Data analysis · EPA SDWIS
What the EPA's data shows about US drinking-water violations
We summarize the EPA's Safe Drinking Water Information System for ~19,000 US cities. Looking across the whole dataset — 1,520,116 violation records from 48,552 community water systems, of which 190,959 are "health-based" — a few patterns stand out. None of this is a safety verdict; it's a read of what utilities reported to the EPA.
The biggest source of health-based violations isn't a pollutant — it's the disinfectant
The two most-cited health-based contaminants, TTHM and HAA5, together account for about 75,000 records — nearly 40% of all health-based violations. Both are disinfection byproducts: they form when the chlorine that makes water microbially safe reacts with natural organic matter. So the single largest category isn't industrial contamination — it's a tradeoff baked into disinfection itself, which is why the EPA regulates the byproducts rather than the chlorine.
Most-cited contaminants (health-based violation records)
Treatment-technique & monitoring rules
A large share of violations aren't a measured chemical at all — they're failures to treat, monitor, or report on schedule under a specific rule. These matter, but they describe a process lapse, not necessarily a substance found in the water.
By state — and why system size matters more than geography
Texas and Oklahoma lead by raw count, but the more telling number is the share of a state's cities with any health-based violation on record. In Oklahoma, 474 of 488 cities have at least one — versus about half in California and New York. States dominated by many small, rural systems tend to rank highest, because small systems have the thinnest budgets for treatment and testing. It's less about where you are than how big your utility is.
| State | Health-based records | Cities affected | |
|---|---|---|---|
| Texas | 35,417 | 802 / 1239 | 65% |
| Oklahoma | 29,120 | 474 / 488 | 97% |
| California | 16,347 | 549 / 911 | 60% |
| Louisiana | 14,578 | 272 / 327 | 83% |
| New Mexico | 7,720 | 204 / 254 | 80% |
| Arkansas | 5,807 | 263 / 401 | 66% |
| New York | 5,243 | 425 / 844 | 50% |
| Pennsylvania | 5,213 | 475 / 776 | 61% |
How to read a "violation"
A violation is a record that a system fell short of a federal rule — it is not a reading that your water is unsafe. Many are monitoring or reporting lapses; others are measured exceedances of a federal limit, often brief and since resolved. A record with no return-to-compliance date can mean an open issue or simply a reporting lag in the EPA's data. For any specific city, the dated, per-system detail is on its TapWaterMap page, and the authoritative record is the EPA's own ECHO and your utility's annual Consumer Confidence Report.
Methodology
Figures are computed from the EPA Envirofacts SDWIS export for active community water systems, retrieved June 2026 (the EPA refreshes roughly quarterly). "Health-based" uses the EPA's own classification. Counts are of violation records, not people or systems, and reflect data as reported by utilities to the EPA. See our Methodology & Sources for the full data chain. Reuse welcome with attribution to TapWaterMap and the EPA.