Reference

The words on these pages, explained

EPA records use a small vocabulary that is precise but unfamiliar. This page defines each term once, in plain English, and keeps the technical meaning intact.

In focus14Terms in plain English

Public records. Clearer context.

ppt and ppbparts per trillion and parts per billion

ppt (parts per trillion, the same as nanograms per liter) is used for PFAS; ppb (parts per billion, the same as micrograms per liter) for lead, copper and lithium. One ppt is roughly one drop of water in twenty Olympic swimming pools; one ppb is a thousand times more.

UCMR 5EPA's fifth national survey of unregulated contaminants

The fifth Unregulated Contaminant Monitoring Rule required every public water system serving more than 3,300 people, plus a random sample of smaller ones, to test for 29 PFAS and lithium between 2023 and 2025. It is a one-time survey, not a compliance program.

MCLmaximum contaminant level: the legal limit

The highest level of a contaminant allowed in drinking water under federal law. The PFAS MCLs adopted in 2024 are enforced on a running annual average from 2029, so a single result above an MCL on this site is a measurement, not a violation.

Health reference levelEPA guidance value, not a limit

A concentration EPA uses to screen unregulated contaminants such as lithium. It is not enforceable and does not by itself trigger any action.

Action levelthe lead or copper level that requires the utility to act

If more than 10% of the homes sampled exceed 15 ppb of lead (1.3 ppm of copper), the utility must take steps such as corrosion control, public education or replacing lead service lines.

90th percentile9 in 10 sampled homes were at or below this value

Lead and copper are measured at a selection of customer taps. EPA records the value that 90% of those homes were at or below; that is the number compared with the action level.

Reporting levelthe smallest amount the laboratory can reliably measure

UCMR 5 minimum reporting levels are laboratory limits set by EPA, not health thresholds. "Not detected" means below this level, not zero.

Complete-set averagethe yearly average EPA uses for the PFAS limits

EPA compares PFAS MCLs with the average of a full set of samples at one sampling location: two half-yearly results for groundwater or four quarterly results for surface water. Non-detects count as zero. Without a complete set, no comparison is made.

Entry pointwhere treated water enters the pipes

UCMR 5 samples are collected where water enters the distribution system, after treatment and before it reaches homes. They are not a test of any single tap.

Community water systema utility that serves people where they live

EPA distinguishes community systems (homes) from non-community systems (schools, workplaces, campgrounds). Only community systems can be the primary supplier for a ZIP code on this site.

PWSIDEPA's permanent ID for a water system

A public water system identifier such as NJ0251001: two letters for the state followed by seven digits. It stays the same if the utility is renamed, which is why system pages use it in their address.

SDWIS and ECHOEPA's compliance database and its public portal

The Safe Drinking Water Information System holds violation, enforcement and lead and copper records reported by states. ECHO is the EPA website that publishes them.

Health-based violationa contaminant, disinfectant or treatment rule was not met

A violation of a maximum contaminant level, a maximum residual disinfectant level or a required treatment technique. Other violations concern monitoring, reporting or public notification and say nothing about the water itself.

ZIP-to-utility matchhow a ZIP code is linked to a water system

ZIP codes and utility service areas do not share boundaries. This site combines EPA service-area maps, the ZIP codes utilities themselves report and Census geography, and publishes the confidence of each match. A water bill names the actual supplier.

For how ZIP codes are matched to utilities and how comparisons are computed, read the methodology.