Manganese is widespread in Cook County wells.
1 widespread, 1 rare out of 2 health checks.
From 11 wells tested in Cook County since 2000; your own well needs its own test. How to read this
Not charted, because fewer than 10 wells were tested: nitrate (4), arsenic (6), uranium (6), radon (3), lead (6).
Rare: under 5%Worth checking: 5 to 15%Widespread: 15%+
Lab results: Water Quality Portal (USGS and EPA)
Who drinks from a well here
About 1 in 102 people 51,542 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 Cook County compares with Illinois and the counties around it.
Each result puts Cook County next to the 8 counties that border it, on the chart's 0 to 50% scale.
This one is widespread in Cook County wells.
Manganese3 of 11 wells over
Too few tested to show: Berrien, MI, DuPage, Kane, Lake, Porter, IN, Will.
This one is rare in Cook County.
Too few Cook County wells were tested for these 5.
Nitrate4 wells tested
Arsenic6 wells tested
Uranium6 wells tested
Radon in water3 wells tested
Lead6 wells tested
None of the 1 Cook County wells tested for bacteria had it, but every well needs a yearly test.
Coliform bacteria0 of 1 wells had it
E. coli0 of 1 wells had it
E. coli means human or animal waste is getting in, so if a test of your Cook County well finds it, boil drinking water until the well is fixed.
Iron reached staining levels in 6 of 11 tested wells, and it isn't a health risk.
Iron stains6 of 11 wells had it
Too few tested to show: Berrien, MI, DuPage, Kane, Lake, Porter, IN, Will.
Cook County has 11 tested wells, and the busiest decade for testing was the 2010s.
Only 1 of the 11 wells have a test from 2020 or later, so a fresh test of your own well tells you more than any of these.
Most Cook 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 Cook County.
- Bacteria
- Nitrate
- Manganese
- Arsenic
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 200 feet.
That comes from 6,775 household wells in Illinois's drilling records. Most wells of every kind here are 155 to 250 feet deep. Wells as deep as Cook 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 77 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 Cook County.
Cook 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 114 town water systems in Cook 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 5.5 ng/L of PFOS. No well in Cook County has 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 Cook County
Is well water in Cook County safe to drink?
3 of 11 wells tested in Cook County had manganese over the limit, about 3 in 10. Across Illinois it's about 1 in 8. That's a bigger share than in 5 of the 6 other Illinois counties with 10 or more tested wells. So test your own well for manganese before you count on it.
Where can I get my well tested in Cook 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, manganese and arsenic, the tests that matter most in Cook County.
How deep are wells in Cook County?
Half the household wells here are shallower than 200 feet. Typical household wells here go deeper than in 77 of the 101 other Illinois counties with enough drilling records.
How does Cook 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 Cook County, at 27%. Lake, IN had the most, at 57%. Groundwater doesn't stop at county lines, so Lake, IN's 57% is worth knowing about if you live near it.
How often should I test my well in Cook County?
None of the 1 wells tested for bacteria in Cook County had it, but the CDC still says to 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 Cook County numbers come from
- People on wells: 51,542 in Cook County, from EPA estimates built on the 2020 Census (Murray et al.).
- Lab tests: 107 well water samples from 11 wells in the Water Quality Portal, run by the USGS and EPA, 2000 to 2021.
- Well depths: 6,775 Illinois drilling records for Cook County, gathered in the US Groundwater Well Database (Lin and others, 2024).
- Neighboring counties: the 8 counties that share a border with Cook County, from the Census Bureau's county adjacency file.
- PFAS: EPA tests of 114 town water systems in Cook County under the Fifth Unregulated Contaminant Monitoring Rule, 2023 to 2025.
- Radon: Cook County is zone 2 on the EPA Map of Radon Zones.
- State programs and links: Illinois Department of Public Health, checked November 1, 2026.