About our data

UCMR 5 occurrence data

What it is

The Fifth Unregulated Contaminant Monitoring Rule required public water systems serving more than 3,300 people — plus a representative sample of smaller ones — to test for 29 PFAS compounds and lithium. Sampling ran from January 2023 to December 2025. We use the complete national release: 1,992,002 individual results across 10,313 systems.

Updates

EPA completed publication in 2026. The dataset is final and will not change.

Limits

It covers 30 substances and no others. Roughly 40,000 small rural systems were outside the programme entirely, and private wells were never in scope. Results are individual samples, not compliance determinations.

Safe Drinking Water Information System

What it is

EPA's national inventory of public water systems. We use it for system names, population served, source water type, and the counties and cities each system serves — the information that turns a federal identifier into somewhere you recognise.

Updates

Refreshed by EPA on a rolling quarterly basis as states report. We re-pull it periodically.

Limits

Service area records are reported by states and vary in completeness. Population figures are estimates maintained by the system, and some are visibly stale.

PWSID-to-ZIP crosswalk

What it is

EPA publishes a mapping of water system identifiers to the ZIP codes they serve alongside the UCMR 5 results. It is what makes a ZIP code lookup possible at all. It covers 20,755 ZIP codes.

Limits

ZIP codes are postal routes, not service boundaries. About a third of the ZIP codes in the crosswalk are served by more than one system, which is why the lookup sometimes asks you to choose. Roughly half of all US ZIP codes do not appear at all, because no system serving them was required to test.

What we do to it

We join the three sources on PWSID, convert concentrations from micrograms per litre to parts per trillion, and take the highest reported value per substance per system, which is the conservative reading. We compare each value to the federal limit where one exists and say plainly when none does. We do not average, weight, rank or score.

Where this record falls short

• Lead is absent, and it is the contaminant most likely to affect an individual household, because it enters between the main and the tap rather than at the treatment plant.
• Nitrate, arsenic, bacteria and disinfection by-products are regulated under separate rules and are not in this dataset.
• Private wells are invisible to every federal dataset. Around 43 million Americans rely on them.
• Sampling is a snapshot. Source water and treatment change; a 2023 sample describes 2023.

Corrections

If a system's details are wrong, or a result does not match what your utility published, we want to know. Send the system name and what you believe is incorrect, and we will check it against the federal source and correct the page or explain why the federal record differs.