Arsenic and nitrate are worth checking in Ada County wells.
2 worth checking, 3 rare out of 5 health checks.
From 170 wells tested in Ada County since 2000; your own well needs its own test. How to read this
Not charted, because fewer than 10 wells were tested: uranium (5), radon (1).
Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+
Lab results: Water Quality Portal (USGS and EPA)
Who drinks from a well here
1 in 9 people 52,199 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
Radon risk (a gas from rock)
Possible here. A home air test is inexpensive.
Radon zones: EPA map
Here's how Ada County compares with Idaho and the counties around it.
Each result puts Ada County next to the 5 counties that border it, on the chart's 0 to 50% scale.
These 2 are worth checking in Ada County.
Arsenic22 of 151 wells over
Nitrate11 of 167 wells over
These 3 are rare in Ada County.
Manganese5 of 162 wells over
Fluoride1 of 165 wells over
Lead0 of 44 wells over
Too few tested to show: Boise.
Too few Ada County wells were tested for these 2.
Uranium5 wells tested
Radon in water1 well tested
Every well needs a bacteria test every year.
No bacteria tests from Ada County wells are in the public data. Test every year anyway.
Iron reached staining levels in 16 of 162 tested wells, and it isn't a health risk.
Iron stains16 of 162 wells had it
Most of the 170 wells tested in Ada County were last tested before 2010.
An old result shows the water as it was then, so treat these 170 wells as a starting point and test your own.
Most Ada County wells in these records were tested for research or because someone suspected a problem, which can push the shares above what a typical home well would show (how we read the numbers).
One mail-in lab test covers the 3 things to check in Ada County.
- Bacteria
- Nitrate
- Arsenic
What Idaho offers well owners
- Free testingDHW offers free private well water testing kits through Idaho Health Tools, limited to one kit per household every five years. See the program
- Certified labsIdaho's list of certified labs
- Well recordsIdaho's well record search
Here's what we know about radon and PFAS in Ada County.
Ada County is in the EPA's middle radon zone. An indoor air test is cheap and tells you whether to look at your water. Radon guide
Between 2023 and 2025 the EPA had 9 town water systems in Ada County test for PFAS. 3 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 28 ng/L of PFOS. No well in Ada County has public PFAS results, so the town systems are the best clue to what's in the groundwater nearby. PFAS guide
Radon zones: EPA Map of Radon Zones. PFAS: EPA tests of town water systems, 2023 to 2025
Questions people ask about well water in Ada County
Is well water in Ada County safe to drink?
22 of 151 wells tested in Ada County had arsenic over the limit, about 1 in 7. Across Idaho it's about 1 in 10. That's a bigger share than in 34 of the 42 other Idaho counties with 10 or more tested wells, leaving out 1 county whose results are likely overstated. So test your own well for arsenic before you count on it.
Can I get my well tested for free in Ada County?
DHW offers free private well water testing kits through Idaho Health Tools, limited to one kit per household every five years. If not, use a lab Idaho has certified for drinking water. Ask for bacteria, nitrate and arsenic, the tests that matter most in Ada County.
How does Ada County compare with the counties around it?
For arsenic, the share of tested wells over the limit: 4 of the 5 neighboring counties with data had a higher share than Ada County, at 15%. Owyhee had the most, at 59%. Groundwater doesn't stop at county lines, so Owyhee's 59% is worth knowing about if you live near it.
How often should I test my well in Ada County?
Ada County has no public bacteria results for wells, so follow the CDC's advice: test for bacteria and nitrate every year, and right away after a flood, a repair, or any change in taste or smell.
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Where the Ada County numbers come from
- People on wells: 52,199 in Ada County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 2,051 well water samples from 170 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2012.
- Neighboring counties: the 5 counties that share a border with Ada County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 9 town water systems in Ada County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Ada County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Department of Health and Welfare, checked November 1, 2026.