Water Quality by County

County-level coverage is how the EPA aggregates Safe Drinking Water Information System (SDWIS) reporting: every Public Water System (PWS) is registered under a primacy agency that maps it to a county FIPS code. That mapping is the basis for the per-county pages on this directory. For each county you can see the number of regulated systems, the population they serve, the count of health-based and monitoring violations on record, and, where the EPA's Unregulated Contaminant Monitoring Rule cycle 5 (UCMR5) sampling has completed, any per-system PFAS occurrence numbers.

Counts shown here are the raw, unweighted totals exactly as published by EPA. Two systems serving the same population are counted as two systems; a single system whose service area crosses county lines is reported in each. Population served is summed across systems in the county, so a household served by both its municipal system and a smaller school or institution system will appear in both numbers. For interpreted, per-capita rankings see the rankings index.

Violation counts include both health-based violations (maximum contaminant level exceedances, treatment technique failures) and monitoring/reporting violations (missed sample submissions, late MCL test reports). Health-based violations are the more direct safety signal; monitoring violations indicate gaps in oversight but do not on their own imply contamination. Click any county to see the violation breakdown by code, system, and date.

Data freshness: EPA refreshes SDWIS quarterly and UCMR5 detection results are added as labs complete analysis. This directory rebuilds on every data refresh, see the methodology page for the snapshot date and the upstream queries used.