Private wells
Idaho well water: who's on a well and what to test for
People on private wells
423,300
Homes on wells
23.9%
Est. above arsenic limit
47,000
Every Idaho 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.
| Ada County | 52,200 | 11% | 16.7% | 2.7 | 140 ft |
| Canyon County | 48,700 | 21% | 34.0% | 3.3 | 149 ft |
| Bingham County | 24,600 | 50% | 7.4% | 3.1 | 120 ft |
| Bonner County | 23,300 | 49% | 7.7% | 0.1 | 184 ft |
| Twin Falls County | 22,800 | 24% | 49.9% | 7.3 | 200 ft |
| Kootenai County | 21,200 | 14% | 11.0% | 0.3 | 300 ft |
| Jefferson County | 20,600 | 65% | 11.6% | 3.5 | 120 ft |
| Bonneville County | 15,400 | 13% | 7.4% | 2.9 | 204 ft |
| Bannock County | 11,700 | 13% | 8.2% | 2.0 | 180 ft |
| Payette County | 11,500 | 44% | 35.7% | 0.7 | 107 ft |
| Cassia County | 11,400 | 44% | 10.4% | 4.6 | 140 ft |
| Gem County | 11,000 | 56% | 24.7% | 1.3 | 123 ft |
| Madison County | 10,800 | 23% | 9.0% | 2.7 | 98 ft |
| Minidoka County | 9,500 | 43% | 6.8% | 4.0 | 157 ft |
| Jerome County | 9,500 | 39% | 8.0% | 3.4 | 280 ft |
| Latah County | 8,600 | 22% | 7.0% | 0.1 | 275 ft |
| Idaho County | 8,100 | 49% | 4.6% | 0.1 | 245 ft |
| Gooding County | 7,100 | 43% | 17.1% | 3.7 | 203 ft |
| Fremont County | 6,700 | 57% | 8.7% | 2.0 | 163 ft |
| Owyhee County | 6,400 | 54% | 45.2% | 0.9 | 200 ft |
| Elmore County | 6,300 | 24% | 16.0% | 1.3 | 263 ft |
| Teton County | 5,900 | 51% | 4.9% | 1.0 | 160 ft |
| Blaine County | 5,800 | 27% | 5.0% | 0.4 | 135 ft |
| Valley County | 5,600 | 47% | 5.3% | 0.1 | 119 ft |
| Washington County | 5,400 | 53% | 16.9% | 0.5 | 101 ft |
| Boundary County | 4,800 | 40% | 7.7% | 0.1 | 186 ft |
| Franklin County | 4,700 | 33% | 11.5% | 0.9 | 180 ft |
| Lemhi County | 4,500 | 56% | 6.7% | 0.3 | 120 ft |
| Benewah County | 4,400 | 46% | 9.5% | 0.0 | 250 ft |
| Nez Perce County | 4,400 | 11% | 6.1% | 0.1 | 240 ft |
| Boise County | 4,300 | 58% | 4.9% | 0.1 | 250 ft |
| Clearwater County | 3,600 | 40% | 4.7% | 0.0 | 280 ft |
| Adams County | 2,600 | 63% | 5.3% | 0.1 | 222 ft |
| Caribou County | 2,600 | 37% | 7.4% | 0.4 | 140 ft |
| Power County | 2,600 | 34% | 7.3% | 1.1 | 218 ft |
| Shoshone County | 2,600 | 21% | 6.8% | 0.0 | 175 ft |
| Lincoln County | 2,500 | 47% | 6.5% | 1.1 | 330 ft |
| Custer County | 2,500 | 59% | 4.5% | 0.2 | 103 ft |
| Oneida County | 2,100 | 47% | 5.4% | 0.9 | 200 ft |
| Bear Lake County | 1,900 | 29% | 6.4% | 0.2 | 145 ft |
| Butte County | 1,400 | 50% | 9.7% | 0.6 | 140 ft |
| Lewis County | 940 | 26% | 5.0% | 0.2 | 240 ft |
| Camas County | 550 | 57% | 9.0% | 0.3 | 140 ft |
| Clark County | 300 | 45% | 6.8% | 0.2 | 161 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
- Canyon County: about 14,000 people
- Twin Falls County: about 7,900 people
- Payette County: about 2,500 people
- Gem County: about 2,100 people
- Jefferson County: about 1,900 people
Highest modeled nitrate
- Twin Falls County: 7.3 mg/L average (limit 10)
- Cassia County: 4.6 mg/L average (limit 10)
- Minidoka County: 4.0 mg/L average (limit 10)
- Gooding County: 3.7 mg/L average (limit 10)
- Jefferson 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.
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-PG 2-stage whole-house filter
Price at Waterdrop, checked Oct 5, 2026
Questions
Which Idaho counties rely most on wells?+
By share of homes: Jefferson County (65%), Adams County (63%), Custer County (59%), Boise County (58%), Camas County (57%). By number of people: Ada County (52,200), Canyon County (48,700), Bingham County (24,600).
Is arsenic a problem in Idaho wells?+
USGS estimates about 47,000 Idaho well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Canyon County, Twin Falls County, Payette 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 Idaho?+
We could not confirm a statewide rule for Idaho. 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).