Most Common Drinking Water Contaminants in America
These are the contaminants most frequently found in violation of EPA standards across American drinking water systems. Their presence in this list means they've exceeded safe levels set by the EPA in the highest number of cities nationwide.
| # | Contaminant | Cities Affected |
|---|---|---|
| 1 | Stage 2 DBP | 2,686 |
| 2 | TTHM | 1,944 |
| 3 | Disinfection Byproducts | 1,930 |
| 4 | E. coli | 1,844 |
| 5 | Chlorine Dioxide | 1,635 |
| 6 | Gross Alpha | 1,290 |
| 7 | Total Haloacetic Acids (HAA5) | 1,265 |
| 8 | Chlorine | 892 |
| 9 | Fecal Coliform | 766 |
| 10 | Barium | 519 |
| 11 | Nitrate | 481 |
| 12 | Selenium | 479 |
| 13 | Coliform (TCR) | 330 |
| 14 | Total Organic Carbon | 298 |
| 15 | Contaminant 2920 | 272 |
| 16 | Bromate | 235 |
| 17 | HAA5 | 223 |
| 18 | Haloacetic Acids | 150 |
| 19 | Nitrate-Nitrite | 114 |
| 20 | Lead | 109 |
Key Takeaways
- Coliform bacteria and disinfection byproducts tend to be the most common violation types nationwide.
- Some contaminants on this list are naturally occurring (like arsenic and radium), while others result from treatment processes or industrial activity.
- The number of cities affected reflects how widespread the issue is, not necessarily the severity of individual violations.
Methodology
Data is sourced from the EPA Safe Drinking Water Information System (SDWIS) via the Envirofacts API. Rankings include community water systems serving 100 or more people. Violation counts reflect historical records in the SDWIS database and include both resolved and unresolved violations. Health-based violations include Maximum Contaminant Level (MCL) violations and Treatment Technique violations. Per-capita calculations use the total population served by tracked water systems in each state.