Well water in Illinois

Name: Private well water quality in Illinois
Creator: Well Water Nerd
License: Public domain (US federal government data)

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Testing recommended

tested well samples in Illinois exceeded the EPA limit for PFOA (4 ng/L), based on public data from USGS Water Quality Portal.

60,373 test results across 15 contaminants from at least 6,644 wells, 1901 to 2026; nitrate has the most at 10,010. Data refreshed September 15, 2026.

Compiled by Nick at Well Water Nerd from public records. How these pages are built

An estimated 1,151,113 people in Illinois get their drinking water from a private well, per USGS 2015 county water use.

Separately, the USGS national arsenic model estimates that 6% of the 1,151,113 people on private wells in Illinois draw water above 10 ug/L (the model's range is 5% to 7%), from geology and well depth rather than local tests.

What individual samples found

Each row summarizes every public sample for that contaminant inside Illinois. Above limit means above the EPA maximum contaminant level; where EPA has no enforceable limit, the row shows a health guideline or nuisance level instead.

Contaminant	Above limit	Samples	Detected	Median	Highest	Limit
PFOA 2019 to 2021	17%	30	37%	0	17	4 ng/L EPA limit
Radium (226 + 228) 1999 to 2019	12%	350	86%	0.72	22	5 pCi/L EPA limit
Arsenic 1973 to 2023	10%	5,098	45%	0	128	10 ug/L EPA limit
Nitrate 1901 to 2026	9%	10,010	63%	0.1	1,170	10 mg/L EPA limit
E. coli 2007 to 2019	4%	75	4%	0	7	0 MPN/100mL EPA limit
PFOS 2019 to 2021	3%	30	13%	0	16	4 ng/L EPA limit
Lead 1971 to 2023	2%	5,517	21%	0	2,000	15 ug/L EPA limit
Fluoride 1931 to 2025	<1%	6,151	97%	0.4	15.5	4 mg/L EPA limit
Copper 1971 to 2023	<1%	5,523	34%	0	13	1.3 mg/L EPA limit
Uranium 1979 to 2023	0%	612	60%	0.051	17	30 ug/L EPA limit
Iron 1901 to 2025	no limit	6,925	89%	260	587,000	300 ug/L nuisance level
Chloride 1901 to 2025	no limit	6,737	97%	16	37,000	250 mg/L nuisance level
Manganese 1914 to 2025	no limit	6,624	86%	20	36,000	300 ug/L health guideline
Sodium 1901 to 2025	no limit	6,462	100%	32.7	15,000	20 mg/L health guideline
Radon 1996 to 2019	no limit	229	97%	210	15,400	2,000 pCi/L health guideline

Over time

Share of samples above the limit by period. A cell needs ten samples to show; a comparison needs twenty in each period. Later periods sample fewer wells, and often the ones already known to have a problem.

Contaminant	before 2000	2000s	2010s	2020s
Nitrate	8%8,254 samples	11%1,085 samples	12%553 samples	12%118 samples
Fluoride	<1%5,622 samples	0%160 samples	<1%269 samples	0%100 samples
Copper	<1%5,091 samples	0%179 samples	0%184 samples	0%69 samples
Lead	2%5,084 samples	0%180 samples	<1%184 samples	0%69 samples
Arsenic	9%4,665 samples	18%180 samples	16%184 samples	17%69 samples
Uranium	0%183 samples	0%176 samples	0%184 samples	0%69 samples

Nitrate, fluoride, copper, lead, arsenic, uranium have held within a few points between the two most recent periods with enough samples.

What this means

The contaminants most often above their limit here are pfoa (17% of 30 samples), radium (226 + 228) (12% of 350 samples), and arsenic (10% of 5,098 samples). A well next door can differ sharply from yours because bedrock fractures, well depth and plumbing all matter, so these rates describe the area, not your tap.

Iron, Chloride, and Manganese are widespread in local groundwater. They are mainly a staining and taste problem, but high manganese also carries a health advisory for infants.

What to test for in Illinois

1
PFOA
17% of 30 local samples were above the limit.
2
Radium (226 + 228)
12% of 350 local samples were above the limit.
3
Arsenic
10% of 5,098 local samples were above the limit.
4
Nitrate
9% of 10,010 local samples were above the limit.
5
E. coli
4% of 75 local samples were above the limit.
6
PFOS
3% of 30 local samples were above the limit.
7
Lead
2% of 5,517 local samples were above the limit.

Test your Illinois well for PFOA, radium (226 + 228) and arsenic

Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.

Order the Tap Score well water test kit, from $209 Already tested? Upload the report

Just want bacteria checked first? Tap Score coliform and E. coli test, $99. Coliform and E. coli are the tests every well needs every year.

Want a quick look before paying for a lab? Home well water test strips on Amazon check hardness, iron, pH and nitrate in minutes. They are a screen, not a result: arsenic, bacteria, uranium and PFAS need a certified lab.

Affiliate links: the merchant pays us a commission and the price is the same for you. It does not change what the data above says. As an Amazon Associate we earn from qualifying purchases.

Where to get a well tested in Illinois: certified labs, state programs and prices.

Already have results back? Get your lab report read against the Illinois numbers, $29. Every line explained, ranked by what matters, with the fix for each.

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Send me the Illinois report

The numbers above, which tests to order and where, and the treatment that fits the local profile, in one email.

The rock under Illinois

Not a principal aquifer

The USGS map places this area outside any principal aquifer, which usually means wells here draw from fractured bedrock or thin local deposits rather than a large regional aquifer. Bedrock wells of that kind are where arsenic, radon and uranium turn up, and yields vary from well to well.

Classification from the USGS Principal Aquifers of the United States (national map, 2003 edition, GIS release 2015), read at the state centroid. A single well can sit in a different unit, especially near a boundary.

Official help in Illinois

Two places to start that are not selling anything: the state program that answers private well questions, and its list of labs certified to run drinking water tests. A certified lab is the difference between a result you can act on and a number.

Private Water & Closed Loop Wells, IDPH
Accredited Labs, Illinois EPA

Treatment that fits the local profile

PFOA

Granular activated carbon whole-house; Point-of-use reverse osmosis; Anion exchange

Radium (226 + 228)

Water softener (cation exchange); Point-of-use reverse osmosis

Arsenic

Point-of-use reverse osmosis; Adsorptive media (iron-based) whole-house; Anion exchange

Nitrate

Point-of-use reverse osmosis; Anion exchange; Distillation

E. coli

Shock chlorination and fix the well cap or casing; UV disinfection whole-house; Chlorination system

Systems we would look at first

Some links below are affiliate links. If you buy through one, the merchant pays us a commission at no extra cost to you. It does not change which products we list or what the data says.

For PFOA

Under-sink reverse osmosis system

under-sink or countertop, about $300

Look for NSF/ANSI 58 certification listing the contaminant you need removed.

For PFOA

Whole-house PFAS carbon system

whole-house, at the point of entry, about $3,000

Granular activated carbon certified to NSF/ANSI 53 for PFOA and PFOS; replace media on schedule.

For PFOA

Aquasana SmartFlow reverse osmosis

under-sink or countertop, about $225

Certified to NSF/ANSI 58 and 53 to reduce arsenic (96%, pentavalent), fluoride (90%), lead (99%), nitrate, nitrite and PFOA/PFOS at the kitchen tap, with a remineralizer.

For arsenic

Whole-house arsenic and uranium filter

whole-house, at the point of entry, about $2,500

Adsorptive iron-based media; sized to your well yield.

For E. coli

UV disinfection system

whole-house, at the point of entry, about $900

Fix the well cap and casing first; UV keeps bacteria from returning.

For E. coli

Aquasana Rhino Well Water with UV

whole-house, at the point of entry, about $2,871

Whole-house system built for wells: sediment pre-filter, KDF and carbon, and a UV lamp rated to kill 99.99% of bacteria and viruses. It does not remove arsenic, nitrate, uranium or radon.

Common questions

Is there arsenic in well water in Illinois?

Arsenic was detected in 45% of 5,098 public well samples in Illinois, and 10% were above the EPA limit of 10 ug/L. Arsenic has no taste or smell; a lab test is the only way to know your level.

How high is radon in well water in Illinois?

The median radon level across 229 public samples in Illinois was 210 pCi/L, with a high of 15,400 pCi/L. New Hampshire recommends treatment above 2,000 pCi/L.

How often should I test a private well in Illinois?

Test for bacteria and nitrate every year. Test once for arsenic, uranium, radon, fluoride, manganese and PFAS, then again every three to five years or after any change in taste, color, or plumbing.

Counties in Illinois

Want the numbers themselves? Download the Illinois dataset as CSV, every county and town, with the sources named.

Sources

USGS Water Quality Portal: 60,373 samples in Illinois. Data from the Water Quality Portal, a service of USGS, EPA and USDA; groundwater samples collected by USGS and cooperators. Retrieved September 3, 2026.
USGS national arsenic model (Ayotte 2017): estimated share of 1,151,113 people on private wells in Illinois above 10 ug/L of arsenic, from geology, well depth and land use. County estimates of domestic well users with arsenic above 10 ug/L from the USGS national logistic regression model (Ayotte, Medalie and Qi, 2017), with private well population from the USGS 2015 county water-use compilation (Dieter and others, 2018). Retrieved September 15, 2026.

Non-detects count as samples and are treated as zero when computing medians. Exactly at the limit counts as not above it. A sample belongs to the town containing the well's reported coordinates.