Quick answer
About 315,000 people in Idaho (20.1% of residents) got their water from private wells in 2010, according to USGS estimates. Idaho Department of Health and Welfare runs the state’s private well guidance. IDHW's Free Idaho Private Well Water Testing program mails free test kits (nitrate, arsenic, uranium, lead, fluoride, manganese and other metals) to residents who sign up through Idaho Health Tools, while funding is available; the page lists it as available fall 2026.
- People on private wells
- 315,241
- Share of residents
- 20.1%
- Rank among states
- 35th
- Well users modeled above 10 µg/L arsenic
- 16.4%
Where to get your well tested in Idaho
- Idaho Department of Health and Welfare: Drinking Water and Private Wells, the state’s guidance for private well owners.
- Certified laboratories in Idaho. Use a lab certified for drinking water, so results are reliable.
Free or low-cost testing
IDHW's Free Idaho Private Well Water Testing program mails free test kits (nitrate, arsenic, uranium, lead, fluoride, manganese and other metals) to residents who sign up through Idaho Health Tools, while funding is available; the page lists it as available fall 2026. (source)
Mail-in lab kits are another option when no local lab is convenient. See well water test kits for what home kits can and can’t tell you.
What to test for
- Every year: total coliform bacteria and nitrate.
- Before buying equipment: pH, hardness, iron, manganese and, if there’s a smell, hydrogen sulfide.
- Based on location: arsenic, uranium and radon, PFAS, and lead in older plumbing.
USGS modeling estimates that about 16.4% of Idaho’s private well users have water above the 10 µg/L arsenic limit. Arsenic has no taste or smell, so include it in your first test.
Private wells by county
Estimated people relying on private wells in each Idaho county in 2010, the share of residents that represents, and the modeled share of well users with arsenic above 10 µg/L. These are USGS model estimates (2010 population), not measurements of any well. Arsenic is hidden for counties with fewer than 500 well users.
| County | People on wells | Share | Arsenic >10 µg/L |
|---|---|---|---|
| Canyon County | 34,625 | 18.3% | 34.0% |
| Ada County | 30,587 | 7.8% | 16.8% |
| Bingham County | 20,577 | 45.1% | 7.4% |
| Twin Falls County | 19,323 | 25.0% | 49.9% |
| Bonner County | 16,221 | 39.7% | 7.6% |
| Bonneville County | 12,511 | 12.0% | 7.5% |
| Jefferson County | 12,192 | 46.6% | 11.5% |
| Kootenai County | 12,079 | 8.7% | 11.1% |
| Madison County | 10,064 | 26.8% | 8.9% |
| Minidoka County | 9,930 | 49.5% | 6.8% |
| Cassia County | 9,788 | 42.6% | 10.6% |
| Jerome County | 9,014 | 40.3% | 8.4% |
Show all 44 counties
| Payette County | 8,206 | 36.3% | 35.5% |
| Elmore County | 8,176 | 30.2% | 15.8% |
| Bannock County | 7,949 | 9.6% | 8.3% |
| Gem County | 7,478 | 44.7% | 24.9% |
| Latah County | 7,329 | 19.7% | 7.0% |
| Gooding County | 6,508 | 42.1% | 17.1% |
| Idaho County | 6,048 | 37.2% | 4.5% |
| Fremont County | 5,821 | 44.0% | 9.0% |
| Owyhee County | 5,643 | 49.0% | 45.2% |
| Blaine County | 5,512 | 25.8% | 5.0% |
| Valley County | 5,355 | 54.3% | 5.2% |
| Teton County | 5,266 | 51.8% | 5.0% |
| Lemhi County | 4,394 | 55.4% | 7.0% |
| Nez Perce County | 3,388 | 8.6% | 6.1% |
| Washington County | 3,343 | 32.8% | 17.0% |
| Boise County | 2,746 | 39.1% | 5.0% |
| Power County | 2,453 | 31.4% | 7.6% |
| Benewah County | 2,376 | 25.6% | 9.3% |
| Boundary County | 2,129 | 19.4% | 7.4% |
| Clearwater County | 2,078 | 23.7% | 4.6% |
| Lincoln County | 2,051 | 39.4% | 6.0% |
| Custer County | 1,946 | 44.5% | 4.0% |
| Oneida County | 1,837 | 42.9% | 5.0% |
| Adams County | 1,826 | 45.9% | 5.0% |
| Franklin County | 1,663 | 13.0% | 11.7% |
| Caribou County | 1,471 | 21.1% | 7.3% |
| Butte County | 1,355 | 46.9% | 10.0% |
| Shoshone County | 1,211 | 9.5% | 6.7% |
| Bear Lake County | 1,153 | 19.3% | 6.4% |
| Lewis County | 677 | 17.7% | 5.0% |
| Camas County | 636 | 56.9% | 9.0% |
| Clark County | 306 | 31.2% | – |
Counties with the highest modeled arsenic share: Twin Falls County (49.9%), Owyhee County (45.2%), Payette County (35.5%), Canyon County (34.0%), Gem County (24.9%).
After you get results
Enter your numbers in the test results tool to see what needs treating and in what order. The guide to reading a well water test explains units and limits.
Common questions
Does Idaho offer free well water testing?
IDHW's Free Idaho Private Well Water Testing program mails free test kits (nitrate, arsenic, uranium, lead, fluoride, manganese and other metals) to residents who sign up through Idaho Health Tools, while funding is available; the page lists it as available fall 2026.
Who oversees private wells in Idaho?
Idaho Department of Health and Welfare (Drinking Water and Private Wells) publishes guidance for private well owners. Private wells aren’t covered by the federal Safe Drinking Water Act, so testing and treatment are up to the owner.
How many people use private wells in Idaho?
USGS estimates about 315,000 people, or 20.1% of the population in 2010. That ranks 35th of the states covered by the estimate.
What should I test my well for in Idaho?
Test every year for total coliform bacteria and nitrate, plus pH, hardness, iron and manganese before buying equipment. Arsenic is a known concern in parts of Idaho, so add it to the first test.
Sources
- Idaho Department of Health and Welfare: Drinking Water and Private Wells
- Idaho certified laboratories
- Idaho free or low-cost testing
- USGS data release: domestic well locations and populations served, 2000 and 2010 (Johnson, Belitz and Lombard, 2019)
- CDC Environmental Public Health Tracking: private well water data (from USGS)