Model documentation

US Water Score Methodology

Current model version: 1.1.0 · Published July 29, 2026.

What the indicator means

The score is a reproducible screening indicator derived from public water-system records. It is not an EPA grade, a regulatory compliance decision, a laboratory result for a home, or a declaration that water is safe or unsafe.

System-level formula

Each active community water system starts at 100. Penalties are capped by group so the final score remains between 0 and 100:

• Violations, maximum 45 points: count and severity of records in the five-year data window, with added weight for health-based and major violations.
• Lead and copper, maximum 25 points: the latest dated EPA ECHO 90th-percentile results, compared with the numerical thresholds documented on each report.
• PFAS, maximum 20 points: UCMR5 occurrence detections and values above numerical MCLs used for comparison. UCMR occurrence data are not themselves compliance determinations.
• Enforcement, maximum 10 points: enforcement-action records in the five-year window.

The implementation calculates score = max(0, 100 − violation penalty − lead/copper penalty − PFAS penalty − enforcement penalty). Grades are A for 85–100, B for 70–84, C for 55–69, D for 35–54, and F below 35.

Geographic aggregation

When multiple active systems map to one ZIP, their scores are weighted by EPA-reported population served. State indicators are population-weighted aggregates of mapped ZIP indicators; state pages also expose the median and unweighted mean when available. Population and service-area mappings may overlap, so state values are comparative indicators rather than population estimates.

Sources and timing

The build uses EPA SDWIS violations and enforcement records, dated EPA ECHO lead and copper results, UCMR5 occurrence data, and public service-area mapping records. The current SDWIS snapshot is July 2026; UCMR5 files are January 2026. Source dates and methodology are monitored for changes.

Known limitations

• Public records can be incomplete, delayed, corrected, or associated with more than one ZIP.
• A ZIP can contain multiple utilities and private wells; a street address may not be served by the system shown.
• Building plumbing and lead service lines can materially change conditions at an individual tap.
• UCMR5 is occurrence monitoring, not a substitute for current compliance records.
• The penalty weights are an editorial screening model, not toxicological risk coefficients.

How to verify your water

Check your utility's current Consumer Confidence Report, official notices, and certified laboratory testing. Report suspected data or mapping errors to david@uswaterscore.com.