Methodology
Methodology—How the data is sourced, scored, and presented.
Every number in a WaterHealthCheck report comes from a named public federal source. This page documents exactly which sources we use, how we score contaminant levels, and—just as importantly—what we don't do with data that isn't in the record.
Data sources
Where the data comes from.
Safe Drinking Water Information System
Quarterly bulk export from the federal database covering every public water system in the U.S. Contains utility-reported monitoring results, violations, and treatment information. This is the primary data source for all contaminant levels in reports.
Unregulated Contaminant Monitoring Rule 5
PFAS and emerging contaminant monitoring data from the 2023–2024 monitoring cycle, compiled under EPA's fifth Unregulated Contaminant Monitoring Rule. The most comprehensive public PFAS dataset ever assembled for U.S. drinking water. Results are now fully incorporated; the dataset is updated if EPA issues corrections or supplements.
EWG Health Guidelines
Used as secondary benchmarks—stricter than EPA Maximum Contaminant Levels, derived from peer-reviewed toxicology. EWG guidelines are not legally enforceable; they represent health-protective limits based on current science. We show both sets of limits so users can see where their utility's readings sit relative to each.
State Supplementary Data
California's MCL for hexavalent chromium (Cr-VI) is implemented—10 ppb effective July 2024 (HSC §116460), shown separately from the federal total chromium MCL which is far less restrictive and does not isolate Cr-VI. Additional state-level MCLs will be added as coverage expands.
Benchmarks
Two limits, not one.
Reports compare each contaminant against two benchmarks, not just the legal limit.
EPA Maximum Contaminant Levels (MCLs) are the legally enforceable limits. Exceeding an MCL is a regulatory violation. MCLs are set by balancing health risk against feasibility and cost—which means some MCLs are set above what toxicology would suggest as a health-protective threshold.
EWG health guidelines are derived from peer-reviewed toxicology without feasibility constraints. They are not legally enforceable, but they represent where the science points independent of what utilities can achieve. For some contaminants—like trihalomethanes and PFAS—EWG guidelines are orders of magnitude stricter than EPA MCLs.
Showing both gives users a complete picture: what's legal, and what the health science recommends.
Status encoding
What the colours mean.
The contaminant was detected at or below both the EPA MCL and the EWG health guideline. Or it was not detected at all.
The contaminant was detected above the EWG health-based guideline but below the EPA Maximum Contaminant Level. Legal, but above the health-protective threshold.
The contaminant was detected above the EPA legal limit. This represents a compliance violation. The report will note the violation record.
The contaminant was tested for but not detected at or above the reporting threshold.
Status colours are used only for data encoding—never for decoration. The colour system is consistent across all report views and export formats.
Utility score
The 0–100 score.
Each utility receives a score from 0 to 100. The score is derived from the contaminant levels detected relative to their limits—weighted by the severity of the contaminant and its detection level.
A score of 100 means all contaminants were either not detected or within all limits. The score decreases as contaminants approach or exceed EWG guidelines, and decreases more sharply for contaminants above EPA MCLs.
The score is a utility-level summary. It is not a health risk score for an individual—household profile, consumption habits, and other exposures all affect actual risk, and those are addressed in the personalised contaminant breakdown rather than the summary score.
What we don't do
No estimates. No interpolation.
If a measurement doesn't exist in the public record, we don't show one. We don't fill gaps with estimates from neighbouring utilities, regional averages, or modelling. A blank field means no data—not zero detected.
Reports reflect utility-level monitoring data as reported to EPA. Point-of-use conditions—older building plumbing, lead service lines not yet replaced — can result in higher contaminant levels at the tap than utility monitoring shows. We note this where relevant, particularly for lead.
Known limitations
Known limitations.
Utility-level data only. Reports reflect monitoring results as reported by the water utility to EPA. Conditions at your specific tap—older building plumbing, a lead service line not yet replaced, a private well on the property—may differ from utility-level data. We note this where relevant, particularly for lead.
Coverage gaps. Not every utility has complete data for every contaminant. Where monitoring data is absent, we show that explicitly rather than substituting an estimate. Gaps are common for smaller utilities and for emerging contaminants not yet subject to federal monitoring requirements.
UCMR5 scope. UCMR5 monitored systems serving 3,300 or more people, plus a representative sample of smaller systems. Smaller utilities outside the sample may not have PFAS data in our system.
EWG guidelines are not regulatory standards. We use them as health-protective benchmarks, clearly labelled as such. They should not be read as legal limits or as official EPA positions.
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