Uranium, radon and 2 more are worth checking in Madera County wells.
4 worth checking, 3 rare out of 7 health checks.
From 87 wells tested in Madera County since 2001; 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 3 people 47,219 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 Madera County compares with California and the counties around it.
Each result puts Madera County next to the 5 counties that border it, on the chart's 0 to 50% scale.
These 4 are worth checking in Madera County.
Uranium12 of 87 wells over
Too few tested to show: Mariposa, Tuolumne.
Radon in water5 of 39 wells over
Too few tested to show: Mariposa, Merced, Mono, Tuolumne.
Arsenic9 of 87 wells over
Too few tested to show: Mariposa, Tuolumne.
Nitrate8 of 86 wells over
Too few tested to show: Mariposa, Tuolumne.
These 3 are rare in Madera County.
Manganese0 of 87 wells over
Too few tested to show: Mariposa, Tuolumne.
Fluoride0 of 86 wells over
Lead0 of 64 wells over
Too few tested to show: Mariposa, Tuolumne.
Coliform bacteria turned up in 4 of 39 tested Madera County wells, so test for it every year.
Coliform bacteria4 of 39 wells had it
Too few tested to show: Mariposa, Mono, Tuolumne.
E. coli0 of 39 wells had it
Too few tested to show: Mariposa, Mono, Tuolumne.
E. coli means human or animal waste is getting in, so if a test of your Madera County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 4 of 87 tested wells, and it isn't a health risk.
Iron stains4 of 87 wells had it
Too few tested to show: Mariposa, Tuolumne.
Madera County has 87 tested wells, and the busiest decade for testing was the 2000s.
Only 10 of the 87 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Madera 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 Madera County.
- Bacteria
- Nitrate
- Uranium
- Radon in water
- Arsenic
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 356 feet.
That comes from 11,965 household wells in California's drilling records. Most wells of every kind here are 282 to 550 feet deep. Wells as deep as Madera 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. That's the deepest typical depth for household wells among the 57 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 Madera County.
Madera 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 7 town water systems in Madera County test for PFAS. 2 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 7.9 ng/L of PFOS. Only 10 wells in Madera 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 Madera County
Is well water in Madera County safe to drink?
12 of 87 wells tested in Madera County had uranium over the limit, about 1 in 7. Across California it's about 1 in 13. That's a bigger share than in 35 of the 43 other California counties with 10 or more tested wells. So test your own well for uranium before you count on it.
Can I get my well tested for free in Madera 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, radon in water and arsenic, the tests that matter most in Madera County.
How deep are wells in Madera County?
Half the household wells here are shallower than 356 feet. That's the deepest typical depth for household wells among the 57 California counties with enough drilling records.
How does Madera County compare with the counties around it?
For uranium, the share of tested wells over the limit: 2 of the 3 neighboring counties with data had a higher share than Madera County, at 14%. Mono had the most, at 36%. Groundwater doesn't stop at county lines, so Mono's 36% is worth knowing about if you live near it.
How often should I test my well in Madera County?
In Madera County, 4 of 39 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 Madera County numbers come from
- People on wells: 47,219 in Madera County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,060 well water samples from 87 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2023.
- Well depths: 11,965 California drilling records for Madera County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 5 counties that share a border with Madera County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 7 town water systems in Madera County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Madera County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.