Drinking-water terms, in plain English

The highest level of a contaminant the EPA allows in drinking water. Set as low as feasible given treatment cost and health risk. If a system's regulated reporting statistic (running annual average or 90th percentile) hits its MCL, that's a violation.

The level the EPA considers ideal for public health — often set to zero for known carcinogens. Not enforceable on its own; it's the target the enforceable MCL is set against.

Used instead of an MCL for lead and copper. If more than 10% of tap-water samples (the 90th percentile) exceed the action level, the utility has to take corrective action — corrosion-control treatment, lead-line replacement, customer education.

When the EPA can't set a numeric MCL for a contaminant, it requires a specific treatment process instead — e.g. filtration for surface water, disinfection for Cryptosporidium. Violations are procedural rather than numeric.

Non-enforceable EPA standards for aesthetic concerns (taste, odor, color, staining). Exceeding an SMCL isn't a health violation — it just means the water might look or taste off.

The maximum level of a residual disinfectant (chlorine, chloramine, chlorine dioxide) allowed in drinking water. Necessary to keep water clean through the distribution system, but too much causes its own health concerns.

Non-enforceable guideline for a contaminant the EPA thinks is risky but hasn't formally regulated yet. Often used for emerging contaminants like certain PFAS compounds — the action level on PFOA/PFOS came from a health advisory before becoming an MCL.

The average of the most recent four quarters of sample results. The EPA enforces TTHM, HAA5, and several other contaminants against the RAA, not against a single high sample — a one-quarter spike alone isn't a violation.

The value below which 90% of samples fall. Used for the lead and copper rule: a system fails if its 90th-percentile result exceeds the action level. This means it's allowed to have a few high samples — the action level isn't on the worst sample, it's on the 90th percentile.

See RAA above.

The single highest result in the reporting period. Reported in CCRs for transparency, but for most contaminants it's NOT the compliance statistic — a single high sample alone doesn't constitute a violation if the RAA stays under the MCL.

A single measurement, not averaged. Some contaminants are regulated this way — nitrate, for instance, where any single result over the MCL is a violation (because of the acute risk to infants).

Samples drawn from customer faucets. Used for lead and copper — the regulation cares about what you actually drink, not what leaves the treatment plant.

Samples drawn from points in the pipes between the treatment plant and the customer. Used for disinfection byproducts (TTHM, HAA5) — those form as the water travels.

Samples drawn at the point water enters the distribution system, after treatment. Used for many regulated contaminants where the concern is what the utility is sending out.

Samples drawn from the well, reservoir, river, or aquifer — before any treatment. Used to characterize raw water quality.

A group of four chemicals — chloroform, bromodichloromethane, dibromochloromethane, bromoform — that form when chlorine reacts with organic matter in source water. The EPA caps total TTHM in drinking water at 80 ppb (μg/L), measured as a running annual average.

Five disinfection byproducts regulated together: monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acid. EPA limit: 60 ppb total, measured as a running annual average.

A family of thousands of synthetic chemicals, nicknamed 'forever chemicals' because they don't break down in the environment. The EPA finalized enforceable MCLs in 2024 for six of them — PFOA, PFOS, PFHxS, PFNA, HFPO-DA, and PFBS — plus a hazard-index limit on mixtures.

Chemicals that form as a side effect of disinfecting drinking water. TTHM and HAA5 are the two regulated DBP groups; bromate and chlorite are others.

A broad class of organic chemicals (benzene, vinyl chloride, trichloroethylene, etc.) that evaporate easily. Often groundwater contaminants from industrial sites or leaking underground storage tanks.

The annual drinking-water report every U.S. public water utility is required by federal law to publish by July 1 each year. Lists every contaminant the utility measured, the level, and the federal limit. The Water Map's dataset is built from these reports.

The federal agency that sets and enforces drinking-water standards under the Safe Drinking Water Act. State agencies typically implement and enforce on the EPA's behalf.

The 1974 federal law (amended 1986 and 1996) that gives the EPA authority to set drinking-water standards and requires utilities to publish annual CCRs.

Common units for trace contaminants. 1 ppb ≈ 1 μg/L in water. To picture it: 1 ppb is about one second in 32 years.

1 ppm ≈ 1 mg/L in water. 1,000× larger than ppb. To picture it: 1 ppm is about one inch in 16 miles.

Unit of radioactivity used for radionuclides in drinking water (radium, uranium, gross alpha). One picocurie = one trillionth of a curie.