We analyzed tap water quality data for hundreds of cities in Illinois to identify the 25 with the most contaminants exceeding health guidelines. This ranking is based on 2026 data from the EPA and EWG databases.
While all public water systems must meet EPA legal limits, many health experts — including the Environmental Working Group — argue that these legal limits are outdated and don't reflect current scientific understanding of safe exposure levels. The cities below have the widest gap between what's in their water and what's considered safe by modern health standards.
Rankings at a Glance
Dunlap, Illinois
Dunlap has 19 contaminants exceeding EWG health guidelines out of 42 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Dunlap water report →Hanna City, Illinois
Hanna City has 19 contaminants exceeding EWG health guidelines out of 41 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Hanna City water report →Machesney Park, Illinois
Machesney Park has 19 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Machesney Park water report →Glasford, Illinois
Glasford has 18 contaminants exceeding EWG health guidelines out of 42 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Glasford water report →Peoria, Illinois
Peoria has 18 contaminants exceeding EWG health guidelines out of 41 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Peoria water report →East Peoria, Illinois
East Peoria has 18 contaminants exceeding EWG health guidelines out of 32 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full East Peoria water report →South Elgin, Illinois
South Elgin has 17 contaminants exceeding EWG health guidelines out of 38 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full South Elgin water report →Aurora, Illinois
Aurora has 17 contaminants exceeding EWG health guidelines out of 33 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Aurora water report →Caseyville, Illinois
Caseyville has 17 contaminants exceeding EWG health guidelines out of 32 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Caseyville water report →Scott Air Force Base, Illinois
Scott Air Force Base has 17 contaminants exceeding EWG health guidelines out of 32 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Scott Air Force Base water report →Belleville, Illinois
Belleville has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Belleville water report →O'fallon, Illinois
O'fallon has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full O'fallon water report →Cahokia Heights, Illinois
Cahokia Heights has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Cahokia Heights water report →Columbia, Illinois
Columbia has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Columbia water report →Waterloo, Illinois
Waterloo has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Waterloo water report →Swansea, Illinois
Swansea has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Swansea water report →East St. Louis, Illinois
East St. Louis has 17 contaminants exceeding EWG health guidelines out of 31 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full East St. Louis water report →Rockford, Illinois
Rockford has 16 contaminants exceeding EWG health guidelines out of 38 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Rockford water report →Elgin, Illinois
Elgin has 16 contaminants exceeding EWG health guidelines out of 36 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Elgin water report →Sleepy Hollow, Illinois
Sleepy Hollow has 16 contaminants exceeding EWG health guidelines out of 36 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Sleepy Hollow water report →Pekin, Illinois
Pekin has 16 contaminants exceeding EWG health guidelines out of 34 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Pekin water report →Jerseyville, Illinois
Jerseyville has 16 contaminants exceeding EWG health guidelines out of 32 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Jerseyville water report →East Galesburg, Illinois
East Galesburg has 16 contaminants exceeding EWG health guidelines out of 28 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full East Galesburg water report →Galesburg, Illinois
Galesburg has 16 contaminants exceeding EWG health guidelines out of 27 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Galesburg water report →Oneida, Illinois
Oneida has 16 contaminants exceeding EWG health guidelines out of 27 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Oneida water report →What We Found: Key Patterns
The most frequently detected contaminants exceeding health guidelines across these cities are Bromodichloromethane, Chloroform, Dibromochloromethane. These appear repeatedly across different water systems and regions.
Water quality varies significantly even within the same state, driven by local geology, agricultural activity, and industrial proximity.
Across all ranked cities, 426 contaminant readings exceed EWG health guidelines, but only 6 exceed EPA legal limits. This gap of 420 highlights how legal standards may not fully protect public health — EPA limits haven't been updated for many contaminants in decades.
Full Rankings Table
| Rank | City | State | Detected | Above Guidelines | Above Legal | People Served |
|---|---|---|---|---|---|---|
| 1 | Dunlap | IL | 42 | 19 | 0 | 1,603 |
| 2 | Hanna City | IL | 41 | 19 | 0 | 1,253 |
| 3 | Machesney Park | IL | 31 | 19 | 2 | 35,580 |
| 4 | Glasford | IL | 42 | 18 | 0 | 2,665 |
| 5 | Peoria | IL | 41 | 18 | 0 | 121,478 |
| 6 | East Peoria | IL | 32 | 18 | 0 | 22,284 |
| 7 | South Elgin | IL | 38 | 17 | 0 | 22,549 |
| 8 | Aurora | IL | 33 | 17 | 0 | 200,000 |
| 9 | Caseyville | IL | 32 | 17 | 0 | 17,000 |
| 10 | Scott Air Force Base | IL | 32 | 17 | 0 | 6,779 |
| 11 | Belleville | IL | 31 | 17 | 0 | 139,879 |
| 12 | O'fallon | IL | 31 | 17 | 0 | 46,603 |
| 13 | Cahokia Heights | IL | 31 | 17 | 0 | 15,250 |
| 14 | Columbia | IL | 31 | 17 | 0 | 11,638 |
| 15 | Waterloo | IL | 31 | 17 | 0 | 10,190 |
| 16 | Swansea | IL | 31 | 17 | 0 | 4,474 |
| 17 | East St. Louis | IL | 31 | 17 | 0 | 450 |
| 18 | Rockford | IL | 38 | 16 | 0 | 147,051 |
| 19 | Elgin | IL | 36 | 16 | 0 | 113,911 |
| 20 | Sleepy Hollow | IL | 36 | 16 | 0 | 3,100 |
| 21 | Pekin | IL | 34 | 16 | 0 | 1,635 |
| 22 | Jerseyville | IL | 32 | 16 | 0 | 12,750 |
| 23 | East Galesburg | IL | 28 | 16 | 1 | 860 |
| 24 | Galesburg | IL | 27 | 16 | 1 | 31,745 |
| 25 | Oneida | IL | 27 | 16 | 2 | 3,460 |
Frequently Asked Questions
What city has the worst tap water in Illinois?
Based on 2026 data, Dunlap, IL has the most contaminants exceeding EWG health guidelines among the cities we analyzed.
How are cities ranked in this report?
Cities are ranked by the number of contaminants exceeding EWG health guidelines. These guidelines are typically stricter than EPA legal limits and reflect current scientific research on safe exposure levels.
Is my tap water safe if my city isn't on this list?
Not necessarily. Even cities not on this list may have contaminants of concern. We recommend checking your specific water quality using our free water scan tool.
What's the best water filter for contaminated water?
A reverse osmosis (RO) system is the most effective for removing the widest range of contaminants. For whole-home protection, a combination of RO for drinking water and a whole-home filter for showers and baths is recommended.
Related Reports
Data sources: Environmental Working Group (EWG) Tap Water Database, U.S. EPA Safe Drinking Water Information System (SDWIS)
Last updated: April 2026
Methodology: Contaminant levels are compared against both EPA legal limits (Maximum Contaminant Levels) and EWG health guidelines, which are often stricter and based on the latest scientific research.
