Nitrate and uranium are widespread in Stanislaus County wells.
2 widespread, 1 worth checking, 4 rare out of 7 health checks.
From 152 wells tested in Stanislaus 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 8 people 68,139 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)
Less likely, but the EPA still says test your air.
Radon zones: EPA map
Here's how Stanislaus County compares with California and the counties around it.
Each result puts Stanislaus County next to the 7 counties that border it, on the chart's 0 to 50% scale.
These 2 are widespread in Stanislaus County wells.
Nitrate35 of 152 wells over
Too few tested to show: Mariposa, Tuolumne.
Uranium21 of 140 wells over
Too few tested to show: Calaveras, Mariposa, Tuolumne.
This one is worth checking in Stanislaus County.
Arsenic8 of 139 wells over
Too few tested to show: Mariposa, Tuolumne.
These 4 are rare in Stanislaus County.
Manganese5 of 141 wells over
Too few tested to show: Mariposa, Tuolumne.
Lead1 of 131 wells over
Too few tested to show: Calaveras, Mariposa, Tuolumne.
Radon in water0 of 23 wells over
Too few tested to show: Calaveras, Mariposa, Merced, Tuolumne.
Fluoride0 of 105 wells over
Too few tested to show: Mariposa, Tuolumne.
Coliform bacteria turned up in 9 of 49 tested Stanislaus County wells, so test for it every year.
Coliform bacteria9 of 49 wells had it
Too few tested to show: Alameda, Calaveras, Mariposa, Tuolumne.
E. coli0 of 49 wells had it
Too few tested to show: Alameda, Calaveras, Mariposa, Tuolumne.
E. coli means human or animal waste is getting in, so if a test of your Stanislaus County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 3 of 105 tested wells, and it isn't a health risk.
Iron stains3 of 105 wells had it
Too few tested to show: Mariposa, Tuolumne.
Most of the 152 wells tested in Stanislaus County were last tested before 2010.
An old result shows the water as it was then, so treat these 152 wells as a starting point and test your own.
Most Stanislaus 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 4 things to check in Stanislaus County.
- Bacteria
- Nitrate
- Uranium
- 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 172 feet.
That comes from 9,084 household wells in California's drilling records. Most wells of every kind here are 130 to 270 feet deep. Wells as deep as Stanislaus 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 39 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 Stanislaus County.
Stanislaus County is in the EPA's lowest radon zone, so radon from well water is unlikely to be a problem. Radon guide
Between 2023 and 2025 the EPA had 15 town water systems in Stanislaus 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 26 ng/L of PFOS. Only 49 wells in Stanislaus 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 Stanislaus County
Is well water in Stanislaus County safe to drink?
35 of 152 wells tested in Stanislaus County had nitrate over the limit, about 1 in 4. Across California it's about 1 in 10. That's a bigger share than in 43 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 Stanislaus 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 and arsenic, the tests that matter most in Stanislaus County.
How deep are wells in Stanislaus County?
Half the household wells here are shallower than 172 feet. Typical household wells here are shallower than in 39 of the 56 other California counties with enough drilling records.
How does Stanislaus County compare with the counties around it?
For nitrate, the share of tested wells over the limit: 1 of the 5 neighboring counties with data had a higher share than Stanislaus County, at 23%. Merced had the most, at 30%. Groundwater doesn't stop at county lines, so Merced's 30% is worth knowing about if you live near it.
How often should I test my well in Stanislaus County?
In Stanislaus County, 9 of 49 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 Stanislaus County numbers come from
- People on wells: 68,139 in Stanislaus County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,879 well water samples from 152 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2021.
- Well depths: 9,084 California drilling records for Stanislaus County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 7 counties that share a border with Stanislaus County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 15 town water systems in Stanislaus County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Stanislaus County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.