Illinois Private Well Water Risk: Arsenic & Nitrate
These figures are USGS statistical estimates for private (domestic) wells in Illinois. They describe the area's modeled risk — not any single well.
Quick answer: Yes — arsenic occurs naturally in Illinois groundwater. USGS modeling estimates about 67,710 Illinois residents rely on private wells with arsenic above the EPA limit of 10 µg/L. The highest modeled exposure is in Will County (~8,733), Kankakee County (~4,374), Kendall County (~3,884). For nitrate, about 0% of Illinois's ~1,120,000 private-well residents are modeled above the 10 mg/L limit. These are area estimates, not a test of your well — arsenic has no taste or smell, so only a certified lab test tells you your level.
This is a modeled probability for the area, not a measurement of your well. USGS statistical models estimate how many private wells in this area are likely to exceed the federal limit — they cannot tell you what is in your water. Two neighbors can have very different results. The only way to know your well is to test it.
Nitrate
0%
of the private-well population is modeled above the 10 mg/L nitrate limit
~296
people on wells estimated above the limit
~1,120,000
total residents on private wells statewide
Source: USGS machine-learning nitrate model, Ransom et al. (2022), domestic-well depth zone.
Find out what's actually in your well
Illinois has one of the higher modeled arsenic exposures in the country — but that number is an area estimate, not your well. Arsenic, nitrate and coliform bacteria are the three most common private-well problems, and none of them has a taste or smell. A certified lab test checks all of them at once; the CDC and EPA recommend testing every well at least once a year.
Basic screening only. Confirm arsenic, nitrate and bacteria with a certified lab.
Find a certified drinking-water labAmazon links are affiliate links — we may earn a commission at no extra cost to you.
Arsenic
Across Illinois, an estimated ~67,710 people rely on private wells with modeled arsenic above the 10 µg/L EPA limit. The highest modeled exposure is in Will County (~8,733 people).
Counties with the highest modeled arsenic exposure
| County | Est. well residents over arsenic limit | 90% range |
|---|---|---|
| Will County | ~8,733 | 7,092–10,811 |
| Kankakee County | ~4,374 | 3,361–5,596 |
| Kendall County | ~3,884 | 3,134–4,816 |
| McHenry County | ~2,904 | 2,420–3,546 |
| Vermilion County | ~2,779 | 2,142–3,548 |
| Sangamon County | ~2,764 | 2,064–3,671 |
| Champaign County | ~2,745 | 2,089–3,531 |
| LaSalle County | ~2,458 | 1,953–3,082 |
| Lake County | ~2,312 | 1,856–2,949 |
| Iroquois County | ~2,138 | 1,680–2,635 |
| Grundy County | ~2,114 | 1,654–2,669 |
| Woodford County | ~1,895 | 1,412–2,484 |
| Livingston County | ~1,819 | 1,411–2,271 |
| Whiteside County | ~1,816 | 1,504–2,248 |
| McLean County | ~1,584 | 1,230–2,020 |
| DeKalb County | ~1,408 | 1,083–1,800 |
| Peoria County | ~1,389 | 1,079–1,782 |
| DuPage County | ~1,104 | 926–1,312 |
| Bureau County | ~1,095 | 831–1,422 |
| Winnebago County | ~1,043 | 888–1,231 |
| Henry County | ~954 | 789–1,156 |
| St. Clair County | ~937 | 738–1,190 |
| Logan County | ~890 | 694–1,132 |
| Ford County | ~742 | 577–924 |
| Edgar County | ~740 | 565–963 |
| Shelby County | ~653 | 503–846 |
| McDonough County | ~628 | 493–795 |
| Douglas County | ~590 | 449–759 |
| Ogle County | ~571 | 478–687 |
| Kane County | ~542 | 436–677 |
| Fulton County | ~541 | 428–685 |
| Boone County | ~524 | 440–623 |
| Piatt County | ~518 | 396–669 |
| Macon County | ~496 | 363–677 |
| De Witt County | ~483 | 371–625 |
| Lee County | ~479 | 388–592 |
| Mercer County | ~461 | 364–582 |
| Tazewell County | ~434 | 344–545 |
| Knox County | ~424 | 348–518 |
| Mason County | ~353 | 267–472 |
| Stark County | ~323 | 253–408 |
| Marshall County | ~319 | 252–402 |
| Moultrie County | ~310 | 237–401 |
| Stephenson County | ~308 | 249–384 |
| Madison County | ~258 | 202–330 |
| Morgan County | ~232 | 184–291 |
| Christian County | ~198 | 127–323 |
| Putnam County | ~191 | 144–250 |
| Warren County | ~190 | 157–232 |
| Carroll County | ~180 | 141–234 |
| Hancock County | ~179 | 139–231 |
| Effingham County | ~146 | 113–189 |
| Cass County | ~142 | 114–178 |
| Cumberland County | ~140 | 111–176 |
| Adams County | ~138 | 95–187 |
| Jo Daviess County | ~137 | 109–176 |
| Fayette County | ~134 | 103–176 |
| Union County | ~123 | 65–209 |
| Monroe County | ~106 | 82–137 |
43 additional Illinois counties have lower modeled arsenic exposure (fewer than 100 estimated well residents over the limit). Of these, 22 have measured USGS well samples: Alexander, Clark, Clay, Coles, Cook, Crawford, Gallatin, Greene, Henderson, Jasper, Lawrence, Menard, Montgomery, Pike, Pulaski, Randolph, Rock Island, Schuyler, Scott, Wabash, Wayne, White.
Source: USGS domestic-well arsenic model, Lombard et al. (2021); county population product (2017).
Measured well tests by county
The USGS has published laboratory results from wells in 76 Illinois counties (at least 5 wells each). These are real measurements of other wells — domestic, public-supply and monitoring wells — not a test of yours. The tables rank counties by the share of sampled wells over each federal threshold.
Nitrate (as N): counties with the most wells > 10 mg/L (EPA MCL)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Kankakee County | 44% | 2.4 mg/L | 9 wells |
| St. Clair County | 20% | 0.4 mg/L | 5 wells |
| Logan County | 13% | 0.3 mg/L | 8 wells |
| Madison County | 9% | 0 mg/L | 11 wells |
| Piatt County | 7% | 0 mg/L | 14 wells |
| McLean County | 5% | 0 mg/L | 22 wells |
| McHenry County | 2% | 0 mg/L | 65 wells |
Manganese: counties with the most wells > 300 µg/L (EPA health advisory)
| County | Wells over | Median | Sampled |
|---|---|---|---|
| Rock Island County | 67% | 898 µg/L | 12 wells |
| Cook County | 50% | 286 µg/L | 56 wells |
| Madison County | 40% | 221 µg/L | 10 wells |
| St. Clair County | 40% | 223 µg/L | 5 wells |
| McDonough County | 40% | 3 µg/L | 5 wells |
| Logan County | 38% | 271 µg/L | 8 wells |
| Champaign County | 20% | 36 µg/L | 15 wells |
| Woodford County | 20% | 108 µg/L | 5 wells |
| Livingston County | 17% | 65 µg/L | 6 wells |
| LaSalle County | 14% | 22 µg/L | 7 wells |
Source: USGS ScienceBase 5f36baa782cee144fb3873e8 (manganese, pH, arsenic) and McMahon et al. (2019), Environ. Sci. Technol. 53:29–38 (nitrate, iron); aggregated by county with Census 2020 boundaries. USGS well samples; not a test of your well.
Match the filter to your test result
Carbon pitchers and faucet filters do not remove arsenic or nitrate. Reverse osmosis is the point-of-use fix; a whole-house system handles the rest of the home.
APEC ROES-50 5-Stage Reverse Osmosis System
5-stage RO, ★ 4.7 — removes arsenic, nitrate, lead, fluoride and PFAS.
Check price on Amazon →Whole-house well water system
Treats every tap, shower and appliance — the right scale for sediment, iron, sulfur and bacteria on a private well.
Compare on Amazon →Waterdrop arsenic-reduction filters
Under-sink units tested for arsenic reduction — the point-of-use fix for a well over the limit.
Shop on Waterdrop direct →Affiliate links — we may earn a commission at no extra cost to you. Re-test after installing to confirm the system is working.
Illinois well water: FAQ
Is there arsenic in well water in Illinois?
Yes. USGS modeling estimates about 67,710 people in Illinois use private wells with arsenic above 10 µg/L, across 102 modeled counties. Arsenic comes from natural bedrock and varies well to well, even between neighbors, so a lab test is the only way to know your own level.
Which Illinois counties have the most arsenic in well water?
By modeled number of well residents over the arsenic limit: Will County (~8,733), Kankakee County (~4,374), Kendall County (~3,884). Counties with more private-well users rank higher, so a smaller county can still have a high share of affected wells.
How do I remove arsenic from well water in Illinois?
Test first. If arsenic is present, a point-of-use reverse-osmosis system under the kitchen sink removes 90%+ of it; boiling does not remove arsenic (it concentrates it), and carbon pitchers and water softeners do not remove it either. Whole-house adsorptive arsenic media is used when levels are high. Re-test after installing and then annually.
Is nitrate a problem in Illinois well water?
USGS models about 0% of Illinois's private-well population (~296 people) above the 10 mg/L nitrate limit. Nitrate is highest in shallow wells near fertilized cropland, septic systems or livestock, and matters most for infants and pregnancy.
Do I need an acid neutralizer for my well in Illinois?
Acidic well water is uncommon in Illinois, but it does occur. In USGS lab results from 1,420 wells across 75 Illinois counties, 7% had pH below 6.5. The most acidic sampled counties are DuPage County (46% of 26 wells), Carroll County (31% of 13 wells), Pulaski County (29% of 7 wells). You only need a neutralizer if your own well tests below about 6.5 — signs include blue-green stains on fixtures and pinhole leaks in copper pipe. A calcite (or calcite/Corosex) neutralizer tank handles mildly acidic water (roughly pH 6–6.5); very acidic water usually needs soda-ash injection. Test first-draw lead and copper too, since acidic water dissolves both from plumbing.
High manganese in Illinois well water — what should I do?
In USGS lab results from 320 wells across 22 Illinois counties, 19% were above the EPA's 300 µg/L health advisory for manganese and 44% were above the 50 µg/L level where it stains fixtures and laundry black or brown. Sampled counties with the highest share over the health advisory: Rock Island County (67% of 12 wells), Cook County (50% of 56 wells), Madison County (40% of 10 wells). If your own test is over 300 µg/L, don't use the water for infant formula and use another source for drinking and cooking until it is treated — high manganese is linked to neurological effects, especially in babies and children. Boiling does not remove it. For the whole house, an oxidizing filter (greensand, Birm or air-injection) handles manganese, usually alongside iron; a water softener can remove low levels. For drinking water, a point-of-use reverse osmosis system reduces manganese at the kitchen tap. Retest after treatment to confirm it worked.