Drinking water quality varies dramatically across the United States. While most Americans assume their tap water is safe, EPA compliance data tells a more nuanced story. We analyzed water quality data for 3,067 US counties using the EPA Safe Drinking Water Information System (SDWIS) to understand where water quality is best, where it falls short, and what drives the differences.
The national average water quality score is 50 out of 100. Of all counties with data, 0 earn an A grade (score 90+), 860 earn a B, 369 earn a C, 614 earn a D, and 1,224 receive a failing grade. The distribution reveals that water quality is far from uniform — geography, infrastructure age, and regulatory enforcement all play roles.
What "Water Quality" Actually Measures
When we talk about water quality by county, we are measuring regulatory compliance — specifically, whether public water systems in a county meet the standards set by the Safe Drinking Water Act (SDWA). The EPA tracks two main categories of violations:
- Health-based violations: These occur when water exceeds Maximum Contaminant Levels (MCLs) for substances like lead, arsenic, nitrates, or disinfection byproducts. These are the most serious violations and carry the most weight in our scoring.
- Monitoring and reporting violations: These occur when water systems fail to test their water on schedule or fail to report results. While not a direct health risk, they indicate gaps in oversight.
- Treatment technique violations: These occur when systems fail to follow required treatment procedures, even if the water itself tests clean.
The Best States for Drinking Water Quality
At the state level, clear geographic patterns emerge. The five states with the highest average county water quality scores are:
| Rank | State | Average Score | Counties |
|---|---|---|---|
| 1 | Hawaii | 78.6 | 4 |
| 2 | Iowa | 71.7 | 99 |
| 3 | North Dakota | 71.6 | 52 |
| 4 | Tennessee | 71.3 | 95 |
| 5 | District of Columbia | 69.3 | 1 |
States with newer water infrastructure, lower population density, and strong state-level environmental agencies tend to score highest. Many top-performing states have abundant natural water sources — groundwater from deep aquifers or surface water from protected watersheds — that require less treatment to meet federal standards.
Counties with the Best Water Quality
The top 10 counties nationally have near-perfect water quality scores, with zero or near-zero health-based violations:
| Rank | County | State | Score | Grade | Health Violations |
|---|---|---|---|---|---|
| 1 | Autauga County, Alabama | AL | 86 | B | 0 |
| 2 | Bullock County, Alabama | AL | 86 | B | 0 |
| 3 | Chambers County, Alabama | AL | 86 | B | 0 |
| 4 | Cherokee County, Alabama | AL | 86 | B | 0 |
| 5 | Chilton County, Alabama | AL | 86 | B | 0 |
| 6 | Cleburne County, Alabama | AL | 86 | B | 0 |
| 7 | Covington County, Alabama | AL | 86 | B | 0 |
| 8 | Crenshaw County, Alabama | AL | 86 | B | 0 |
| 9 | Cullman County, Alabama | AL | 86 | B | 0 |
| 10 | Dallas County, Alabama | AL | 86 | B | 0 |
Counties with the Most Water Quality Concerns
On the other end of the spectrum, some counties have persistent compliance issues. These are not necessarily places where tap water is unsafe to drink — most violations are addressed — but they indicate systemic challenges with water infrastructure or regulatory compliance:
| Rank | County | State | Score | Grade | Health Violations |
|---|---|---|---|---|---|
| 1 | Foard County, Texas | TX | 0 | F | 170 |
| 2 | Borden County, Texas | TX | 0 | F | 60 |
| 3 | McCulloch County, Texas | TX | 0.1 | F | 500 |
| 4 | Edwards County, Texas | TX | 0.1 | F | 167 |
| 5 | Briscoe County, Texas | TX | 0.1 | F | 107 |
| 6 | San Augustine County, Texas | TX | 0.2 | F | 440 |
| 7 | Treasure County, Montana | MT | 0.2 | F | 19 |
| 8 | Taliaferro County, Georgia | GA | 0.2 | F | 42 |
| 9 | Choctaw County, Oklahoma | OK | 0.3 | F | 573 |
| 10 | Yukon-Koyukuk Census Area, Alaska | AK | 0.3 | F | 314 |
Note
A low water quality score does not mean tap water is currently unsafe. It reflects historical compliance issues reported to the EPA. Many violations are resolved quickly, and public water systems are required to notify customers of any health-based concerns.
What Drives Water Quality Differences?
Several factors explain why water quality varies so much between counties:
- Infrastructure age: Counties with older water systems — particularly those built before 1980 — face more frequent violations due to aging pipes, lead service lines, and outdated treatment facilities.
- Source water type: Groundwater from deep, protected aquifers typically requires less treatment than surface water from rivers and reservoirs. Counties relying on surface water may face more seasonal contamination challenges.
- Agricultural activity: Counties with intensive farming may see higher nitrate levels from fertilizer runoff, leading to more health-based violations.
- Regulatory enforcement: State-level enforcement varies. Some states inspect and enforce more aggressively than others, leading to higher reported violation counts even if underlying water quality is similar.
- Population density: Larger systems serving denser populations face different challenges than small rural systems, but both can have compliance issues.
Methodology
Water quality scores are calculated using EPA SDWIS data. Each county is scored using percentile-rank methodology — a score of 85 means the county has better compliance than 85% of all US counties. Scores weight health-based violations most heavily, with monitoring and treatment technique violations as secondary factors. Letter grades map to score ranges: A (90-100), B (75-89), C (60-74), D (40-59), F (0-39). For full details, see our methodology page.
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.