Uranium, arsenic and 2 more are worth checking in Pima County wells.
4 worth checking, 3 rare out of 7 health checks.
From 173 wells tested in Pima County since 2000; your own well needs its own test. How to read this
Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+
Lab results: Water Quality Portal (USGS and EPA)
Who drinks from a well here
About 1 in 15 people 70,129 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Deep wells: fewer surface germs, more minerals from the rock.
Well depths: US Groundwater Well Database
Radon risk (a gas from rock)
Possible here. A home air test is inexpensive.
Radon zones: EPA map
Here's how Pima County compares with Arizona and the counties around it.
Each result puts Pima County next to the 6 counties that border it, on the chart's 0 to 50% scale.
These 4 are worth checking in Pima County.
Uranium3 of 27 wells over
Too few tested to show: Graham, Pinal, Santa Cruz, Yuma.
Arsenic13 of 134 wells over
Manganese14 of 160 wells over
These 3 are rare in Pima County.
Radon in water2 of 48 wells over
Nitrate2 of 107 wells over
Lead1 of 134 wells over
Coliform bacteria turned up in 4 of 12 tested Pima County wells, so test for it every year.
Coliform bacteria4 of 12 wells had it
Too few tested to show: Cochise, Graham, Pinal, Santa Cruz, Yuma.
E. coli0 of 10 wells had it
Too few tested to show: Cochise, Graham, Pinal, Santa Cruz, Yuma.
E. coli means human or animal waste is getting in, so if a test of your Pima County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 4 of 144 tested wells, and it isn't a health risk.
Iron stains4 of 144 wells had it
Most of the 173 wells tested in Pima County were last tested before 2010.
An old result shows the water as it was then, so treat these 173 wells as a starting point and test your own.
Most Pima 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 6 things to check in Pima County.
- Bacteria
- Nitrate
- Uranium
- Arsenic
- Manganese
- Fluoride
What Arizona offers well owners
- Free testingWe didn't find a free program for all of Arizona. Arizona's private well page
- Certified labsArizona's list of certified labs
- Well recordsWe didn't find an online search.
Half the household wells here are shallower than 380 feet.
That comes from 6,130 household wells in Arizona's drilling records. Most wells of every kind here are 200 to 500 feet deep. Wells as deep as Pima County's are usually better protected from fertilizer and bacteria at the surface, but they can pick up more of what's in the rock, like arsenic and uranium. Typical household wells here go deeper than in 12 of the 14 other Arizona counties with enough drilling records.
How to find your own well's depth
Well depths: US Groundwater Well Database
Here's what we know about radon and PFAS in Pima County.
Pima 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 28 town water systems in Pima County test for PFAS. 4 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 23 ng/L of PFOS. Only 97 wells in Pima County have 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 Pima County
Is well water in Pima County safe to drink?
3 of 27 wells tested in Pima County had uranium over the limit, about 1 in 9. Across Arizona it's about 1 in 10. That's a bigger share than in 4 of the 6 other Arizona counties with 10 or more tested wells. So test your own well for uranium before you count on it.
Where can I get my well tested in Pima County?
Use a lab Arizona has certified for drinking water. Arizona's list is linked above, in the box about testing. Ask for bacteria, nitrate, uranium, arsenic, manganese and fluoride, the tests that matter most in Pima County.
How deep are wells in Pima County?
Half the household wells here are shallower than 380 feet. Typical household wells here go deeper than in 12 of the 14 other Arizona counties with enough drilling records.
How does Pima County compare with the counties around it?
For uranium, the share of tested wells over the limit: 1 of the 2 neighboring counties with data had a higher share than Pima County, at 11%. Maricopa had the most, at 17%. Groundwater doesn't stop at county lines, so Maricopa's 17% is worth knowing about if you live near it.
How often should I test my well in Pima County?
In Pima County, 4 of 12 tested wells had coliform bacteria, so the CDC's advice matters here: 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 Pima County numbers come from
- People on wells: 70,129 in Pima County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,034 well water samples from 173 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2026.
- Well depths: 6,130 Arizona drilling records for Pima County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with Pima County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 28 town water systems in Pima County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Pima County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Department of Health Services, checked November 1, 2026.