What you're looking at
This list ranks public water systems by the population they serve, drawn from the EPA's SDWIS record (data cutoff December 31, 2024; snapshot September 16, 2026). It exists to orient you to the systems whose monitoring records affect the most people — not to rank them by quality. Population figures are as reported to the EPA and can lag real service-area changes.
At the top of the list
The largest system on record is the New York City System (PWSID NY7003493), reported serving about 8,271,000 people, with a Purity Score of 86 out of 100. It is followed by the Los Angeles Department of Water & Power (CA1910067, about 3,875,566 served, score 95) and the City of Houston (TX1010013, about 2,970,543 served, score 91). The scores differ even among the three biggest systems, which is the point: size and score are separate measures.
Why size is not a quality signal
A large utility typically monitors a broad set of contaminants, so its score reflects more measurements — not necessarily a worse or better outcome than a small system that tested less. That is also why two large systems can carry noticeably different scores. PurityMap reports confidence separately from the score so that breadth of testing is visible rather than hidden inside the number.
Population figures are as reported to the EPA and can lag real service-area changes. Size is not a quality signal.
What this means for your tap
Even the largest system's record describes treated water leaving the utility, not the water at your own fixture. Lead in particular usually enters through a home's service line or plumbing, so a big system's score says nothing definitive about an individual older tap. Confirm your supplier on your utility bill before drawing a conclusion for your address.
How to use the ranking
Open the full national water rankings to see all 25 of the largest systems with their populations and scores, then click any system through to its city water report to see which contaminants were measured and when. To see the extremes, compare the cities with the highest Purity Scores and the cities with the lowest Purity Scores, and read how the Purity Score is calculated. For your own city, check its PFAS and lead findings directly.
This article summarizes public EPA records and a scoring method built on them. It is not medical advice and does not declare any water fit or unfit for a specific person. For health questions about lead, PFAS or other contaminants, consult a licensed healthcare professional and, for your own address, use a certified laboratory test.