Top Water Contaminants
Health-based drinking water violations by contaminant, from 12,049,741 violations across 433,347 water systems.
Health-based violations occur when a public water system fails to meet the EPA's maximum contaminant level (MCL) for a regulated substance — meaning the water delivered to customers may have posed a risk to human health. The contaminants listed here represent the most frequently violated drinking water standards in the United States. A high violation count reflects both the prevalence of the contaminant and the number of water systems that have struggled to meet the standard over time.
Most Frequently Violated Contaminants
Health-based violations ranked by total violation count across all water systems.
| # | Contaminant | Violations | % of Health Violations |
|---|---|---|---|
| 1 | Coliform (TCR) | 632,869 | 38.8% |
| 2 | TTHM | 203,703 | 12.5% |
| 3 | Arsenic | 106,569 | 6.5% |
| 4 | Nitrate | 92,628 | 5.7% |
| 5 | Total Haloacetic Acids (HAA5) | 91,815 | 5.6% |
| 6 | Surface Water Treatment Rule | 81,115 | 5% |
| 7 | Combined Radium (-226 and -228) | 69,422 | 4.3% |
| 8 | Revised Total Coliform Rule | 60,925 | 3.7% |
| 9 | Groundwater Rule | 53,827 | 3.3% |
| 10 | Fluoride | 35,736 | 2.2% |
| 11 | Gross Alpha, Excl. Radon and U | 34,296 | 2.1% |
| 12 | Interim Enhanced Surface Water Treatment Rule | 29,055 | 1.8% |
| 13 | CARBON, TOTAL | 24,423 | 1.5% |
| 14 | Lead and Copper Rule | 23,685 | 1.5% |
| 15 | Combined Uranium | 21,743 | 1.3% |
| 16 | Nitrate-Nitrite | 20,605 | 1.3% |
| 17 | LEAD AND COPPER RULE REVISIONS | 11,151 | 0.7% |
| 18 | Stage 1 Disinfectants and Disinfection Byproducts Rule | 8,743 | 0.5% |
| 19 | Long Term 2 Enhanced Surface Water Treatment Rule | 7,525 | 0.5% |
| 20 | Selenium | 3,168 | 0.2% |
| 21 | Barium | 1,941 | 0.1% |
| 22 | Trichloroethylene | 1,818 | 0.1% |
| 23 | Tetrachloroethylene | 1,749 | 0.1% |
| 24 | Nitrite | 1,739 | 0.1% |
| 25 | Atrazine | 1,178 | 0.1% |
| 26 | Di(2-ethylhexyl) phthalate | 1,146 | 0.1% |
| 27 | 1,1-Dichloroethylene | 920 | 0.1% |
| 28 | Cadmium | 751 | 0% |
| 29 | ETHYLENE DIBROMIDE | 661 | 0% |
| 30 | Benzene | 588 | 0% |
Only health-based violations are shown. Health-based violations indicate that contaminant levels exceeded EPA's maximum contaminant level (MCL) or treatment technique requirements. Total health-based violations in database: 1,632,684.
Understanding Drinking Water Violations
Health-Based vs. Monitoring Violations
The EPA tracks two main types of drinking water violations. Health-based violations occur when a contaminant level actually exceeds a regulatory limit, posing a potential health risk. Monitoring and reporting violations occur when a water system fails to test for contaminants as required or fails to report results on time — even if the water itself is safe. Health-based violations are more serious and are the focus of this contaminant list. Both types require public notification to customers.
Common Contaminants Explained
Total coliform and E. coli violations are among the most frequent and often indicate problems with water distribution system integrity or source water protection. Nitrate violations are common in agricultural areas where fertilizer runoff contaminates groundwater wells. Lead violations surged in public reporting after the Flint, Michigan crisis, though lead contamination typically originates in household plumbing rather than the water system itself. Turbidity violations reflect issues with filtration effectiveness at the treatment plant.
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