Why we publish this page
A water quality rating system is only as trustworthy as its inputs. We publish the full source list — with links — so you can verify any specific data point against the original source. If you see something on a TapWaterSafety page that doesn’t match what an authoritative source says, contact us and we will correct it.
Primary federal sources
EPA Safe Drinking Water Information System (SDWIS)
The federal master record of every public water system in the United States. We pull utility identity (name, owner type, source water type), service area (counties, cities, ZIPs served), and population served from this source.
- Source: US Environmental Protection Agency
- Public URL: epa.gov/enviro/sdwis-overview
- Bulk data: echo.epa.gov/files/echo_data/sdwa_downloads.html
- Refresh: Quarterly
- Coverage: All ~150,000 US public water systems, including ~50,000 community water systems that serve residents year-round
- What we use from it: PWSID, utility name, state, county, cities served, ZIPs served, population, owner type, source water type, contact info
EPA Enforcement and Compliance History Online (ECHO)
The federal record of every Safe Drinking Water Act violation, enforcement action, and resolution. Our Legal Compliance score component comes from this source.
- Source: US Environmental Protection Agency
- Public URL: echo.epa.gov
- Refresh: Quarterly
- Coverage: All SDWA violations going back decades
- What we use from it: MCL violations (health-based), TT violations (treatment technique), monitoring violations, reporting violations, public notification violations, resolution status
EPA UCMR (Unregulated Contaminant Monitoring Rule)
The federal sampling program for contaminants not yet regulated under the Safe Drinking Water Act. UCMR 5 (2023-2025) is the first round to include all six PFAS compounds. We use UCMR data for PFAS detection where available.
- Source: US Environmental Protection Agency
- Public URL: epa.gov/dwucmr
- Refresh: Per UCMR cycle (every 5 years)
- What we use from it: PFAS, PFOA, PFOS, PFNA, PFHxS, PFBS, GenX, and other emerging contaminant detection levels
Primary nonprofit source
EWG Tap Water Database
The Environmental Working Group’s database of contaminant detections per public water system, compared against EWG’s health-based guidelines. EWG’s guidelines are typically stricter than EPA’s legal limits and reflect modern toxicology research. Our Health Guideline Performance score component (40% of total) is based on this source.
- Source: Environmental Working Group, a 501(c)(3) nonprofit
- Public URL: ewg.org/tapwater
- Refresh: Annually
- Coverage: ~50,000 US community water systems
- What we use from it: Detected contaminant levels per utility, EWG health guidelines, exceedance ratios
Note on attribution: We attribute EWG as a source on every page that uses their data. For commercial-scale use of EWG data, we recommend reaching out to EWG at info@ewg.org for formal data partnership.
Utility-published sources
Consumer Confidence Reports (CCRs)
Every US community water system is legally required by the EPA to publish an annual Consumer Confidence Report listing detected contaminants, treatment processes, and any violations. We link to each utility’s CCR from its utility page so readers can see the original. Where EWG data is unavailable or stale, CCR data is the fallback.
- Source: The utility itself (legally mandated reporting)
- Refresh: Annually (utilities publish by July 1 each year)
- Coverage: All US community water systems
- What we use from it: Contaminant levels, treatment process information, recent violations
Supporting data
US Census Bureau American Community Survey (ACS)
Used for service-area population and demographic context.
- Source: US Census Bureau
- Public URL: census.gov/programs-surveys/acs
- Refresh: Annually (5-year ACS estimates)
- What we use from it: Population counts, demographic context for service areas
How we combine these sources
Our scoring methodology applies a weighted formula across five components, each drawing from one or more of these sources:
| Component | Weight | Primary sources |
|---|---|---|
| Health Guideline Performance | 40% | EWG Tap Water Database, utility CCRs |
| Legal Compliance | 20% | EPA ECHO, EPA SDWIS violations |
| High-Risk Contaminant Presence | 20% | EWG, EPA UCMR 5 (PFAS), CCRs |
| Compliance & Reporting History | 10% | EPA ECHO |
| Source Water Vulnerability | 10% | EPA SDWIS, state Source Water Assessment Programs |
See the full methodology page for the exact formula and weighting.
On every utility page
Every utility’s grade card lists which specific sources contributed to that utility’s score, with the date the data was fetched. If a utility shows “Sample data” or “Stub data” in the source list, that means the live source data hasn’t been pulled yet and the displayed values are illustrative placeholders, not real measurements. Real source attribution looks like: “EPA SDWIS (fetched 2026-04-10), EWG Tap Water Database (fetched 2026-04-12), City of Tampa CCR 2024.”
What we do not source
We do not test water ourselves. We do not have proprietary sensors. We do not measure water at any specific tap. All of our numbers come from the sources listed above, and you can verify any of them against the originals.
For your specific home’s tap water — particularly if you’re concerned about lead from premise plumbing — we recommend ordering an EPA-certified laboratory test. We list certified labs in our filter buying guide.
Corrections
If you find a data point on TapWaterSafety that doesn’t match the source we cite, please tell us with the page URL, the specific data point disputed, and the source you believe is correct. We respond to data corrections within 30 days. Confirmed corrections are noted in the utility’s changelog.