What 42,081 US water systems actually report
We assembled the measured contaminant levels of 42,081 US community water systems from raw state monitoring records, and graded every one of them against the health-based level in the research alongside the legal limit. Everything built on it is free to read, with the full method behind every figure.
We did not measure any of this water. We gathered the utilities' numbers and graded them.
people served by those systems
274 million
- 42,081 water systems in the panel set
- 38,332 community systems the reports cover
- 214,088 measured results in the graded overview
Look up your own water
Enter a ZIP code or address to see the PFAS monitoring record for the water system that serves it, drawn from the same federal and state records these reports are built on. It is free and needs no email.
Start here
One page covering the whole graded panel at once: 214,088 measured results across 23,927 water systems. It says what each contaminant is, where it comes from, and what is measured but not graded.
The national numbers, one contaminant at a time
Each contaminant has its own page: what the research shows, the health-based and legal lines, and our national measurements for it. The measurements come from the same 38,332 community water systems and the same grading rules, so you can read them against each other. Every page names its own denominator and labels how each value was obtained.
- Science page · national data
Total trihalomethanes (TTHM)
3,049 of 17,538 systems average at or above the 41 ppb level where pooled human data shows cancer risk rising. 221 are at or above the 80 ppb legal limit. - Science page · national data
PFOA and PFOS
12,940 systems have a PFOA result and 2,685 detected it. 666 average at or above the 4.0 ppt federal limit that binds from 2029. - Science page · national data
Arsenic
3,532 of 15,115 systems report arsenic at or above the 2 ppb reporting floor; 188 are at or above the 10 ppb federal limit.
Regional benchmark: 28 Southern California districts, compared as a county
One regional benchmark so far, covering all 28 retail water districts in one Southern California county. It is a county-level benchmark built on the districts’ own published water quality reports, and it names no district. Your district’s own report is on its website, and the state posts every district’s report too. It covers one county so far, and we add counties one at a time.
What is this dataset?
It is 42,081 US community water systems with measured contaminant levels.
We assembled it from state drinking-water monitoring records, utility reports and federal monitoring rounds into one panel set. Then we graded each system against health-based thresholds drawn from the cited primary literature.
We took none of these measurements ourselves. Outside the 28 Southern California districts, we computed every value as a reconstruction from raw monitoring records. Inside them, we used the utility’s own published figures. We label which kind you are reading, so you always know.
Everything here stops at the water system, so it does not describe any single house. Lead, which enters at the plumbing between the main and your tap, is structurally understated by utility data, so these figures carry no national lead number. The tap-water overview explains why.
Can I get the raw data?
No. The row-level extract took months to build out of several hundred separate state and federal sources, and the product is built on it. We do publish every finding, the full method, the denominators, the limits and the primary source behind every threshold, so you can check the work without the compilation itself.
You can check any figure yourself. If you doubt a Southern California number, open your district’s own report: it is on the district’s website, and the state posts every district’s report too. If you doubt a national number, put one address into the lookup and compare what it returns against the utility’s published report.
Researchers and reporters who need specific figures should write to data@stasishome.com and say what they are working on. If a figure here disagrees with a utility’s published report, the published report is more likely to be right and we want to know that too.
Coverage, contaminant by contaminant
National coverage for each contaminant with a full data block: how many systems have a result, how many detect it, how many sit above the health-based line, and where the full accounting lives.
| Contaminant | Systems with a result | Detected | Above the health line | States ranked | Held back, low coverage |
|---|---|---|---|---|---|
| Total trihalomethanes (TTHM) | 17,538 | 13,130 | 3,049 | 27 | 20 |
| PFOA and PFOS | 12,940 | 2,685 | 666 | 31 | 18 |
| Arsenic | 15,115 | 6,665 | 3,532 | 22 | 14 |
A state is ranked only where at least 25% of its community systems have a result, so a state held back is simply one we hold too few results for. “Above the health line” means different things by contaminant. For trihalomethanes it is at or above 41 ppb. For PFOA it is at or above 4.0 ppt. For arsenic it is at or above the 2 ppb reporting floor, because its health goal sits below what any laboratory can measure. Only 62% of community systems have any graded result at all, the ceiling under every figure here.
Sources cited on this page
Peer-reviewed