Methodology
Water Quality Score
Each city receives a water quality score from 0 to 100, calculated based on four factors from EPA compliance data:
- Contaminants above EPA limits (MCL): Each contaminant exceeding its Maximum Contaminant Level reduces the score by 15 points (maximum deduction: 45 points).
- Active health-based violations: Each unresolved health-based violation reduces the score by 20 points (maximum deduction: 40 points).
- PFAS above health advisories: Each PFAS compound detected above EPA health advisory levels reduces the score by 10 points (maximum deduction: 30 points).
- Monitoring violations: Each unresolved monitoring or reporting violation reduces the score by 5 points (maximum deduction: 15 points).
Score Ranges
| Score | Rating | Meaning |
|---|---|---|
| 90-100 | Excellent | Meets or exceeds all standards |
| 70-89 | Good | Minor issues, generally safe |
| 50-69 | Fair | Some contaminants or violations present |
| 30-49 | Poor | Multiple contaminants above limits |
| 0-29 | Very Poor | Significant contamination or violations |
Data Sources & Freshness
Our data comes from two primary EPA sources updated on different schedules:
- SDWIS: Updated quarterly by EPA. Contains violation history and compliance data.
- UCMR5: Released in rounds as sampling is completed (2023-2026). Contains PFAS and other unregulated contaminant monitoring data.
We refresh our database within one week of each new EPA data release. The "last updated" date on each page reflects when we last processed data for that water system.
Limitations
- Our scores reflect the most recent available EPA data, which may lag behind current conditions.
- Water quality can vary within a city depending on which water system serves a specific address.
- Not all contaminants are monitored by EPA. Some emerging contaminants may not appear in our data.
- Scores do not account for the condition of household plumbing, which can introduce lead and other contaminants.