Methodology

All water quality data comes from the EPA’s Unregulated Contaminant Monitoring Rule, 5th cycle (UCMR 5). Under this program, water systems serving more than 3,300 people and a representative sample of smaller systems were required to test for 30 contaminants, including 29 PFAS compounds, between 2023 and 2026.

Every water system receives a letter grade (A through F) based on how its PFAS levels compare to EPA Maximum Contaminant Levels (MCLs).

For compounds without an established MCL, grades are assigned based on detection count.

These are the first-ever federal limits on PFAS in drinking water, finalized in April 2024.

Some databases use proprietary health guidelines that are significantly stricter than EPA standards. We use EPA MCLs because they are:

• The legally enforceable federal standards that water systems must comply with
• Based on extensive scientific review by the EPA
• Transparent and publicly documented
• The standard against which compliance actions and enforcement are measured

Using stricter proprietary guidelines would result in virtually every water system receiving a failing grade, which would reduce the tool’s ability to distinguish between systems with genuine exceedances and those that meet federal requirements.

All concentrations are displayed in parts per trillion (ppt), equivalent to nanograms per liter (ng/L). For reference: 1 part per trillion is approximately one drop of water in 20 Olympic swimming pools.

Raw UCMR 5 data is processed as follows:

1. Non-detect results (samples below the Minimum Reporting Level) are excluded from concentration averages.
2. For systems with multiple sampling events, concentrations are averaged across all detected samples.
3. The MCL ratio is calculated as: average detected concentration ÷ EPA MCL.
4. Grades are assigned based on the highest MCL ratio across all regulated compounds.

• Data reflects testing conducted between 2023 and 2026 and may not represent current water quality.
• Not all water systems were tested — smaller systems serving fewer than 3,300 people and private wells are generally not included.
• PFAS is just one category of drinking water contaminant; a good PFAS grade does not mean water is free of all contaminants.
• Concentration averages may not represent peak contamination events.
• Some systems may have installed treatment since testing was conducted.
• The Hazard Index for PFAS mixtures is not yet incorporated into grading.

The complete grading algorithm is available on GitHub. If you believe a grade is incorrect, please contact data@checkyourwater.org with the PWSID and your analysis.