What the score is
The PurityMap Purity Score is a reproducible number from 0 to 100 for a water system. It is computed only over the contaminants that system actually measured: each finding is scored against a health-based reference — usually the federal limit and, where relevant, the health goal — and the per-contaminant results roll up into one overall number. Because the inputs are fixed public records, the same inputs always produce the same score.
What goes into it
Findings are grouped into categories such as PFAS and metals (which includes lead), and each is compared to its own reference standard from the current federal rules. The score uses the 2024-federal benchmark, so PFAS is scored against the 2024 limits and lead against the Lead and Copper Rule action level. Every finding on a report links to the dated EPA source it came from.
What it deliberately leaves out
The score is health-based, so water hardness is excluded — it is an aesthetic and plumbing matter, not a regulated health contaminant. Contaminants that were never tested are shown as pending, never scored as if they passed. And the score is reported separately from confidence, so a thorough testing program is not penalized for surfacing more measurements.
A missing category is marked pending, not counted as clean. Absence of a measurement is never treated as evidence of absence.
A worked example: Tucson
For Tucson Water (PWSID AZ0410112), the 2024 PFAS round found PFOA and PFOS not detected, so the PFAS category scored 100. The most recent lead result, 0.65 µg/L, is about 4% of the 15 µg/L action level, giving a metals score of 98. Those roll up to an overall 99 out of 100 — but at moderate confidence, because two of the four categories (hardness and disinfectant type) were not in the federal data and are still pending.
Why we separate score and confidence
A system that tests widely can post a lower score than one that tests little, simply because more was measured. Collapsing that into a single number would reward thin testing. Reporting confidence alongside the score lets you see both how clean the measured record is and how complete the testing was.
How to use it
Treat the score as a fast summary and then read the evidence beneath it. From any city water report you can open each finding with its source and date, compare against the highest and lowest Purity Scores nationally, and see which contaminants — PFAS, lead and others — drove the number for your city.
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, for your own address, use a certified laboratory test.