# UCMR 5 — PFAS in US public water systems, per system (built 2026-10-31)

Source: US EPA — UCMR 5 (2023–2025) occurrence data, final dataset — https://www.epa.gov/dwucmr/occurrence-data-unregulated-contaminant-monitoring-rule — ucmr5-occurrence-data.zip (UCMR5_All.txt, UCMR5_ZIPCodes.txt dated 2026-10-15), retrieved 2026-10-31. Public domain (US federal work).

Method: Per public water system (PWSID): for each of the 29 PFAS analytes (lithium excluded), the number of results, the number reported as detections (AnalyticalResultsSign "="), and the maximum detected value in µg/L; the sampling window; ZIP codes served from UCMR5_ZIPCodes.txt. Non-detects are results below the UCMR 5 minimum reporting level (MRL), which for PFOA and PFOS is 0.004 µg/L = 4 ng/L. Nothing is modelled or interpolated.

Files: `summary.json` (counts + provenance) · `zip/NNN.json` (ZIP → PWSIDs, by 3-digit prefix) · `pws/ST.json` (systems by state: name, size, sample count, window, tested analytes, detections {analyte: [max µg/L, detects, results]}).

Counts: 10313 systems · 20755 ZIPs · 3595 systems with ≥1 PFAS detection · PFOA detected (≥ 4 ng/L) in 1305 · PFOS detected (≥ 4 ng/L) in 1352.

Rebuild: download the zip, run the aggregation (session 2026-10-31 handoff has the script); never edit these files by hand. Dated history: keep each release under its file date; this is the 2026-10-15 final release.