Nitrate and uranium are widespread in Fresno County wells.
2 widespread, 2 worth checking, 3 rare out of 7 health checks.
From 149 wells tested in Fresno 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
1 in 7 people 149,804 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 Fresno County compares with California and the counties around it.
Each result puts Fresno County next to the 8 counties that border it, on the chart's 0 to 50% scale.
These 2 are widespread in Fresno County wells.
Nitrate43 of 149 wells over
Uranium37 of 147 wells over
These 2 are worth checking in Fresno County.
Arsenic11 of 148 wells over
Manganese10 of 147 wells over
These 3 are rare in Fresno County.
Radon in water1 of 60 wells over
Too few tested to show: Inyo, Kings, Merced, Mono, San Benito.
Fluoride0 of 149 wells over
Lead0 of 112 wells over
Coliform bacteria turned up in 24 of 60 tested Fresno County wells, so test for it every year.
Coliform bacteria24 of 60 wells had it
Too few tested to show: Inyo, Mono.
E. coli1 of 60 wells had it
Too few tested to show: Inyo, Mono.
E. coli means human or animal waste is getting in, so if a test of your Fresno County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 6 of 148 tested wells, and it isn't a health risk.
Iron stains6 of 148 wells had it
Fresno County has 149 tested wells, and the busiest decade for testing was the 2010s.
Only 16 of the 149 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Fresno 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 5 things to check in Fresno County.
- Bacteria
- Nitrate
- Uranium
- Arsenic
- Manganese
What California offers well owners
- Free testingThe State and Regional Water Boards fund or regulate 11 domestic well programs that offer free well testing in their service areas. See the program
- Certified labsCalifornia's list of certified labs
- Well recordsCalifornia's well record search
Half the household wells here are shallower than 180 feet.
That comes from 24,025 household wells in California's drilling records. Most wells of every kind here are 135 to 340 feet deep. Wells as deep as Fresno 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 are shallower than in 36 of the 56 other California counties with enough drilling records.
Look up your own well's depth in California's well record search, or see how to find it.
Well depths: US Groundwater Well Database
Here's what we know about radon and PFAS in Fresno County.
Fresno 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 20 town water systems in Fresno County test for PFAS. 6 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 41 ng/L of PFOS. Only 10 wells in Fresno 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 Fresno County
Is well water in Fresno County safe to drink?
43 of 149 wells tested in Fresno County had nitrate over the limit, about 3 in 10. Across California it's about 1 in 10. That's a bigger share than in 44 of the 46 other California counties with 10 or more tested wells. So test your own well for nitrate before you count on it.
Can I get my well tested for free in Fresno County?
The State and Regional Water Boards fund or regulate 11 domestic well programs that offer free well testing in their service areas. If not, use a lab California has certified for drinking water. Ask for bacteria, nitrate, uranium, arsenic and manganese, the tests that matter most in Fresno County.
How deep are wells in Fresno County?
Half the household wells here are shallower than 180 feet. Typical household wells here are shallower than in 36 of the 56 other California counties with enough drilling records.
How does Fresno County compare with the counties around it?
For nitrate, the share of tested wells over the limit: 2 of the 8 neighboring counties with data had a higher share than Fresno County, at 29%. Tulare had the most, at 37%. Groundwater doesn't stop at county lines, so Tulare's 37% is worth knowing about if you live near it.
How often should I test my well in Fresno County?
In Fresno County, 24 of 60 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 Fresno County numbers come from
- People on wells: 149,804 in Fresno County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 2,265 well water samples from 149 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2023.
- Well depths: 24,025 California drilling records for Fresno County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 8 counties that share a border with Fresno County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 20 town water systems in Fresno County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Fresno County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.