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Water AwarenessFoundation
Methodology

How we make the reports — and what they don't tell you.

This page is the full audit trail. It explains where our data comes from, how the Water Score is calculated, what the score does and doesn't measure, and the limits we openly acknowledge. If you spot a problem, write to corrections@waterawarenessfoundation.com.

Data sources

Primary: Environmental Working Group's Tap Water Database

Every ZIP report draws first from the EWG Tap Water Database, which compiles utility-reported contaminant sampling data covering nearly every public water system in the United States. EWG's health-protective guidelines — typically stricter than federal legal limits — are the reference points we display against measured concentrations on every report.

We cache successful lookups for 30 days. The first visitor for a given ZIP triggers a live lookup; subsequent visitors get a sub-100ms cached response.

Secondary sources, integrated into article content

Articles and contaminant pages additionally draw on:

  • CDC environmental health publications
  • U.S. Geological Survey water-quality surveys (especially the 2023 PFAS national tap-water study)
  • NIEHS, ATSDR, and NTP toxicological assessments
  • WHO Guidelines for Drinking-water Quality (4th ed.)
  • Peer-reviewed primary literature via PubMed
  • U.S. federal regulations (Safe Drinking Water Act, Lead and Copper Rule, etc.) cited for legal-limit context alongside the EWG health-protective guidelines we treat as the reference for actual safety.

Every numeric claim and citation on the site maps to an entry in our source registry.

How contaminants are flagged

For each utility, we display every contaminant EWG has measured against two reference values:

  • The EWG health-protective guideline — typically based on California OEHHA public-health goals, peer-reviewed cancer-risk thresholds, or EWG's own analyses. These are far stricter than federal legal limits in nearly every case.
  • The federal legal limit (MCL) — shown for context. "Above guideline" does not mean the water is in legal violation; it means the level exceeds what EWG considers health-protective.

A contaminant is flagged on the report when the measured level exceeds the EWG guideline. Severity tiers:

  • Severe: 100× or more above the guideline
  • Elevated: 10×–100× above the guideline
  • Above: 1×–10× above the guideline
  • Detected: present but below the guideline

What the report does NOT measure

This is the most important section of this page. Please read it.

EWG's data is utility-reported sampling, averaged and made comparable across systems. It is more health- protective than legal-compliance data, but it is still not a lab test of the water at your specific kitchen tap.

Specifically, the report does not measure:

  • Lead in your own home. Lead enters water from service lines, lead solder, and brass fittings inside your house — after the utility's responsibility ends. Two homes on the same block can have wildly different lead levels.
  • PFAS exposure in real-time. The 2024 federal PFAS rule triggered new utility sampling that is still rolling out. Reports through 2026 may undercount PFAS at your utility.
  • Contaminants without federal sampling requirements. Microplastics and most pharmaceuticals are not routinely measured at the utility scale.
  • Seasonal variation. Many contaminants — especially disinfection byproducts and nitrate — vary significantly through the year. Annual averages can hide real spikes.

Our recommendation to readers: the report is the start of the conversation, not the end. If you want certainty about your own tap, the only path is a certified laboratory test. Most state health departments maintain a list of certified labs.

Reporting lag

Contaminant-sampling data lags actual sampling by 1–6 quarters depending on the state. The latest report you see for a given utility reflects what that utility has reported to its state regulator and what has been ingested into our upstream data source. When we say "recent," we mean recent as reported, which is roughly the past 4–5 years of sampling.

Updates and corrections

We publish every correction we make. The first correction log will appear here once we have made one. The policy:

  • Article text is updated in place, with the updated date published in the article header.
  • Substantive corrections (numeric values, conclusions) are logged on this page with the original wording, the corrected wording, and the date.
  • Stylistic edits and minor copy fixes are not logged.
  • We do not silently edit articles to change their conclusions. If we update our position, we say so.

Source registry

Every numeric claim, regulatory limit, and health effect cited on the site traces back to one of these primary sources. New sources are added as articles require them.

  1. Safe Drinking Water Act overview — U.S. Environmental Protection Agency (accessed 2026-05)
  2. National Primary Drinking Water Regulations — U.S. Environmental Protection Agency (accessed 2026-05)
  3. Safe Drinking Water Information System (SDWIS) Federal Reports — U.S. Environmental Protection Agency (accessed 2026-05)
  4. PFAS National Primary Drinking Water Regulation (Final Rule, April 2024) — U.S. Environmental Protection Agency (accessed 2026-05)
  5. Lead and Copper Rule Improvements (Final Rule, October 2024) — U.S. Environmental Protection Agency (accessed 2026-05)
  6. Sources of Lead Exposure — Centers for Disease Control and Prevention (accessed 2026-05)
  7. Community Water Fluoridation — Centers for Disease Control and Prevention (accessed 2026-05)
  8. Guidelines for Drinking-water Quality, Fourth Edition — World Health Organization (accessed 2026-05)
  9. Perfluorinated Chemicals (PFAS) — National Institute of Environmental Health Sciences (accessed 2026-05)
  10. Toxicological Profile for Perfluoroalkyls — Agency for Toxic Substances and Disease Registry (accessed 2026-05)
  11. NTP Monograph on the State of the Science Concerning Fluoride Exposure and Neurodevelopmental and Cognitive Health Effects (2024) — National Toxicology Program (accessed 2026-05)
  12. PFAS in US Tapwater (2023) — U.S. Geological Survey (accessed 2026-05)
  13. Tap Water Database — Environmental Working Group (accessed 2026-05)
  14. PFAS Contamination in the U.S. — Environmental Working Group (accessed 2026-05)
  15. Public Drinking Water Systems: Facts and Figures — U.S. Environmental Protection Agency (accessed 2026-05)
  16. Lead Service Line Inventory — U.S. Environmental Protection Agency (accessed 2026-05)
  17. Estimated Use of Water in the United States — U.S. Geological Survey (accessed 2026-05)
  18. Microplastics Research — U.S. Environmental Protection Agency (accessed 2026-05)
  19. Flint Water Advisory Task Force Final Report — State of Michigan (accessed 2026-05)
  20. Lead Pipes Are Widespread and Used in Every State — Natural Resources Defense Council (accessed 2026-05)
  21. Chlorine in Drinking Water — U.S. Environmental Protection Agency (accessed 2026-05)
  22. Drinking Water Treatment Unit Standards (NSF/ANSI 42, 53, 58, 401, P473) — NSF International (accessed 2026-05)