Reference
Water quality, in plain English.
This glossary defines the terms you'll encounter in our reports and articles — written for people who want to understand their water without needing a chemistry degree. Every term has a direct link (e.g. /glossary#mcl) so articles can link straight to a definition.
A
Action Level
A concentration of a contaminant in drinking water that, when exceeded in more than 10% of tap samples, triggers required corrective actions by the water utility. Established under EPA's Lead and Copper Rule (40 CFR Part 141). An Action Level exceedance is not itself an MCL violation—it is a threshold that requires the utility to respond.
In simple terms
If too many homes in a utility's sample test above this number, the water company must take action—but crossing it doesn't automatically mean the water is in violation of the law.
Where you'll see this—Lead data in water reports·Methodology
Aquifer
An underground layer of permeable rock, sediment, or soil that holds and transmits groundwater. Water systems drawing from aquifers are classified as groundwater systems by EPA, and face different treatment requirements than surface water systems.
In simple terms
A layer of porous rock underground that acts like a giant natural reservoir—water utilities pump from it the same way you'd draw from a well.
Where you'll see this—Source water type descriptions in city reports
C
CCR
Consumer Confidence Report
An annual water quality report that federally regulated public water systems are required to deliver to customers by 1 July each year, under EPA regulations at 40 CFR Part 141 Subpart O. Also called the Annual Water Quality Report. It must disclose detected contaminants, their levels, and the health effects of any exceeding the MCL.
In simple terms
The yearly report your water company is legally required to send you explaining what was found in your water—though most people never read it.
Where you'll see this—Methodology·City report pages—utility source links
Cation Exchange
A water treatment process in which positively charged ions (cations)—such as calcium, magnesium, lead, or barium—are removed by exchanging them for sodium or hydrogen ions as water passes through a resin bed. Widely used in water softeners and in certain point-of-use filters for heavy metal reduction.
In simple terms
A filter method that swaps out unwanted dissolved minerals for harmless ones—the mechanism behind most water softeners.
Where you'll see this—Water softener vs filter guide
Contaminant
Under the Safe Drinking Water Act, any physical, chemical, biological, or radiological substance or matter in water. The presence of a contaminant does not indicate a health risk—the significance depends on the specific substance, its concentration, and the duration and route of exposure.
In simple terms
Anything that ends up in water that wasn't originally there. The word doesn't mean dangerous—it just means "something extra is present."
Where you'll see this—All city water reports·Contaminant library
D
Disinfection Byproduct
DBP
A chemical compound formed when disinfectants—typically chlorine or chloramine—react with naturally occurring organic matter in water. The two most regulated DBP groups are total trihalomethanes (TTHMs) and haloacetic acids (HAA5), both regulated under EPA's Stage 2 Disinfectants and Disinfection Byproducts Rule.
In simple terms
When utilities add chlorine to kill bacteria, it can react with naturally occurring material in the water and produce a new set of unintended chemicals as a side effect.
Where you'll see this—TTHM definition·HAA5 definition·TTHM contaminant page
E
Electrolysis
A process that uses direct electrical current to drive a non-spontaneous chemical reaction. In water ionizers, electrolysis splits water across electrode plates to produce alkaline (electron-rich) water at one outlet and acidic water at the other, altering pH and ORP without adding chemicals.
In simple terms
Passing electricity through water to split it into an alkaline stream and an acidic stream—the mechanism behind water ionizers.
Where you'll see this—What is a water ionizer·ORP definition
H
HAA5
Haloacetic Acids
A group of five haloacetic acid compounds—monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid—regulated collectively under EPA's Stage 2 Disinfectants and Disinfection Byproducts Rule. The EPA MCL for HAA5 is 60 µg/L (60 ppb). HAA5 form when chlorine used in disinfection reacts with organic matter in source water.
In simple terms
A group of five chemicals that form as a byproduct of water disinfection with chlorine. They're measured and regulated together because they tend to appear together.
Where you'll see this—HAA5 contaminant page·Disinfection Byproduct definition·City water reports—contaminant tables
Hard Water
Water with an elevated concentration of dissolved calcium and magnesium ions, typically measured in grains per gallon (gpg) or milligrams per litre (mg/L) as calcium carbonate. EPA's secondary standard for hardness is non-enforceable. Hard water is not a regulated health concern.
In simple terms
Water that contains a lot of dissolved minerals from the ground it passed through. It leaves scale on kettles and spots on dishes—but hardness itself is not a health issue.
Where you'll see this—Water softener vs filter guide
Health Guideline
A non-enforceable concentration limit derived from toxicological data, intended to represent a health-protective threshold independent of feasibility constraints. WaterHealthCheck uses EWG health guidelines as secondary benchmarks alongside EPA MCLs. EWG guidelines are typically derived from cancer risk models at 1-in-1,000,000 lifetime risk and are often orders of magnitude stricter than enforceable MCLs.
In simple terms
A recommended safe level based purely on health science—stricter than the legal limit, but not itself a law. Think of it as where scientists say the risk approaches zero.
Where you'll see this—Methodology·Status colour encoding in all reports·MCL definition
M
MCL
Maximum Contaminant Level
The highest permissible concentration of a contaminant in drinking water delivered to any user of a public water system, established and enforced under the Safe Drinking Water Act. MCLs are legally binding. They are set as close to the MCLG as is technically and economically feasible—meaning the MCL reflects what utilities can achieve, not necessarily what toxicology would recommend.
In simple terms
The legal limit. If a utility exceeds this, it's a violation. It's set by balancing health science against what water systems can realistically achieve—so it isn't always the same as "safe."
Where you'll see this—Methodology·All city water reports—status encoding·MCLG definition
MCLG
Maximum Contaminant Level Goal
A non-enforceable public health goal set by EPA at the concentration of a contaminant at which no known or anticipated adverse health effects occur, with an adequate margin of safety. For carcinogens with no established safe threshold—including lead, arsenic, and the PFAS compounds regulated since 2024—EPA sets the MCLG at zero.
In simple terms
The ideal target—the level at which there's no known health risk. Unlike the MCL, it's not legally enforceable. When you see an MCLG of zero, it means scientists haven't identified a completely safe level.
Where you'll see this—Methodology·Contaminant detail pages·MCL definition
N
NSF 42
An NSF International / ANSI standard certifying that a water treatment device reduces aesthetic contaminants—primarily chlorine taste and odour, and particulate matter at Class I (≥0.5 µm). NSF 42 certification does not cover health-related contaminants. Certification applies to a specific product model tested under stated conditions, not to a brand as a whole.
In simple terms
A certification that a filter makes water taste and smell better—not a certification that it removes anything harmful. Most basic pitcher filters carry this.
Where you'll see this—Does Brita filter lead?·NSF 53 definition
NSF 53
An NSF International / ANSI standard certifying that a water treatment device reduces specific health-related contaminants—including lead, certain VOCs, and Cryptosporidium—at the tested flow rate and filter capacity. Certification is per product model, not per brand. A filter with NSF 53 for lead is tested to demonstrate lead reduction to below 10 µg/L from a 150 µg/L challenge solution.
In simple terms
The certification that actually matters for safety—it proves a specific filter model removes health-related contaminants like lead under controlled test conditions.
Where you'll see this—Does Brita filter lead?·NSF 42 definition·NSF 58 definition
NSF 58
An NSF International / ANSI standard certifying the performance of reverse osmosis systems, including contaminant reduction claims for arsenic, nitrate, PFAS, TDS, and other substances. Certification applies to a specific product configuration tested at stated water pressure and temperature. NSF 58 is the relevant certification to look for when evaluating an RO system's PFAS reduction claims.
In simple terms
The certification specifically for reverse osmosis systems—confirming they actually remove the contaminants on their label, including PFAS and nitrate.
Where you'll see this—PFAS water filter guide·Does Brita filter lead?·Reverse Osmosis definition
O
ORP
Oxidation-Reduction Potential
A measurement, expressed in millivolts (mV), of the tendency of a solution to gain or lose electrons. A positive ORP indicates an oxidising environment; a negative ORP indicates a reducing environment. Water ionizers are commonly marketed as producing water with a negative ORP, which proponents describe as an indicator of antioxidant character.
In simple terms
A measurement of how chemically reactive a liquid is. Often cited in water ionizer marketing—but ORP in a glass of water is not the same as antioxidant activity inside the body.
Where you'll see this—What is a water ionizer·Electrolysis definition
P
PFAS
Per- and Polyfluoroalkyl Substances
A group of more than 12,000 synthetic chemical compounds characterised by strong carbon-fluorine bonds that resist breakdown in the environment and the human body. Some PFAS compounds accumulate in tissue over time. EPA's 2024 drinking water rule set enforceable MCLs for six PFAS—PFOA, PFOS, PFNA, PFHxS, HFPO-DA (GenX), and a hazard index for PFNA, PFHxS, PFBS, and HFPO-DA in combination.
In simple terms
A large family of man-made chemicals sometimes called "forever chemicals" because they break down very slowly. Some have been linked to health effects at very low concentrations—hence why the EPA now regulates several of them in drinking water.
Where you'll see this—PFOA contaminant page·PFAS water filter guide·UCMR5 definition
ppb
parts per billion
A unit of concentration equivalent to micrograms per litre (µg/L) in water. One ppb is one microgram of a substance dissolved in one litre of water. Most EPA MCLs for regulated contaminants are expressed in ppb. One ppb is approximately equivalent to one drop of food colouring in an Olympic-size swimming pool (2.5 million litres).
In simple terms
A way of expressing a tiny concentration—one part per billion means one drop of something in about 500 barrels of water. Most contaminant limits in our reports are set at the ppb level.
Where you'll see this—All city water reports—contaminant level columns·ppt definition
ppt
parts per trillion
A unit of concentration equivalent to nanograms per litre (ng/L) in water—one thousand times smaller than ppb. EPA's 2024 MCLs for PFOA and PFOS are set at 4 ppt each, reflecting evidence of health effects at very low concentrations. Analytical methods capable of detecting at the ppt level became widely available only in the 2010s.
In simple terms
A concentration one thousand times smaller than ppb—used for PFAS because these compounds can have health effects at vanishingly small amounts.
Where you'll see this—PFOA contaminant page·ppb definition
Primacy Agency
The authority—typically a state environmental or health agency—that EPA has granted primary enforcement responsibility for the Safe Drinking Water Act within its jurisdiction. Forty-nine states and one territory hold primacy. Primacy agencies must adopt drinking water regulations at least as stringent as federal EPA standards, and may set stricter limits. California's chromium-6 MCL is an example of a state standard stricter than the federal equivalent.
In simple terms
The state agency that acts as the local enforcer of drinking water rules—it can be stricter than federal standards, but never more lenient.
Where you'll see this—Methodology·State limit notes on contaminant pages
PWS
Public Water System
A system that provides water for human consumption through pipes or other constructed conveyances to at least 25 people or 15 service connections for at least 60 days per year, as defined under the Safe Drinking Water Act. Each PWS is assigned a unique PWSID (Public Water System Identification number) by EPA.
In simple terms
Any water system that serves at least 25 people—from a small community well to a city-wide municipal supply. If you're on tap water, you're served by a PWS.
Where you'll see this—Report headers—EPA PWSID field·Methodology
R
Reverse Osmosis
A water purification process that forces water under pressure through a semi-permeable membrane with pores small enough to block dissolved ions, molecules, and larger contaminants. Effective against PFAS, nitrate, arsenic, lead, fluoride, and TDS. NSF 58 certifies RO system performance. Typical rejection rates are 90–99% for most regulated contaminants. RO systems also produce a waste stream (reject water).
In simple terms
A filtration method that pushes water through an extremely fine membrane under pressure, blocking most dissolved contaminants. It's one of the few treatments that reliably removes PFAS.
Where you'll see this—PFAS water filter guide·NSF 58 definition·Filter recommendation sections in city reports
S
SDWIS
Safe Drinking Water Information System
EPA's federal database containing records for all public water systems regulated under the Safe Drinking Water Act, including monitoring results, violations, enforcement actions, system characteristics, and treatment information. WaterHealthCheck syncs from SDWIS weekly via EPA's bulk data export.
In simple terms
EPA's master database of what every public water system in the US has found in its water—the primary source for everything in our reports.
Where you'll see this—Methodology·All city water reports—data source badge
T
Total Dissolved Solids
TDS
A measure of the combined content of all inorganic and organic substances dissolved in water, expressed in milligrams per litre (mg/L) or parts per million (ppm). TDS includes minerals, salts, metals, and ions. EPA's secondary standard for TDS is 500 mg/L—a non-enforceable aesthetic guideline based on taste, not a health limit.
In simple terms
A rough measure of everything dissolved in your water. High TDS can affect taste, but a high TDS reading alone doesn't mean the water is harmful—it depends what the dissolved substances actually are.
Where you'll see this—Water softener vs filter guide·Reverse Osmosis definition
TTHM
Total Trihalomethanes
The sum of four trihalomethane compounds—chloroform, bromodichloromethane, dibromochloromethane, and bromoform—regulated collectively under EPA's Stage 2 Disinfectants and Disinfection Byproducts Rule. The EPA MCL for TTHMs is 80 µg/L (80 ppb), calculated as a localised running annual average. TTHMs form when chlorine reacts with naturally occurring organic matter in source water.
In simple terms
A group of four chemicals that form as a side effect of chlorine disinfection. They're measured together because they tend to co-occur, and the MCL covers all four combined.
Where you'll see this—TTHM contaminant page·Disinfection Byproduct definition·City water reports—contaminant tables
U
UCMR5
Unregulated Contaminant Monitoring Rule 5
EPA's fifth Unregulated Contaminant Monitoring Rule, requiring public water systems serving 3,300 or more people—and a statistically representative sample of smaller systems—to monitor for 29 PFAS compounds and lithium between 2023 and 2025. UCMR5 produced the most comprehensive PFAS dataset ever assembled for US drinking water and directly informed EPA's 2024 PFAS MCL rulemaking.
In simple terms
The EPA programme that required thousands of utilities to test for PFAS for the first time, producing the dataset that led to the first federal PFAS drinking water limits in 2024.
Where you'll see this—Methodology·PFAS contaminant page·City water reports—data source badge
Unregulated Contaminant
A substance detected in drinking water for which EPA has not yet established an enforceable MCL. Unregulated contaminants may still be monitored under UCMR programmes and can appear in water reports. Many—including microplastics and most PFAS variants beyond the six now regulated—remain without federal MCLs despite emerging evidence of health effects.
In simple terms
A substance that turns up in water tests but doesn't yet have a legal limit. The absence of a limit usually means the regulatory process hasn't caught up—not that it's been proven safe.
Where you'll see this—PFOA contaminant page·UCMR5 definition·Microplastics in contaminant library
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