We analyzed tap water quality data for hundreds of cities in Idaho 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EPA legal limits, many health experts — including the Environmental Working Group — argue that these legal limits are outdated and don't reflect current scientific understanding of safe exposure levels.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
Boise, Idaho
Boise has 16 contaminants exceeding EWG health guidelines out of 35 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Boise water report →Mountain Home, Idaho
Mountain Home has 15 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 Mountain Home water report →Twin Falls, Idaho
Twin Falls has 15 contaminants exceeding EWG health guidelines out of 19 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Twin Falls water report →Meridian, Idaho
Meridian has 13 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 Meridian water report →Idaho Falls, Idaho
Idaho Falls has 13 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 Idaho Falls water report →Kuna, Idaho
Kuna has 13 contaminants exceeding EWG health guidelines out of 21 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Kuna water report →Garden City, Idaho
Garden City has 12 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 Garden City water report →Nampa, Idaho
Nampa has 12 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 Nampa water report →Weiser, Idaho
Weiser has 11 contaminants exceeding EWG health guidelines out of 13 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Weiser water report →Mountain Home Afb, Idaho
Mountain Home Afb has 10 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 Mountain Home Afb water report →Marsing, Idaho
Marsing has 10 contaminants exceeding EWG health guidelines out of 11 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Marsing water report →Glenns Ferry, Idaho
Glenns Ferry has 10 contaminants exceeding EWG health guidelines out of 10 detected — significantly above the national average. Multiple known carcinogens were found at levels of concern.
See full Glenns Ferry water report →Idaho City, Idaho
Idaho City has 9 contaminants exceeding EWG health guidelines out of 19 total detected, placing it among the most concerning water supplies analyzed.
See full Idaho City water report →Chubbuck, Idaho
Chubbuck has 9 contaminants exceeding EWG health guidelines out of 16 total detected, placing it among the most concerning water supplies analyzed.
See full Chubbuck water report →Coeur D Alene, Idaho
Coeur D Alene has 8 contaminants exceeding EWG health guidelines out of 15 total detected, placing it among the most concerning water supplies analyzed.
See full Coeur D Alene water report →Burley, Idaho
Burley has 8 contaminants exceeding EWG health guidelines out of 13 total detected, placing it among the most concerning water supplies analyzed.
See full Burley water report →Pocatello, Idaho
Pocatello has 7 contaminants exceeding EWG health guidelines out of 16 total detected, placing it among the most concerning water supplies analyzed.
See full Pocatello water report →Juliaetta, Idaho
Juliaetta has 7 contaminants exceeding EWG health guidelines out of 13 total detected, placing it among the most concerning water supplies analyzed.
See full Juliaetta water report →Donnelly, Idaho
Donnelly has 7 contaminants exceeding EWG health guidelines out of 9 total detected, placing it among the most concerning water supplies analyzed.
See full Donnelly water report →Jerome, Idaho
Jerome has 7 contaminants exceeding EWG health guidelines out of 7 total detected, placing it among the most concerning water supplies analyzed.
See full Jerome water report →Horseshoe Bend, Idaho
Horseshoe Bend has 7 contaminants exceeding EWG health guidelines out of 7 total detected, placing it among the most concerning water supplies analyzed.
See full Horseshoe Bend water report →Hollister, Idaho
Hollister has 7 contaminants exceeding EWG health guidelines out of 7 total detected, placing it among the most concerning water supplies analyzed.
See full Hollister water report →Yellow Pine, Idaho
Yellow Pine has 7 contaminants exceeding EWG health guidelines out of 7 total detected, placing it among the most concerning water supplies analyzed.
See full Yellow Pine water report →Rexburg, Idaho
Rexburg has 6 contaminants exceeding EWG health guidelines out of 16 total detected, placing it among the most concerning water supplies analyzed.
See full Rexburg water report →Star, Idaho
Star has 6 contaminants exceeding EWG health guidelines out of 12 total detected, placing it among the most concerning water supplies analyzed.
See full Star water report →What We Found: Key Patterns
The most frequently detected contaminants exceeding health guidelines across these cities are Total trihalomethanes (TTHMs), Haloacetic acids (HAA5), Radium, combined (-226 and -228). 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, 245 contaminant readings exceed EWG health guidelines, but only 0 exceed EPA legal limits. This gap of 245 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 | Boise | ID | 35 | 16 | 0 | 250,799 |
| 2 | Mountain Home | ID | 24 | 15 | 0 | 14,000 |
| 3 | Twin Falls | ID | 19 | 15 | 0 | 50,197 |
| 4 | Meridian | ID | 30 | 13 | 0 | 138,620 |
| 5 | Idaho Falls | ID | 22 | 13 | 0 | 69,450 |
| 6 | Kuna | ID | 21 | 13 | 0 | 32,038 |
| 7 | Garden City | ID | 28 | 12 | 0 | 12,500 |
| 8 | Nampa | ID | 22 | 12 | 0 | 100,200 |
| 9 | Weiser | ID | 13 | 11 | 0 | 5,630 |
| 10 | Mountain Home Afb | ID | 18 | 10 | 0 | 7,500 |
| 11 | Marsing | ID | 11 | 10 | 0 | 1,300 |
| 12 | Glenns Ferry | ID | 10 | 10 | 0 | 0 |
| 13 | Idaho City | ID | 19 | 9 | 0 | 478 |
| 14 | Chubbuck | ID | 16 | 9 | 0 | 15,842 |
| 15 | Coeur D Alene | ID | 15 | 8 | 0 | 54,115 |
| 16 | Burley | ID | 13 | 8 | 0 | 10,345 |
| 17 | Pocatello | ID | 16 | 7 | 0 | 57,092 |
| 18 | Juliaetta | ID | 13 | 7 | 0 | 0 |
| 19 | Donnelly | ID | 9 | 7 | 0 | 237 |
| 20 | Jerome | ID | 7 | 7 | 0 | 11,500 |
| 21 | Horseshoe Bend | ID | 7 | 7 | 0 | 750 |
| 22 | Hollister | ID | 7 | 7 | 0 | 220 |
| 23 | Yellow Pine | ID | 7 | 7 | 0 | 72 |
| 24 | Rexburg | ID | 16 | 6 | 0 | 39,409 |
| 25 | Star | ID | 12 | 6 | 0 | 14,000 |
Frequently Asked Questions
What city has the worst tap water in Idaho?
Based on 2026 data, Boise, ID 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.
