How every number is made

The data

EPA SDWIS, through the ECHO bulk download, edition 2026 Q2. It lists every public water system, the areas it serves, its lead and copper 90th-percentile results, and its violations. TapBuff uses active community water systems: 49,356 of them, reporting 331 million people served.

EPA UCMR 5, the fifth Unregulated Contaminant Monitoring Rule, occurrence data file of Aug. 27, 2026. It holds results for 29 PFAS and lithium from nearly every utility serving more than 3,300 people, and the ZIP codes each one serves.

The Census Bureau’s 2020 ZIP code to county relationship file, used only when EPA ties no utility to a ZIP code.

The site is rebuilt from fresh files when EPA publishes them. Each report names the edition it was built from.

Matching your ZIP code

A ZIP code is matched to every utility that lists it, from UCMR 5’s ZIP code file and SDWIS’s service areas. That covers 20,797 ZIP codes. For 14,692 more, no utility lists the ZIP, so TapBuff shows the largest utilities serving the county that holds most of its land, and says so.

ZIP codes and utility service areas do not line up neatly. A ZIP can be served by several utilities, and some homes in it may be on private wells. Your water bill names your utility.

Which utilities get a report

Every active community system EPA sampled under UCMR 5, plus any other serving 3,300 people or more: 10,433 reports. Smaller systems appear in ZIP results with their violation count, because federal data alone is too thin to fill a report honestly.

Measuring a result

Every result is compared with two federal lines: the Maximum Contaminant Level Goal, the health goal below which EPA expects no risk, and the Maximum Contaminant Level, the enforceable limit. Lead and copper use the Lead and Copper Rule’s action levels instead of a limit. Where neither exists, as for lithium, a non-enforceable EPA benchmark is shown and labeled as one.

UCMR 5 results are shown as the highest single sample, with every detection dated. Compliance with a limit is judged on averages over time, not single samples, so a result above a limit is not by itself a violation, and every report says so.

“Not found” means below EPA’s minimum reporting level in every sample. It does not mean zero.

Lead and copper results are the utility’s 90th percentile across the homes it sampled in a monitoring period. They describe the system, not your home.

Violations are health-based violations from Jan. 1, 2016 on, as EPA records them, with their status. Monitoring and reporting violations are counted but not listed.

Limits change. Each contaminant page states its current status with dates, including proposed changes that are not yet final.

Standards and certifications

What each NSF/ANSI standard covers comes from NSF’s own published descriptions. A filter is listed for a claim only when a certifier’s own listing (NSF, WQA, IAPMO, CSA or UL) shows that standard and that claim. Makers’ advertised claims that could not be found on a listing are not used.

Each certification on the site links to the listing it came from and carries the date it was checked.

Five-year cost

System and cartridge prices are each maker’s own list price, with the date checked. Cartridges are replaced at the maker’s gallon or month rating, whichever comes first for the household entered. Assumptions are stated on the calculator and can be changed.

What this cannot do

Federal bulk data does not carry most routine results, such as nitrate, arsenic or disinfection byproduct levels, unless a violation occurred. Your utility’s own annual Consumer Confidence Report has them.

No public data describes your own plumbing. A certified laboratory test of your tap is the only way to know your water.

Corrections

If a report is wrong because EPA’s data is wrong, the fix belongs with the utility or the state, which submit it; TapBuff will show the correction once EPA publishes it. If TapBuff got something wrong, it is corrected and the page’s update date changes.