Methodology
Everything on this site comes from public federal data. This page lists each source, how we process it and what it cannot tell you.
Sources and dates
| What | Source | Version used |
|---|---|---|
| Water systems, service areas, violations, lead and copper | EPA ECHO Safe Drinking Water Act data download (from SDWIS) | 2026Q3, published October 2026 |
| PFAS test results and ZIP codes served | EPA Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) | Final data, August 2026 |
| County radon zones | EPA Map of Radon Zones | County list, June 2024 file |
| Which system serves a city | EPA Community Water System Service Area Boundaries | Version 3 |
| Population, city and county boundaries, ZIP areas | US Census Bureau | Vintage 2024 estimates; 2020 ZCTA to county file |
How a city page is built
We place each city's official center point on EPA's service area map and take the largest community water system there as the city's main supplier. Large cities often have smaller systems at the edges, so your home may be served by another one: the ZIP lookup on each page shows every system that reported your ZIP in EPA's PFAS testing. A city gets a page only when its main system has a lead sampling record and PFAS test results; 10 of the 300 largest US cities qualify today.
PFAS
Results come from UCMR 5, EPA's screening of 29 PFAS compounds in public water systems between 2023 and 2025. We show the highest result for each compound. EPA set enforceable limits of 4 ng/L (parts per trillion) for PFOA and PFOS in April 2024; water systems must comply by April 2029, and in 2026 EPA proposed letting some systems apply for two more years. EPA has also proposed withdrawing the 2024 limits for PFHxS, PFNA, GenX chemicals and PFAS mixtures, so we compare only PFOA and PFOS with a limit. UCMR 5 is screening data, not a compliance test.
Lead and copper
Utilities sample tap water in homes most likely to have lead and report the 90th percentile: the level that 90% of samples were at or below. The federal action level for lead is 15 ppb and for copper 1,300 ppb. A result below the action level does not mean every home is below it; lead mostly comes from a building's own service line and plumbing.
Violations
We count health-based violations only: exceeding a maximum contaminant level, a maximum residual disinfectant level, or breaking a treatment technique rule. Monitoring, reporting and notice violations are excluded. We show violations whose compliance period began on or after January 2016. Status follows EPA's record: resolved, closed (archived), being addressed or unresolved.
Radon zones
EPA's radon map, first published in 1993 and available as a county list (we use the June 2024 file), assigns each county to Zone 1 (predicted average above 4 pCi/L), Zone 2 (2 to 4) or Zone 3 (below 2). Connecticut replaced its counties with planning regions in 2022; for those regions we use the zone of the former county that holds the region's main city, and say so on the page. EPA recommends testing every home regardless of zone.
What this site does not do
We do not test water or air, sell products or services, or rate a specific address. We are not affiliated with EPA, any state or any utility. If you find an error, tell us and we will check it against the source and correct it.