Methodology · Version 1.0
How we grade US drinking water
The full algorithm behind every TapWaterSafety grade is published here. Our methodology is intentionally open so that anyone — utilities, journalists, researchers, residents — can verify our scores or challenge our reasoning.
The score in one sentence
Each public water system receives a score from 0 to 100, derived from a weighted formula across five components, mapped to a letter grade A through F.
The five components
Health Guideline Performance
How detected contaminant levels compare to the Environmental Working Group's (EWG) health-based guidelines, which are typically stricter than legal limits. This is the largest single component because it most directly corresponds to real-world health risk.
Source: EWG Tap Water Database, utility Consumer Confidence Reports
Legal Compliance
EPA Safe Drinking Water Act (SDWA) violations in the past 60 months — Maximum Contaminant Level (MCL) violations and Treatment Technique (TT) violations. Recent violations carry more weight than older ones.
Source: EPA ECHO, EPA SDWIS Federal Reporting Services
High-Risk Contaminant Presence
Whether specific high-concern contaminants are detected at all — PFAS, lead, arsenic, chromium-6, radium, nitrate above caution threshold, and disinfection byproducts above 40 ppb.
Source: EWG Tap Water Database, EPA UCMR 5 (PFAS), CCRs
Compliance & Reporting History
Monitoring violations, reporting failures, and public notification failures over the past 36 months. Utilities that fail to test, report, or notify the public are operationally weak even if their water is currently clean.
Source: EPA ECHO
Source Water Vulnerability
Source type (surface, groundwater, etc.) and state Source Water Assessment Program (SWAP) findings where available. A utility with a vulnerable source has to work harder to deliver clean water than one with a pristine source.
Source: EPA SDWIS, State SWAP reports
Grade boundaries
| Score | Grade | Interpretation |
|---|---|---|
| 85–100 | A | Excellent — minimal contamination, full compliance, low-risk source |
| 75–84 | B | Good — minor concerns but no major issues |
| 65–74 | C | Average — some contaminants above health guidelines, generally compliant |
| 55–64 | D | Below average — multiple concerns, possibly some violations |
| 0–54 | F | Poor — significant contamination, violations, or vulnerable source |
Top Concerns logic
Each utility page displays up to 5 "Top Concerns" identified by these rules, in priority order:
- Any PFAS compound detected
- Lead detected at source above 1 ppb
- Any contaminant detected at more than 10x the EWG health guideline
- Any health-based violation in the last 24 months
- Total trihalomethanes (TTHM) or haloacetic acids (HAA5) above 40 ppb
- Arsenic, chromium-6, or radium detected at any level
- Three or more monitoring/reporting violations in last 36 months
Filter recommendations
When we recommend filters on a utility page, we match the filter technology to that water's specific top concerns. We do not recommend the same filter for every utility — different contaminants require different technologies.
We earn affiliate commissions when readers buy filters through our links. This is disclosed on every page and never influences our scoring.
Data confidence indicators
- High confidence: Utility serves more than 10,000 people, has CCR data from the last 24 months, present in EWG database
- Medium confidence: Some data sources missing but core EPA SDWIS/ECHO data complete
- Low confidence: Small utility (under 3,300 people served), limited contaminant data available
Update cadence
Scores are recalculated quarterly, in line with EPA SDWIS and ECHO data refresh cycles. The EWG database refreshes annually. Significant violations or contamination events that occur between cycles can trigger off-cycle updates.
What this methodology does not do
- We do not test water ourselves. All data comes from regulatory and nonprofit sources.
- We do not measure lead at the tap. Lead contamination often comes from household plumbing, not the utility.
- We do not predict future contamination events. Scores reflect documented data from the recent past.
- We do not adjust for treatment changes that haven't yet appeared in monitoring data.
International standards comparison
Every utility and city page on TapWaterSafety includes a side-by-side comparison of detected contaminant levels against five different drinking water standards: US EPA legal limits, the EU Drinking Water Directive 2020/2184, the World Health Organization's Guidelines for Drinking-water Quality, California's Public Health Goals (often the strictest US benchmark), and EWG's health-based guidelines.
We do this because US EPA limits are decades out of date and substantially weaker than the standards most modern democracies use. Some examples of the gap:
- Lead: EPA action level is 15 ppb. EU's final limit (by 2036) is 5 ppb — three times stricter. California's Public Health Goal is 0.2 ppb, 75 times stricter than EPA.
- Total chromium: EPA limit is 100 ppb. EU's final limit is 25 ppb — four times stricter. Many European countries effectively regulate chromium-6 separately, which EPA does not.
- Pesticides: EU caps every individual pesticide at 0.1 ppb regardless of compound. EPA's pesticide limits are typically 10-30 times higher per compound.
- Vinyl chloride: EPA limit 2 ppb, EU 0.5 ppb, WHO 0.3 ppb. The US allows roughly 7x more of this known human carcinogen.
- Benzene: EPA 5 ppb, EU 1 ppb. The US allows 5x more.
- PFOA and PFOS: EU caps the sum of 20 PFAS compounds at 100 ng/L. EPA's 2024 rule is stricter on individual PFOA/PFOS (4 ng/L each) but doesn't cap the sum, allowing more total PFAS exposure.
For some contaminants the US is actually stricter — for example, US nitrate limits are tighter than European ones. We show both so readers can see the full picture rather than cherry-picked comparisons.
If a utility's water exceeds the EU Drinking Water Directive, we display a "Would fail European Union drinking water standards" notice prominently on the page. This is meant as a factual comparison, not a legal judgment — US utilities follow US law and are not obligated to meet EU standards. The point is to make the gap visible.
Sources for our comparison values:
- EPA: epa.gov/dwstandardsregulations — National Primary Drinking Water Regulations
- EU DWD: Directive (EU) 2020/2184 — Drinking Water Directive recast
- WHO: Guidelines for Drinking-water Quality 4th edition + addendum
- California PHG: Office of Environmental Health Hazard Assessment
- EWG: EWG Tap Water Database health guidelines
The full standards database we use is published at data sources and is open for verification or correction.
Disputes and corrections
We take methodology criticism seriously. If a utility, journalist, or resident believes we have a score wrong, we publish a public dispute process. See our contact page for how to reach us.
Open methodology principle
The full scoring algorithm is published. The data sources are public. Anyone can reproduce our scores. This is intentional: a methodology that can be verified by anyone is harder to attack and easier to defend.