Is Your City’s Tap Water Safe? The 100 Largest US Water Systems, by the EPA Data

The 100 largest US public water systems serve about 93,230,625 people (EPA SDWIS). In their federal records, 41 have something above a current EPA limit or action level, 53 more have something worth a look (a detection under the limit, an older exceedance, or violations on record), and 6 are clear on every measure we grade. These are the systems’ own reported results — occurrence records, not compliance verdicts, and never a reading of any individual tap.

All 100 systems, largest first

These are the systems’ own EPA UCMR5 monitoring results — “reported above the limit,” not “in violation.” The EPA limit (4 ppt for PFOA and for PFOS) is a running annual average, so a system counts here when the AVERAGE of a full set of results at one of its sampling locations is above the limit — EPA’s own method. That is an occurrence flag, not a compliance finding: real compliance is determined from quarterly samples reported to the state. Enforcement phases in through 2029-2031. A metro area is usually served by more than one water system — this page covers the area’s largest. Suburbs and neighboring towns often have their own systems with their own results; check your exact ZIP for yours.

See also water quality by state.