We analyzed tap water quality data for hundreds of cities in Colorado 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EPA legal limits, many health experts — including the Environmental Working Group — argue that these legal limits are outdated and don't reflect current scientific understanding of safe exposure levels.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
Thornton, Colorado
Thornton has 22 contaminants exceeding EWG health guidelines out of 37 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Thornton water report →Brighton, Colorado
Brighton has 20 contaminants exceeding EWG health guidelines out of 40 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Brighton water report →Farmington Hills, Colorado
Farmington Hills has 20 contaminants exceeding EWG health guidelines out of 40 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Farmington Hills water report →Aurora, Colorado
Aurora has 18 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 Aurora water report →Englewood, Colorado
Englewood has 18 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 Englewood water report →Fountain, Colorado
Fountain 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 Fountain water report →Sterling, Colorado
Sterling has 18 contaminants exceeding EWG health guidelines out of 30 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Sterling water report →Indian Hills, Colorado
Indian Hills has 18 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 Indian Hills water report →Frisco, Colorado
Frisco has 18 contaminants exceeding EWG health guidelines out of 26 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Frisco water report →Castle Rock, Colorado
Castle Rock has 17 contaminants exceeding EWG health guidelines out of 29 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Castle Rock water report →Keenesburg, Colorado
Keenesburg has 17 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 Keenesburg water report →Rocky Ford, Colorado
Rocky Ford has 17 contaminants exceeding EWG health guidelines out of 25 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Rocky Ford water report →Fowler, Colorado
Fowler has 17 contaminants exceeding EWG health guidelines out of 25 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Fowler water report →Vail, Colorado
Vail 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 Vail water report →Pueblo West, Colorado
Pueblo West has 16 contaminants exceeding EWG health guidelines out of 29 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Pueblo West water report →Black Hawk, Colorado
Black Hawk has 16 contaminants exceeding EWG health guidelines out of 29 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Black Hawk water report →Lochbuie, Colorado
Lochbuie has 16 contaminants exceeding EWG health guidelines out of 26 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Lochbuie water report →Keystone, Colorado
Keystone has 16 contaminants exceeding EWG health guidelines out of 22 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Keystone water report →Parker, Colorado
Parker has 15 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 Parker water report →Commerce City, Colorado
Commerce City has 15 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 Commerce City water report →Avondale, Colorado
Avondale has 15 contaminants exceeding EWG health guidelines out of 23 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Avondale water report →La Junta, Colorado
La Junta has 15 contaminants exceeding EWG health guidelines out of 18 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full La Junta water report →Wiggins, Colorado
Wiggins has 15 contaminants exceeding EWG health guidelines out of 18 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Wiggins water report →Colorado City, Colorado
Colorado City has 14 contaminants exceeding EWG health guidelines out of 24 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Colorado City water report →Gunnison, Colorado
Gunnison has 14 contaminants exceeding EWG health guidelines out of 22 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Gunnison water report →What We Found: Key Patterns
The most frequently detected contaminants exceeding health guidelines across these cities are Bromodichloromethane, Dibromoacetic acid, 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, 421 contaminant readings exceed EWG health guidelines, but only 18 exceed EPA legal limits. This gap of 403 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 | Thornton | CO | 37 | 22 | 0 | 155,700 |
| 2 | Brighton | CO | 40 | 20 | 2 | 55,201 |
| 3 | Farmington Hills | CO | 40 | 20 | 2 | 1,500 |
| 4 | Aurora | CO | 38 | 18 | 0 | 314,346 |
| 5 | Englewood | CO | 33 | 18 | 0 | 57,332 |
| 6 | Fountain | CO | 32 | 18 | 3 | 25,130 |
| 7 | Sterling | CO | 30 | 18 | 1 | 15,100 |
| 8 | Indian Hills | CO | 28 | 18 | 2 | 1,300 |
| 9 | Frisco | CO | 26 | 18 | 0 | 4,495 |
| 10 | Castle Rock | CO | 29 | 17 | 0 | 99,757 |
| 11 | Keenesburg | CO | 27 | 17 | 1 | 3,600 |
| 12 | Rocky Ford | CO | 25 | 17 | 1 | 4,443 |
| 13 | Fowler | CO | 25 | 17 | 0 | 0 |
| 14 | Vail | CO | 32 | 16 | 0 | 19,351 |
| 15 | Pueblo West | CO | 29 | 16 | 0 | 30,000 |
| 16 | Black Hawk | CO | 29 | 16 | 0 | 15,167 |
| 17 | Lochbuie | CO | 26 | 16 | 2 | 6,830 |
| 18 | Keystone | CO | 22 | 16 | 1 | 9,900 |
| 19 | Parker | CO | 38 | 15 | 0 | 59,781 |
| 20 | Commerce City | CO | 32 | 15 | 0 | 68,603 |
| 21 | Avondale | CO | 23 | 15 | 0 | 1,615 |
| 22 | La Junta | CO | 18 | 15 | 1 | 9,200 |
| 23 | Wiggins | CO | 18 | 15 | 0 | 135 |
| 24 | Colorado City | CO | 24 | 14 | 2 | 2,175 |
| 25 | Gunnison | CO | 22 | 14 | 0 | 9,610 |
Frequently Asked Questions
What city has the worst tap water in Colorado?
Based on 2026 data, Thornton, CO 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.
