Rankings
Large water systems with the highest PFOA or PFOS
Water systems serving 10,000 or more people, ranked by how far their highest entry-point average of PFOA or PFOS is above the 4 ppt federal limit (EPA UCMR 5, 2023–2025).
Biggest water systems with PFAS above the federal limit
Every water system with at least one PFAS entry-point average above a 2024 federal limit, largest first.
Water systems with the most health-based violations since 2016
Community water systems ranked by the number of health-based violations (a limit exceeded or a required treatment step missed) since January 2016.
Large water systems with the highest lead levels
Water systems serving 10,000+ people, ranked by their most recent lead 90th percentile (periods ending 2022 or later). The action level is 15 ppb, falling to 10 ppb in November 2027.
Water systems with the most lithium
Highest single lithium results in EPA's first national lithium testing (UCMR 5). There is no federal limit, and EPA's screening level is 10 ppb.