Our Methodology

WaterSafety analyzes drinking water quality for every US public water system by examining EPA violation records, contaminant testing results, and enforcement actions. We believe everyone deserves to know what is in their tap water.

Data Sources

Our primary data source is the EPA Safe Drinking Water Information System (SDWIS), the federal database of drinking water quality violations for all public water systems in the United States. SDWIS tracks:

• Health-based violations — Exceedances of Maximum Contaminant Levels (MCLs) for specific contaminants
• Monitoring violations — Failure to test water as required
• Treatment technique violations — Failure to follow required treatment processes
• Enforcement actions — State and federal enforcement responses to violations

SDWIS data is updated quarterly as states submit compliance data to the EPA.

How We Calculate the Water Safety Score

Every water system receives a Water Safety Score on a 0-100 scale (A-F) based on its compliance record:

• Health-Based Violations — Highest weight. Violations where contaminant levels exceed the Maximum Contaminant Level (MCL) are the most serious. These mean the water contained a contaminant at levels the EPA considers a health risk. Recent violations are weighted more heavily than older ones.
• Monitoring Violations — Moderate weight. Failure to test water as required is concerning because it means contamination could go undetected. Systems that do not test cannot prove their water is safe.
• Violation History and Recurrence. Systems with repeated violations over multiple years score lower than those with isolated incidents. A pattern of violations suggests systemic problems rather than one-time issues.
• Enforcement Actions. Formal enforcement actions (administrative orders, consent agreements, civil penalties) by state or federal authorities indicate the violations were serious enough to warrant regulatory intervention.

Letter grades: A (80-100) indicates a clean compliance record; F (0-34) indicates multiple health-based violations or persistent non-compliance.

Contaminant Profiles

For key contaminants of public concern (lead, PFAS, arsenic, nitrate, disinfection byproducts), we provide dedicated pages explaining what the contaminant is, the EPA's legal limit, health effects, and which water systems have violations for that specific contaminant.

Data Collection Process

We query SDWIS data for all public water systems (community water systems and non-transient non-community water systems), extract violation records, and calculate our composite Water Safety Score. Systems are mapped to zip codes so users can look up their local water system.

Update Frequency

EPA SDWIS data is updated quarterly. We refresh our database within two weeks of each quarterly update to capture new violations, resolved violations, and enforcement actions.

Known Limitations

• SDWIS only covers public water systems. Private wells (serving approximately 43 million Americans) are not regulated under the Safe Drinking Water Act and have no federal monitoring data.
• Lead contamination often occurs in household service lines and plumbing, not at the treatment plant. System-level testing may not reflect the lead level at your specific tap.
• Some emerging contaminants (PFAS, microplastics, certain pharmaceuticals) do not yet have enforceable federal MCLs, meaning violations cannot be recorded even if the contaminant is present.
• Monitoring violations mean the system failed to test — which could mean the water is fine but untested, or could mean contamination went undetected. We treat both health-based and monitoring violations as concerning.
• The Water Safety Score is our own composite metric, not an EPA designation.

How to Cite This Data

If you use data from WaterSafety, please cite:

WaterSafety. "[Water System Name] Safety Data." iswatersafe.com, 2026. Accessed [date].

Underlying data is sourced from the EPA Safe Drinking Water Information System (SDWIS) and is in the public domain.