Nitrate is widespread in Merced County wells.
1 widespread, 3 worth checking, 2 rare out of 6 health checks.
From 89 wells tested in Merced County since 2001; your own well needs its own test. How to read this
Not charted, because fewer than 10 wells were tested: radon (7).
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 6 people 50,415 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 Merced County compares with California and the counties around it.
Each result puts Merced County next to the 6 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in Merced County wells.
Nitrate26 of 88 wells over
Too few tested to show: Mariposa.
These 3 are worth checking in Merced County.
Arsenic11 of 74 wells over
Too few tested to show: Mariposa.
Manganese12 of 86 wells over
Too few tested to show: Mariposa.
Uranium8 of 74 wells over
Too few tested to show: Mariposa.
These 2 are rare in Merced County.
Fluoride0 of 87 wells over
Too few tested to show: Mariposa.
Lead0 of 69 wells over
Too few tested to show: Mariposa.
Too few Merced County wells were tested for this one.
Radon in water7 wells tested
Coliform bacteria turned up in 7 of 36 tested Merced County wells, so test for it every year.
Coliform bacteria7 of 36 wells had it
Too few tested to show: Mariposa.
E. coli0 of 36 wells had it
Too few tested to show: Mariposa.
E. coli means human or animal waste is getting in, so if a test of your Merced County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 3 of 87 tested wells, and it isn't a health risk.
Iron stains3 of 87 wells had it
Too few tested to show: Mariposa.
About 4 in 10 of the 89 wells tested in Merced County have a test from 2020 or later.
With 35 wells tested since 2020, age matters less here than who did the testing and why.
Most Merced 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 Merced County.
- Bacteria
- Nitrate
- Arsenic
- Manganese
- Uranium
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 200 feet.
That comes from 7,643 household wells in California's drilling records. Most wells of every kind here are 140 to 302 feet deep. Wells as deep as Merced 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 27 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 Merced County.
Merced 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 11 town water systems in Merced County test for PFAS. 5 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 45 ng/L of PFOS. Only 35 wells in Merced 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 Merced County
Is well water in Merced County safe to drink?
26 of 88 wells tested in Merced 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 45 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 Merced 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, arsenic, manganese and uranium, the tests that matter most in Merced County.
How deep are wells in Merced County?
Half the household wells here are shallower than 200 feet. Typical household wells here are shallower than in 27 of the 56 other California counties with enough drilling records.
How does Merced County compare with the counties around it?
For nitrate, the share of tested wells over the limit: Merced County, at 30%, had a higher share than all 5 neighboring counties with data. Groundwater doesn't stop at county lines, so Fresno's 29% is worth knowing about if you live near it.
How often should I test my well in Merced County?
In Merced County, 7 of 36 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 Merced County numbers come from
- People on wells: 50,415 in Merced County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,179 well water samples from 89 wells in the Water Quality Portal, run by the USGS and EPA, 2001 to 2023.
- Well depths: 7,643 California drilling records for Merced County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 6 counties that share a border with Merced County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 11 town water systems in Merced County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Merced County is zone 3 on the EPA Map of Radon Zones.
- State programs and links: State Water Resources Control Board, checked November 1, 2026.