Water data · Overview
What is actually in US tap water
We pulled every contaminant result we could find for US community water systems out of state testing records and federal monitoring rounds, and put them in one place: 214,088 measured results, across 23,927 water systems. The table below is the graded slice of that: one row for each contaminant our threshold registry can hold to a health-based or legal line, the same registry that grades a customer’s own report.
Each row says what the contaminant is, where it comes from, how many systems tested for it, how many found any, and how many are over the line that matters. Most tap water in America is fine. The gap worth reading is between what is legal and what the health research supports.
What we found
- Most systems are clean for most things. The single most common result in this dataset is a non-detect. That is worth saying first, because almost nobody says it.
- Legal limits and safety are set decades apart. For several contaminants the enforceable limit is a cost-and-feasibility compromise set decades ago, and the health research points lower. Trihalomethanes are the clearest case: 3,049 systems are above the health line and only 221 are above the legal one.
- What you have depends on where your water starts. Rivers and reservoirs bring chlorination byproducts, wells bring arsenic, radium and nitrate, and an industrial history adds solvents and forever chemicals. Very few places have all of it.
- The biggest gap is missing tests. Of 38,332 community water systems, only 23,927 have any result at all on record. A system with no data is not a clean system, it is an unknown one.
- Lead is not in this table. Lead comes from the pipe between the street and your tap, not from the water a utility delivers, so a national figure for it would describe nothing. See the lead section below.
What this means for your house
Almost nobody needs to worry about everything in this table. What matters is the two or three rows that apply to where your water comes from, and that is knowable for free in a couple of minutes.
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Start with where your water comes from.
Surface water points to chlorination byproducts, groundwater to arsenic, radium and nitrate. A city with industrial history adds solvents and PFAS. Enter your address and we will name your system and its source
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Read your utility’s annual report against the health line.
Every US water system publishes one. It will tell you it met every standard, and that will be true. The useful question is what the numbers are. A system can meet every standard and still carry a level worth reading. Every contaminant brief we have written
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Match the filter to what you actually have.
Carbon handles chlorination byproducts and solvents, reverse osmosis handles arsenic, nitrate, chromium-6 and PFAS, and a softener handles neither. Buying before you know is how people end up with the wrong device. What each method actually removes · What does not work
Every contaminant we grade, and what each one is
Ordered by how many systems have a result, which tracks how widely each contaminant is monitored more than how common it is. Nitrate leads at 19,069 systems because it is on almost every routine panel. Chromium-6 sits near the bottom at 2,550 because only California requires it.
These rows are the graded set, not the measured universe. The panel set behind them holds thousands of distinct analyte keys. One compound can appear under several names, regulated sums sit beside the species they add up, and some keys are not contaminants at all, like alkalinity or sodium. A contaminant appears here once our registry holds a health-based or legal line for it in the unit the data arrives in.
| Contaminant | Systems with a result | Found any | Over the line |
|---|---|---|---|
| Nitrate farm runoff and septic · measured in ppm · legal limit 10 | 19,069 | 69%13,245 systems, typically 0.79 ppm | 99 at or above 10 ppm, which is also the legal limit |
| Fluoride added, and natural · measured in ppm · legal limit 4 | 17,671 | 80%14,120 systems, typically 0.4 ppm | 199 at or above 2 ppm · 12 over the legal limit |
| Trihalomethanes chlorination byproduct · measured in ppb · legal limit 80 | 17,538 | 75%13,130 systems, typically 19.1 ppb | 3,049 at or above 41 ppb · 221 over the legal limit |
| Haloacetic acids chlorination byproduct · measured in ppb · legal limit 60 | 16,031 | 68%10,857 systems, typically 12.4 ppb | 159 at or above 60 ppb, which is also the legal limit |
| Arsenic natural, from rock · measured in ppb · legal limit 10 | 15,115 | 44%6,665 systems, typically 2 ppb | 3,532 detected at or above 2 ppb · 188 over the legal limit |
| Tetrachloroethylene (PCE) dry-cleaning solvent · measured in ppb · legal limit 5 | 14,480 | 11%1,548 systems, typically 0.5 ppb | 1,482 detected at or above 0.5 ppb · 7 over the legal limit |
| Trichloroethylene (TCE) industrial solvent · measured in ppb · legal limit 5 | 14,450 | 10%1,480 systems, typically 0.5 ppb | 25 at or above 1.7 ppb · 5 over the legal limit |
| 1,1-dichloroethylene industrial solvent · measured in ppb · legal limit 7 | 14,282 | 10%1,386 systems, typically 0.5 ppb | 0 at or above 6 ppb |
| PFOS forever chemical · measured in ppt · legal limit 4 from 2029 | 12,956 | 22%2,822 systems, typically 2 ppt | 726 at or above 4 ppt, which is also the legal limit |
| PFOA forever chemical · measured in ppt · legal limit 4 from 2029 | 12,940 | 21%2,685 systems, typically 2 ppt | 666 at or above 4 ppt, which is also the legal limit |
| PFHxS forever chemical · measured in ppt · no federal limit | 12,802 | 16%2,034 systems, typically 2 ppt | 457 at or above 3 ppt |
| PFNA forever chemical · measured in ppt · no federal limit | 12,508 | 6%694 systems, typically 2 ppt | 7 at or above 10 ppt |
| Manganese natural, from rock · measured in ppb · no federal limit | 10,849 | 51%5,488 systems, typically 14 ppb | 797 at or above 100 ppb |
| Gross alpha radiation natural, from rock · measured in pCi/L · legal limit 15 from 2029 | 10,556 | 56%5,867 systems, typically 3 pCi/L | 3,281 detected at or above 3 pCi/L · 240 over the legal limit |
| Radium natural, from rock · measured in pCi/L · legal limit 5 from 2029 | 7,212 | 50%3,581 systems, typically 1 pCi/L | 1,846 detected at or above 1 pCi/L · 87 over the legal limit |
| Perchlorate rocket fuel and fireworks · measured in ppb · no federal limit | 2,970 | 17%496 systems, typically 0.372 ppb | 132 at or above 1 ppb |
| Chromium-6 natural and industrial · measured in ppb · no federal limit | 2,550 | 73%1,873 systems, typically 0.598 ppb | 1,581 detected at or above 0.1 ppb |
| 1,4-dioxane industrial solvent · measured in ppb · no federal limit | 109 | 70%76 systems, typically 0.249 ppb | 6 at or above 1 ppb |
“Over the line” means one of two things, printed on each row. For most contaminants it is a health-based concentration. For arsenic, chromium-6, PCE, radium and gross alpha the health goal sits below what any laboratory can measure, so the count is a detection at or above the reporting floor, and those rows are marked detected. A count of systems over the enforceable legal limit is shown separately wherever one exists.
What is not in this table, and why
Lead and copper. Both are reported as a 90th-percentile figure from a small, deliberately high-risk sample of homes, often 20 to 100 addresses for an entire city. Both enter at the plumbing between the street and your tap rather than in the water the utility delivers. A row here would be read as a level in the water, which it is not, so there is no row. For the same reason, no figure built on this dataset will ever be a national lead number: for lead, the only real answer is a test at your own kitchen tap. The lead brief
Uranium. We hold readings for it, but they arrive in picocuries per litre and the only uranium health and legal lines in our registry are set in micrograms per litre, a different unit that this dataset does not populate. Converting would mean guessing, so the rows are held and not graded.
Everything nobody tested. The largest gap on this page is coverage, not any one contaminant. 14,405 of the 38,332 community water systems in scope have no result here for any of these graded contaminants. Most are small systems, and none has been tested for these contaminants.
Read any of these in full
Each science page carries what the research shows, the health-based and legal lines, our national measurements with their state ranking, and the full method for how those numbers were built.
- Trihalomethanes: 3,049 of 17,538 systems above the health line, and why the legal limit sits at almost twice that.
- PFAS: what 12,940 systems report, and the 2029 deadline that has not arrived yet.
- Arsenic : the geology map, and the private wells nobody tests.
- What Orange County’s water districts report : every contaminant 28 districts publish, from their own water quality reports, counted at county level.
How this was built, and what it cannot tell you
Every figure on this page is regenerated from one dataset of 38,332 community water systems by one script, so any number here can be checked by re-running it. What follows is that accounting.
- Records
- 214,088 contaminant results across 23,927 community water systems, drawn from 38,332 systems that passed every filter. The full panel set holds 42,081 systems and thousands of distinct analyte keys. The rows here are the ones our threshold registry can grade against a health or legal line. Because one contaminant can carry several keys, this page counts results and systems, not contaminants.
- Sources
- State drinking-water monitoring records, utility reports and federal monitoring rounds, assembled into one panel set dated 2026-06-16. System type, population served and purchased-water status: EPA SDWIS, vintage 2026Q1. Health and legal lines: the Stasis threshold registry, version 2026-08-08, the same registry behind your own report.
- Provenance
- Outside the 28 Orange County districts we hand-read, every value in this dataset is a reconstruction we computed from raw state monitoring records, rather than a figure lifted from a utility’s published report. We do not measure the water. We gather these numbers and interpret them.
- Scoring rule
- Systems are counted against the health-based level where one exists below the legal limit, and against a laboratory detection where the health goal sits below what any instrument can measure. Both lines are printed on every row. Non-detects are counted as zero and stay in the denominator, so a share is never inflated by dropping clean systems.
- Known limitation: coverage is not uniform
- A contaminant near the bottom of this table is often less-monitored rather than genuinely rare. Chromium-6 has 2,550 results because California is effectively the only state that requires the test, so its numbers describe California more than the country. Read the systems-with-a-result column before reading anything else in a row.
- Known limitation: purchased water
- A system that buys finished water often has its plant-level chemistry sampled under the seller’s permit number, so our record reads “not reported” where the water was tested upstream. The size of that hole is analyte-specific: arsenic reporting runs 43.5% among systems that treat their own water against 22.6% among systems that buy, while trihalomethanes run 45.6% against 46.3%. For that reason, every share on this page is stated against the systems that actually reported.
- Raw data
- Not published. The row-level extract is a compilation of several hundred separate state and federal sources. Every finding, method, denominator and limit is on this page, and every threshold links to its public primary source. Researchers and reporters who need specific figures can write to data@stasishome.com.
- Corrections
- If a figure here disagrees with your utility’s published report, the published report is more likely to be right and we want to know. Write to data@stasishome.com with the system name and we will check it against the source and correct the record.