Private wells
Illinois well water: who's on a well and what to test for
People on private wells
1,227,200
Homes on wells
9.5%
Est. above arsenic limit
67,700
Every Illinois county
Sorted by people on private wells. Arsenic and nitrate are modeled for typical wells in the county, not measured at yours; depth is the median of domestic well records.
| Lake County | 100,600 | 15% | 6.1% | 0.0 | 155 ft |
| McHenry County | 79,800 | 27% | 10.1% | 0.1 | 125 ft |
| Will County | 68,800 | 11% | 12.6% | 0.0 | 160 ft |
| Cook County | 51,500 | 1% | 3.8% | 0.0 | 200 ft |
| DuPage County | 49,200 | 5% | 5.1% | 0.0 | 175 ft |
| Kane County | 47,500 | 9% | 14.8% | 0.0 | 212 ft |
| Winnebago County | 41,400 | 14% | 5.1% | 0.6 | 148 ft |
| LaSalle County | 24,800 | 20% | 11.7% | 0.0 | 177 ft |
| Kankakee County | 22,800 | 22% | 10.9% | 0.1 | 125 ft |
| Madison County | 20,100 | 7% | 3.7% | 1.7 | 41 ft |
| Ogle County | 19,400 | 37% | 5.8% | 0.4 | 205 ft |
| Rock Island County | 19,100 | 13% | 7.8% | 0.3 | 260 ft |
| Peoria County | 18,900 | 9% | 14.9% | 0.8 | 65 ft |
| Whiteside County | 18,600 | 32% | 4.7% | 0.5 | 117 ft |
| Tazewell County | 18,000 | 13% | 21.0% | 0.4 | 138 ft |
| Adams County | 15,700 | 22% | 3.0% | 0.4 | 185 ft |
| Kendall County | 15,300 | 13% | 9.4% | 0.0 | 205 ft |
| Sangamon County | 15,100 | 7% | 15.4% | 1.5 | 40 ft |
| St. Clair County | 15,000 | 5% | 9.5% | 0.8 | 76 ft |
| Boone County | 15,000 | 29% | 5.3% | 0.5 | 168 ft |
| Champaign County | 15,000 | 7% | 28.3% | 0.0 | 121 ft |
| Vermilion County | 14,300 | 19% | 16.9% | 0.1 | 89 ft |
| McLean County | 13,400 | 7% | 42.3% | 0.1 | 152 ft |
| Woodford County | 13,200 | 33% | 35.2% | 0.1 | 141 ft |
| Henry County | 12,500 | 24% | 13.3% | 0.3 | 264 ft |
| DeKalb County | 12,500 | 12% | 11.6% | 0.0 | 205 ft |
| Macon County | 12,500 | 11% | 56.9% | 0.2 | 91 ft |
| Jefferson County | 12,500 | 32% | 6.7% | 0.6 | 70 ft |
| Williamson County | 12,400 | 16% | 4.0% | 0.1 | 148 ft |
| Stephenson County | 12,300 | 26% | 3.1% | 0.9 | 190 ft |
| Macoupin County | 11,300 | 25% | 14.0% | 1.0 | 41 ft |
| Lee County | 11,300 | 34% | 9.1% | 0.1 | 205 ft |
| Effingham County | 11,100 | 30% | 3.1% | 0.7 | 46 ft |
| Franklin County | 11,000 | 29% | 3.7% | 0.4 | 177 ft |
| Jo Daviess County | 10,100 | 40% | 11.6% | 0.8 | 230 ft |
| Jackson County | 10,000 | 16% | 6.7% | 0.2 | 210 ft |
| Grundy County | 9,600 | 19% | 8.6% | 0.0 | 160 ft |
| Fayette County | 9,500 | 47% | 6.2% | 1.4 | 41 ft |
| Iroquois County | 9,500 | 34% | 13.1% | 0.0 | 137 ft |
| Christian County | 9,300 | 19% | 12.1% | 0.9 | 41 ft |
| Saline County | 9,100 | 36% | 7.7% | 0.1 | 130 ft |
| Monroe County | 9,100 | 26% | 3.5% | 0.1 | 210 ft |
| Marion County | 9,000 | 23% | 7.3% | 0.7 | 42 ft |
| Fulton County | 8,800 | 27% | 12.3% | 0.5 | 65 ft |
| Clinton County | 8,700 | 25% | 6.0% | 1.1 | 35 ft |
| Bureau County | 8,600 | 26% | 12.2% | 0.2 | 202 ft |
| Shelby County | 8,600 | 40% | 32.9% | 1.0 | 41 ft |
| Livingston County | 8,300 | 21% | 7.1% | 0.1 | 84 ft |
| Randolph County | 7,900 | 28% | 5.0% | 0.7 | 58 ft |
| Knox County | 7,700 | 15% | 9.5% | 0.2 | 168 ft |
| Wayne County | 7,300 | 42% | 8.1% | 0.1 | 120 ft |
| Crawford County | 7,300 | 32% | 3.4% | 1.2 | 54 ft |
| Morgan County | 6,700 | 20% | 20.8% | 1.9 | 41 ft |
| Hancock County | 6,700 | 37% | 4.4% | 0.2 | 240 ft |
| Union County | 6,700 | 39% | 3.3% | 0.1 | 240 ft |
| Montgomery County | 6,500 | 24% | 7.1% | 1.2 | 40 ft |
| Hamilton County | 6,300 | 78% | 5.9% | 0.0 | 278 ft |
| Bond County | 6,100 | 40% | 3.8% | 1.2 | 40 ft |
| Clark County | 6,000 | 37% | 3.0% | 0.5 | 65 ft |
| Perry County | 6,000 | 30% | 7.0% | 1.2 | 38 ft |
| Coles County | 6,000 | 12% | 20.4% | 0.2 | 65 ft |
| Jersey County | 6,000 | 29% | 3.4% | 1.1 | 74 ft |
| Mercer County | 5,900 | 37% | 10.6% | 0.4 | 402 ft |
| Washington County | 5,700 | 41% | 8.7% | 1.1 | 41 ft |
| Richland County | 5,600 | 33% | 5.1% | 0.1 | 180 ft |
| Jasper County | 5,600 | 57% | 4.6% | 0.3 | 40 ft |
| Logan County | 5,500 | 20% | 46.4% | 0.8 | 60 ft |
| Carroll County | 5,500 | 40% | 5.9% | 0.2 | 260 ft |
| McDonough County | 5,400 | 18% | 16.5% | 0.1 | 235 ft |
| Clay County | 5,400 | 39% | 12.4% | 0.2 | 86 ft |
| Mason County | 5,300 | 41% | 14.1% | 0.9 | 72 ft |
| Cumberland County | 5,200 | 46% | 6.3% | 0.5 | 42 ft |
| Douglas County | 5,200 | 22% | 31.3% | 0.1 | 72 ft |
| Edgar County | 5,000 | 28% | 38.5% | 0.4 | 60 ft |
| Warren County | 4,900 | 30% | 9.3% | 0.3 | 189 ft |
| Massac County | 4,800 | 32% | 7.9% | 0.2 | 120 ft |
| Lawrence County | 4,800 | 35% | 3.2% | 0.5 | 135 ft |
| Johnson County | 4,700 | 39% | 4.1% | 0.1 | 240 ft |
| Moultrie County | 4,500 | 26% | 34.3% | 0.1 | 82 ft |
| Pike County | 4,400 | 31% | 4.1% | 1.0 | 130 ft |
| White County | 4,200 | 29% | 7.3% | 0.1 | 180 ft |
| Piatt County | 4,000 | 23% | 39.4% | 0.0 | 155 ft |
| De Witt County | 4,000 | 24% | 67.6% | 0.1 | 170 ft |
| Menard County | 3,900 | 30% | 22.7% | 2.4 | 45 ft |
| Greene County | 3,600 | 29% | 12.2% | 1.5 | 45 ft |
| Henderson County | 3,300 | 52% | 2.6% | 1.1 | 110 ft |
| Marshall County | 3,300 | 28% | 24.8% | 0.4 | 106 ft |
| Wabash County | 3,200 | 26% | 3.3% | 0.2 | 240 ft |
| Cass County | 3,100 | 25% | 11.6% | 1.1 | 59 ft |
| Ford County | 2,900 | 20% | 14.7% | 0.0 | 126 ft |
| Calhoun County | 2,900 | 67% | 3.2% | 0.3 | 140 ft |
| Schuyler County | 2,600 | 41% | 7.8% | 1.1 | 51 ft |
| Pulaski County | 2,500 | 42% | 6.2% | 0.1 | 74 ft |
| Pope County | 2,300 | 63% | 2.8% | 0.1 | 202 ft |
| Edwards County | 2,200 | 35% | 11.7% | 0.6 | 58 ft |
| Stark County | 2,000 | 36% | 35.7% | 0.3 | 75 ft |
| Hardin County | 1,900 | 51% | 3.8% | 0.1 | 210 ft |
| Putnam County | 1,900 | 37% | 14.2% | 0.3 | 180 ft |
| Scott County | 1,800 | 36% | 7.9% | 2.0 | 50 ft |
| Brown County | 1,700 | 38% | 3.6% | 0.8 | 270 ft |
| Gallatin County | 1,600 | 29% | 27.0% | 0.1 | 100 ft |
| Alexander County | 1,600 | 29% | 3.5% | 0.0 | 148 ft |
Shading: red is among the highest counties nationally, amber the middle, green the lower range. Click a column to sort.
Most well users above the arsenic limit
- Will County: about 8,700 people
- Kankakee County: about 4,400 people
- Kendall County: about 3,900 people
- McHenry County: about 2,900 people
- Vermilion County: about 2,800 people
Test first, then treat what the test finds
We may earn a commission from links below, at no cost to you. Picks follow the testing order, not who pays. How we make money.
1. Certified lab well test
Start here, every year
A mail-in kit analyzed by a certified lab measures bacteria, nitrate, arsenic, metals and more. Test strips cannot measure arsenic or uranium reliably.
Mail-in certified lab well water test
Mid-range2. Sediment pre-filter
Sand, silt or cloudy water
A 5-micron cartridge after the pressure tank catches sand, silt and rust before they reach fixtures or other filters. Cartridges are cheap and swap every few months.
5-micron whole-house sediment filter
Budget3. Iron and sulfur filter
Orange stains or rotten-egg smell
Air-injection or oxidizing-media filters remove orange staining and rotten-egg smell. Size one from a lab result for iron, manganese and hydrogen sulfide.
Air-injection iron and sulfur filter
Premium4. UV disinfection system
Coliform bacteria in a test
Inactivates bacteria and viruses; usually for private wells or after boil notices.
12 GPM whole-house UV purifier
Premium5. Under-sink reverse osmosis
Arsenic, nitrate or uranium in a test
The most thorough option for drinking water: PFAS, lead, nitrate, arsenic and more. Look for NSF/ANSI 58.
Waterdrop G3P600 tankless RO
Price at Waterdrop, checked Oct 5, 2026or compare on Amazon
iSpring RCC7AK reverse osmosis
Mid-range6. Whole-house well filter
Several problems at once
Multi-stage sediment and carbon for every tap. Add UV if bacteria shows up, and choose stages from your lab results, not a guess.
Waterdrop WHF21 2-stage whole-house filter
Mid-rangeQuestions
Which Illinois counties rely most on wells?+
By share of homes: Hamilton County (78%), Calhoun County (67%), Pope County (63%), Jasper County (57%), Henderson County (52%). By number of people: Lake County (100,600), McHenry County (79,800), Will County (68,800).
Is arsenic a problem in Illinois wells?+
USGS estimates about 67,700 Illinois well users have water above the 10 microgram per liter arsenic limit that applies to public systems, the largest numbers in Will County, Kankakee County, Kendall County. Arsenic has no taste or smell, so only a lab test finds it.
Do I have to test my well when I sell a house in Illinois?+
We could not confirm a statewide rule for Illinois. Check with your county health department. Buyers can still make a lab test a condition of the offer.
How often should I test my well?+
No federal law covers private wells. EPA's drinking water limits apply to public water systems, so testing and treating a private well is up to the owner. EPA and CDC recommend testing at least once a year for coliform bacteria, nitrate, pH and total dissolved solids, and testing for arsenic, uranium, radon and other local concerns based on where you live.
Well-user counts, arsenic and nitrate figures are each a modeled estimate built from national datasets, not a count of tested wells, and well depths summarize drilling records rather than every well. Your own well can be very different from the county average, which is why the only answer for your home is a lab test. Sources: EPA Office of Research and Development, Water Source 2020 (private well estimates by census block); CDC Environmental Public Health Tracking, Well Water Index (from USGS nitrate and arsenic models); USGS, county-level domestic well population with arsenic above 10 micrograms per liter (Ayotte and others, 2017); USGS Synthesized Domestic Well Database for the Conterminous United States (SynDWell-US), McCarthy and others, 2025 (depth, where a county has 30+ usable records).