Drinking water quality varies not just by county, but by state. Some states have consistently high scores across nearly all their counties, while others struggle with widespread compliance challenges. Understanding state-level patterns helps contextualize county data and reveals where systemic investment may be needed.
We calculated the average water quality score for every US state by aggregating all county-level EPA SDWIS scores within each state.
States with the Best Average Water Quality
These states have the highest average water quality scores across all their counties:
| Rank | State | Avg Score | Grade | Counties Scored |
|---|---|---|---|---|
| 1 | Hawaii | 78.6 | B | 4 |
| 2 | Iowa | 71.7 | C | 99 |
| 3 | North Dakota | 71.6 | C | 52 |
| 4 | Tennessee | 71.3 | C | 95 |
| 5 | District of Columbia | 69.3 | C | 1 |
| 6 | Massachusetts | 69.1 | C | 14 |
| 7 | Alabama | 68.8 | C | 67 |
| 8 | Minnesota | 67.2 | C | 87 |
| 9 | Delaware | 66.1 | C | 3 |
| 10 | Rhode Island | 65 | C | 5 |
| 11 | South Carolina | 64.9 | C | 46 |
| 12 | Georgia | 64.4 | C | 159 |
| 13 | Kentucky | 64.2 | C | 118 |
| 14 | Washington | 59.3 | D | 39 |
| 15 | Michigan | 58.5 | D | 83 |
States with the Worst Average Water Quality
These states have the lowest average water quality scores across all their counties:
| Rank | State | Avg Score | Grade | Counties Scored |
|---|---|---|---|---|
| 1 | Oklahoma | 15.8 | F | 77 |
| 2 | Alaska | 19.8 | F | 22 |
| 3 | New Mexico | 21.7 | F | 32 |
| 4 | Arizona | 28.6 | F | 15 |
| 5 | Maine | 30.1 | F | 16 |
| 6 | Louisiana | 30.1 | F | 63 |
| 7 | Texas | 30.4 | F | 254 |
| 8 | Idaho | 32.6 | F | 44 |
| 9 | New Hampshire | 33.2 | F | 10 |
| 10 | West Virginia | 35.2 | F | 54 |
| 11 | Wyoming | 36.6 | F | 23 |
| 12 | Colorado | 38.7 | F | 64 |
| 13 | Pennsylvania | 38.9 | F | 67 |
| 14 | Wisconsin | 39.5 | F | 71 |
| 15 | New York | 40.1 | D | 57 |
Methodology
State averages are calculated by summing all county water quality scores within each state and dividing by the number of counties with data. Water quality scores use percentile-rank methodology based on EPA SDWIS violation data. Letter grades map to 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.