While most US counties have acceptable drinking water quality, some face persistent compliance challenges. Using EPA SDWIS data, we identified the 25 counties with the worst water quality scores — places where water systems have struggled with health-based violations, monitoring gaps, or aging infrastructure.
A low score does not necessarily mean tap water is currently unsafe. It reflects historical compliance issues and indicates systemic challenges that residents should be aware of.
The 25 Counties with the Worst Water Quality
These counties have the lowest water quality scores in the nation:
Common Factors in Low-Ranking Counties
The lowest-ranking counties share several characteristics. Texas (10 counties), Oklahoma (6 counties), Alaska (3 counties), Georgia (2 counties), Colorado (2 counties) are most represented. Common challenges include aging infrastructure, small underfunded rural water systems, agricultural runoff, and inconsistent monitoring.
Note
A low water quality score reflects historical compliance issues, not necessarily current water safety. Public water systems are required to notify customers of any immediate health risks. Check your specific water system's Consumer Confidence Report for the most accurate information.
Methodology
Water quality scores are calculated using EPA SDWIS data with percentile-rank methodology. Health-based violations carry the most weight, followed by monitoring and treatment technique violations. Letter grades map to score ranges: A (90-100), B (75-89), C (60-74), D (40-59), F (0-39). Counties are ranked by water score in ascending order.
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.