PFAS Watchlist Methodology
How this watchlist sources, validates, and presents official PFAS drinking water records — and what it will never claim or infer.
Core Principles
Official government data only
Every displayed record comes from EPA UCMR 5, EPA SDWIS, EPA ECHO, or another official government dataset with a stable .gov URL.
No risk scoring
This system generates no PFAS risk score, safety label, or contamination rating. Only official source data is displayed.
No compliance inference
UCMR 5 monitoring results are presented as monitoring data only. This system never determines whether a result constitutes a regulatory violation.
No safe/unsafe labels
Words like 'safe,' 'unsafe,' 'contaminated,' or 'dangerous' are never applied by this system unless they appear verbatim in a cited official source.
Missing ≠ absent
Absence of a record means no qualifying official record was located. It does not establish that PFAS are absent from that system's water.
Full provenance
Every record shows its source dataset, source URL, and retrieval date. Raw source payloads are retained for audit.
Allowed Data Sources
EPA UCMR 5
EPA Unregulated Contaminant Monitoring Rule 5 Occurrence Data
Primary PFAS monitoring data source. Used for all displayed PFAS occurrence records.
Refresh: Published by EPA; updated as EPA releases final occurrence data.
EPA SDWIS
EPA Safe Drinking Water Information System
Canonical utility registry. Used to match PWSID to utility name, location, and system type.
Refresh: Ingested quarterly.
EPA ECHO
EPA Enforcement and Compliance History Online
Supplemental official enforcement context where available.
Refresh: Referenced by direct link; not bulk-ingested.
Disallowed sources
This watchlist never uses commercial databases, NGOs, news stories, utility marketing pages, crowdsourced data, university summaries unless republished by a government source, or modeled/inferred contamination risk data unless published by a government authority and clearly labeled as such.
Language Rules
Allowed output language
- Official PFAS record
- Official detection
- Official non-detect
- Official monitoring result
- Official enforcement context
- No qualifying official PFAS record located
Forbidden output language
- Safe
- Unsafe
- Toxic
- Dangerous
- Failed
- Contaminated
- Poisoned
- Risk score (PFAS-specific)
- Contamination level
- Exceeds safe limit (unless exact source wording)
Record Validation
Every PFAS record must pass all of the following validations before it is published. Records that fail any check are not displayed and are queued for review.
Official government source validation — source URL resolves to a .gov or official EPA endpoint
Schema validation — all required fields present and correctly typed
Utility linkage validation — PWSID resolves to a known public water system in SDWIS
Date validation — sample date is within UCMR 5 monitoring window (2023–2025)
Unit preservation — result unit matches source exactly (no conversion without documentation)
Key-field completeness — analyte code, result value, detection flag, and source URL all present
UCMR 5 Interpretation Rules
UCMR 5 is monitoring data, not compliance data. EPA required public water systems to monitor for PFAS under UCMR 5 to gather occurrence data. A result — whether a detection or non-detect — is a monitoring result, not a compliance determination.
The EPA PFAS MCL rule (finalized April 2024) sets enforceable limits for PFOA, PFOS, PFNA, PFHxS, HFPO-DA, and a PFAS mixture. Where this watchlist shows a result alongside an EPA MCL, that comparison is informational only — this system does not determine whether any system is in compliance or violation.
Detection flag semantics: "Detected above MRL" means the result exceeded the minimum reporting limit. "Not Detected above MRL" means the result was below the MRL. Both are presented exactly as reported in the UCMR 5 dataset.
Data Freshness
Each record displays its source retrieval date — the date this system last fetched the record from the official source. Each page also shows the source publication/update date where EPA provides it. This system is not responsible for delays in EPA data releases.