Arsenic is widespread in McHenry County wells.
1 widespread, 2 worth checking, 4 rare out of 7 health checks.
From 74 wells tested in McHenry 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 4 people 79,789 people
Nobody else tests these wells. Only you do.
People on wells: EPA researchers' 2020 estimate
How deep wells go here
Middle depth: some shelter from the surface, not complete.
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 McHenry County compares with Illinois and the counties around it.
Each result puts McHenry County next to the 7 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in McHenry County wells.
Arsenic13 of 58 wells over
These 2 are worth checking in McHenry County.
Manganese4 of 71 wells over
Too few tested to show: DeKalb, Kane, Kenosha, WI, Lake, Walworth, WI.
Nitrate4 of 74 wells over
Too few tested to show: Boone, Cook, DeKalb, Kane, Kenosha, WI, Lake.
These 4 are rare in McHenry County.
Radon in water0 of 14 wells over
Lead0 of 58 wells over
Fluoride0 of 71 wells over
Uranium0 of 58 wells over
Every well needs a bacteria test every year.
No bacteria tests from McHenry County wells are in the public data. Test every year anyway.
Iron reached staining levels in 62 of 71 tested wells, and it isn't a health risk.
Iron stains62 of 71 wells had it
Too few tested to show: DeKalb, Kane, Kenosha, WI, Lake, Walworth, WI.
About 6 in 10 of the 74 wells tested in McHenry County have a test from 2020 or later.
With 47 wells tested since 2020, age matters less here than who did the testing and why.
Most McHenry 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 McHenry County.
- Bacteria
- Nitrate
- Arsenic
- Manganese
What Illinois offers well owners
- Free testingWe didn't find a free program for all of Illinois. Illinois's private well page
- Certified labsIllinois's list of certified labs
- Well recordsIllinois's well record search
Half the household wells here are shallower than 125 feet.
That comes from 19,801 household wells in Illinois's drilling records. Most wells of every kind here are 75 to 200 feet deep. Wells as deep as McHenry 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 50 of the 101 other Illinois counties with enough drilling records.
Look up your own well's depth in Illinois'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 McHenry County.
McHenry 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 12 town water systems in McHenry County test for PFAS. 1 of them had at least one test above 4 ng/L, the new EPA limit for PFOA and PFOS. The highest single result was 8.3 ng/L of PFOA. Only 3 wells in McHenry 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 McHenry County
Is well water in McHenry County safe to drink?
13 of 58 wells tested in McHenry County had arsenic over the limit, about 2 in 10. Across Illinois it's about 2 in 10. So test your own well for arsenic before you count on it.
Where can I get my well tested in McHenry County?
Use a lab Illinois has certified for drinking water. Illinois's list is linked above, in the box about testing. Ask for bacteria, nitrate, arsenic and manganese, the tests that matter most in McHenry County.
How deep are wells in McHenry County?
Half the household wells here are shallower than 125 feet. Typical household wells here go deeper than in 50 of the 101 other Illinois counties with enough drilling records.
How does McHenry County compare with the counties around it?
For manganese, the share of tested wells over the limit: 1 of the 2 neighboring counties with data had a higher share than McHenry County, at 6%. Cook had the most, at 27%. Groundwater doesn't stop at county lines, so Cook's 27% is worth knowing about if you live near it.
How often should I test my well in McHenry County?
McHenry County has no public bacteria results for wells, so follow the CDC's advice: 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 McHenry County numbers come from
- People on wells: 79,789 in McHenry County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 1,074 well water samples from 74 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2021.
- Well depths: 19,801 Illinois drilling records for McHenry County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 7 counties that share a border with McHenry County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 12 town water systems in McHenry County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: McHenry County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Illinois Department of Public Health, checked November 1, 2026.