Frequently asked questions

For most people, yes. US public water systems are treated, monitored and regulated to a standard much of the world does not have. But safety is not uniform, and the variation is measurable rather than hypothetical.

In the federal testing programme that concluded in 2026, 60.1% of the 10,313 systems tested detected at least one of 30 monitored substances, and 1,734 systems recorded at least one sample above a health-based federal limit. Whether your own system falls into either group is a specific fact, and it takes a ZIP code to find out.

The results of a federal monitoring programme called UCMR 5, which required public water systems to test for 29 PFAS compounds and lithium between 2023 and 2025. We show the highest reported value for each substance found at your system, the federal limit where one exists, and the date of the most recent sample.

We do not model, estimate or score. If a number appears on this site, a laboratory measured it and EPA published it.

Per- and polyfluoroalkyl substances: a family of thousands of synthetic chemicals built around carbon–fluorine bonds, used since the 1940s in non-stick coatings, firefighting foam, stain resistance and waterproofing. The bond that makes them useful also makes them almost impossible to break down, which is why they are commonly called forever chemicals and why they turn up in water supplies decades after the industrial activity that released them.

Parts per trillion — roughly one drop in twenty Olympic swimming pools. The limits are set this low because these compounds accumulate in the body rather than passing through, so the relevant exposure is cumulative over years, not the dose in a single glass.

No, and this distinction matters. UCMR 5 results are individual sample measurements. Compliance with a Maximum Contaminant Level is judged on a running annual average at each entry point to the distribution system, not on any single sample.

Compliance is also not yet required. The deadline is April 2029, and EPA has proposed extending it to April 2031 for systems that apply and qualify. A result above a limit on this site means a measurement exceeded that number — not that anyone has violated anything.

It means no system serving that ZIP was required to test under UCMR 5 — not that the water is clean, and not that it is contaminated.

Testing was required of systems serving more than 3,300 people, plus a sample of smaller ones. Roughly 40,000 small rural systems were outside the programme. And if you are on a private well, no federal testing programme covers you at all.

It means none of the 30 monitored substances were found above the laboratory's reporting limit in the samples taken. That is a real and reassuring result, but it is narrow.

UCMR 5 did not test for lead, nitrate, arsenic, bacteria or disinfection by-products. Those are regulated under separate rules and reported in your utility's annual Consumer Confidence Report.

Lead is the most important contaminant this dataset does not cover, and the reason is structural: lead almost never comes from the treatment plant. It comes from the service line and household plumbing between the water main and your tap.

That makes it a property-level question rather than a system-level one. A system with a spotless record can still deliver lead into a specific kitchen. Housing built before 1986 is the risk factor that matters, and the answer comes from testing at your own tap.

No. Private wells are not covered by the Safe Drinking Water Act, are never federally tested, and are entirely the owner's responsibility. Around 43 million Americans are in this position.

The general guidance is to test annually for bacteria and nitrate, and periodically for a broader panel, through a laboratory certified by your state.

Reverse osmosis and properly certified activated carbon systems reduce PFAS. A standard pitcher filter sold for taste and odour generally does not.

The thing to look for is certification to NSF/ANSI 53 or NSF/ANSI 58 that explicitly names the contaminant being reduced. A filter certified for chlorine taste tells you nothing about its performance on PFOA.

ZIP codes are postal routes, not water service areas, and the two rarely align. About a third of the ZIP codes in this dataset are served by more than one water system.

When that happens we ask you to choose. Your water system is named on your water bill.

UCMR 5 sampling ran from 2023 to 2025 and EPA completed publication of the results in 2026. That dataset is now final and will not change.

We re-check the underlying system records periodically, and we review the regulatory position quarterly, because two EPA proposals affecting these limits are currently pending.

The federal data is in the public domain and you need no permission from us to use it. Our own analysis and writing are not public domain. If you cite a figure we calculated, please attribute it and link to the page, and we would ask you to reproduce the caveats along with the number.