Glossary

The chemicals

PFASPer- and polyfluoroalkyl substances

A family of thousands of synthetic fluorinated chemicals used since the 1940s in non-stick coatings, firefighting foam, food packaging and waterproof fabric. Commonly called forever chemicals because the carbon–fluorine bond resists breaking down in the environment or the body.

PFOAPerfluorooctanoic acid

One of the two most studied PFAS compounds and among the most widely detected. Classified by the International Agency for Research on Cancer in 2023 as carcinogenic to humans (Group 1), citing renal cell carcinoma and testicular cancer. Federal drinking water limit: 4.0 ppt.

PFOSPerfluorooctanesulfonic acid

The other principal regulated compound, historically used in stain repellents and firefighting foam. Classified by IARC as possibly carcinogenic to humans (Group 2B). Federal drinking water limit: 4.0 ppt.

PFHxS, PFNA, HFPO-DAThree further regulated compounds

HFPO-DA is widely known as GenX. Each carries a federal limit of 10 ppt under the 2024 rule, though EPA has proposed rescinding all three.

Hazard IndexA limit for mixtures

A unitless calculation applied to mixtures of PFHxS, PFNA, HFPO-DA and PFBS, recognising that several compounds below their individual limits may still pose combined risk. A value above 1 exceeds the standard.

Measurement

pptParts per trillion

The unit used for PFAS, equivalent to nanograms per litre. Approximately one drop in twenty Olympic swimming pools. EPA publishes UCMR 5 values in micrograms per litre; we convert to ppt, the unit used in the regulation.

MRLMinimum reporting level

The lowest concentration a laboratory will report as a confirmed measurement. Below it, a result is recorded as a non-detect — which means not measurable, not necessarily absent.

Non-detect

A sample in which a substance was not found above the reporting limit. Shown on this site as nothing detected, and distinct from a claim that the substance is absent.

Entry point

The location where treated water enters the distribution system. Compliance sampling happens here, which is why system-level results say nothing about what your household plumbing adds.

Regulation

MCLMaximum Contaminant Level

The enforceable legal limit for a contaminant in public drinking water. Set with reference to health science but also to treatment feasibility and cost, so it is not purely a biological threshold.

MCLGMaximum Contaminant Level Goal

The non-enforceable level at which no known adverse health effect occurs, with a margin of safety. For PFOA and PFOS the goal is zero, which is why the enforceable limit of 4.0 ppt reflects feasibility rather than an all-clear.

Running annual average

The method by which MCL compliance is actually determined — an average of a year's samples at each entry point, not any single result. A system can record one high sample and remain compliant.

SDWASafe Drinking Water Act

The 1974 federal law governing public drinking water in the United States. It does not cover private wells.

Primacy agency

The state or tribal authority that administers drinking water regulation on EPA's behalf. Nearly all states hold primacy, which is part of why enforcement and sampling intensity vary between them.

CCRConsumer Confidence Report

The annual water quality report every community water system must provide to its customers, covering the regulated contaminants UCMR 5 did not measure.

Systems and data

PWSIDPublic Water System Identification number

The unique federal identifier for each water system, and the key that lets UCMR 5 results be joined to system names, service areas and population.

Community water system

A public system serving the same population year-round — the category that supplies homes, as distinct from systems serving schools, campgrounds or workplaces.

UCMRUnregulated Contaminant Monitoring Rule

A recurring EPA programme requiring nationwide testing for substances not yet regulated, to establish whether regulation is warranted. UCMR 5 covered 29 PFAS compounds and lithium.

SDWISSafe Drinking Water Information System

EPA's national inventory of public water systems, holding names, ownership, source water type, population served and violation history.

Groundwater / surface water

The two principal source types. Groundwater is drawn from wells and aquifers; surface water from rivers, lakes and reservoirs. In UCMR 5, groundwater systems detected PFAS slightly more often, while surface water systems exceeded limits more often.

Treatment

Reverse osmosisRO

A treatment method forcing water through a semi-permeable membrane. Effective against PFAS when the system is certified for it and maintained.

Activated carbonGAC

Filtration through carbon media with very high surface area. Effective against PFAS in properly certified and regularly replaced form; largely ineffective in filters sold only for taste and odour.

NSF/ANSI 53 and 58

The certification standards for drinking water treatment units — 53 for health-related contaminant reduction, 58 for reverse osmosis. Certification is contaminant-specific: check that PFOA and PFOS are named.

Disinfection by-productsDBPs

Compounds formed when disinfectants react with organic matter in source water. Regulated separately, and a genuine trade-off against the far larger risk of untreated pathogens.

Service line

The pipe connecting the water main to a building. In older housing it may be lead, which is why lead is a property-level question rather than a system-level one.