What the Purity Score measures
The Purity Score is computed only over the contaminants a system actually measured, scored per-contaminant against health-based references and rolled up to a 0–100 number. It is reproducible: the same inputs always give the same score. For the full method, see how the Purity Score is calculated.
Why 'lowest' needs a caveat
A low score means more or more-severe measured exceedances — often a PFAS sample above the 2024 EPA limits, or lead. But a city that tests thoroughly can score lower than one that tests very little, simply because more was measured. Score and confidence are reported separately for exactly this reason. In the September 16, 2026 ranking snapshot, taken before the current score, the lowest-ranked system was the Spalding County Water System in Georgia (PWSID GA2550036) at 0 out of 100, with 5 flagged contaminants on record.
A low score is a reason to read the evidence, not a verdict. Every finding on PurityMap links to its dated EPA source.
How to use the rankings
Open the full national water rankings to see the lowest- and highest-scoring systems, each with its metric and data cutoff, and compare against the cities with the highest Purity Scores. Then click through to a city water report to see which contaminants drove the number, and whether a certified filter addresses them.
This article explains a scoring method built on public EPA monitoring data. It is not medical advice and does not declare any water fit or unfit for a specific person. For health questions about specific contaminants, consult a licensed healthcare professional, and use a certified laboratory to test your own water.