Absence of evidence, not evidence of absence

A water system with zero violations on record has either genuinely met every standard it was tested against, or has simply not been tested as often. EPA's own monitoring schedule scales with population served, so the two explanations are not equally likely across every system size.

What to weigh alongside it

Population served on the system's own page is the fastest signal: systems serving fewer than about 10,000 people test on a sparser schedule under EPA rules. That does not mean the water is worse, only that a clean record carries less statistical weight.

Where to go next

Real systems this plays out on: PORTLAND WATER BUREAU, OR, CITY OF HOUSTON, TX, SFPUC CITY DISTRIBUTION DIVISION, CA.

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