arizona 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.
| Maricopa County | 146,800 | 3% | 51.4% | 3.5 | 300 ft |
| Pinal County | 98,500 | 21% | 43.3% | 2.9 | 340 ft |
| Pima County | 70,100 | 6% | 28.9% | 3.5 | 306 ft |
| Yavapai County | 69,600 | 29% | 39.3% | 0.7 | 280 ft |
| Navajo County | 55,900 | 41% | 11.1% | 0.2 | 340 ft |
| Apache County | 54,100 | 79% | 10.7% | 0.3 | 260 ft |
| Coconino County | 32,300 | 22% | 13.0% | 0.6 | 188 ft |
| Cochise County | 28,200 | 23% | 9.6% | 1.6 | 250 ft |
| Yuma County | 27,400 | 12% | 15.9% | 0.8 | 180 ft |
| Gila County | 21,400 | 37% | 7.1% | 0.7 | 163 ft |
| Mohave County | 20,000 | 11% | 15.4% | 1.4 | 220 ft |
| Graham County | 10,300 | 26% | 11.4% | 1.4 | 110 ft |
| La Paz County | 10,300 | 50% | 25.5% | 1.5 | 200 ft |
| Santa Cruz County | 4,200 | 13% | 6.4% | 1.6 | 250 ft |
| Greenlee County | 2,400 | 27% | 16.2% | 0.9 | 120 ft |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Maricopa County: about 3,900 people
- Yavapai County: about 3,800 people
- Yuma County: about 1,900 people
- Apache County: about 1,600 people
- Cochise County: about 1,300 people
Highest modeled nitrate
- Pima County: 3.5 mg/L average (limit 10)
- Maricopa County: 3.5 mg/L average (limit 10)
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.
2. 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.
3. 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.
4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
5. 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.
6. 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.
Questions
Which Arizona counties rely most on wells?+
By share of homes: Apache County (79%), La Paz County (50%), Navajo County (41%), Gila County (37%), Yavapai County (29%). By number of people: Maricopa County (146,800), Pinal County (98,500), Pima County (70,100).
Is arsenic a problem in Arizona wells?+
USGS estimates about 17,000 Arizona well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Maricopa County, Yavapai County, Yuma County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Arizona?+
We could not confirm a statewide rule for Arizona. 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).