Water quality grades provide an intuitive way to understand EPA compliance data. Rather than interpreting raw scores, grades translate percentile rankings into familiar letter grades — from A (excellent) to F (failing). But how many counties earn each grade? And what do the grades actually mean?
We analyzed all 3,067 US counties with water quality data to understand the national grade distribution.
National Water Quality Grade Distribution
Here is how counties are distributed across the five grade levels:
| Grade | Counties | Percentage | Score Range | Description |
|---|---|---|---|---|
| A | 0 | 0% | 90-100 | Excellent water quality |
| B | 860 | 28% | 75-89 | Good water quality |
| C | 369 | 12% | 60-74 | Moderate water quality |
| D | 614 | 20% | 40-59 | Poor water quality |
| F | 1,224 | 40% | 0-39 | Failing water quality |
What Each Grade Means
Water quality grades are based on percentile-rank scores derived from EPA SDWIS violation data:
- A (90-100): Better compliance than 90% of all US counties. Typically zero or near-zero health violations.
- B (75-89): Better than 75% of counties. Minor compliance issues, no serious health risks.
- C (60-74): Moderate compliance. Some violations, generally addressed promptly.
- D (40-59): Below average. Multiple violations or serious infrastructure challenges.
- F (0-39): In the bottom 40% nationally. Persistent compliance issues requiring attention.
Note
Grades reflect historical compliance, not real-time water safety. A county with a C grade may have perfectly safe tap water today if past violations have been resolved. Always check your water system's most recent Consumer Confidence Report.
Methodology
Grade distribution is calculated by assigning each county a letter grade based on its water quality score, then counting counties per grade. Water quality scores use percentile-rank methodology based on EPA SDWIS violation data. Score ranges: A (90-100), B (75-89), C (60-74), D (40-59), F (0-39).
Data source: U.S. Environmental Protection Agency, Safe Drinking Water Information System (SDWIS) Federal Reporting Services, accessed via ECHO API. All figures are estimates based on publicly available compliance data and may differ from other published analyses due to methodology differences.
Data Editor
Evan BrooksData Editor
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