Maximum Contaminant Level (MCL)
A maximum contaminant level is the highest concentration of a contaminant that the EPA allows in drinking water delivered to any user of a public water system, set as close to the health-based MCLG as is feasible given available treatment technology and cost, and codified at 40 CFR § 141.2.
Where you'll see it on a CCR
Every Consumer Confidence Report is required to list any regulated contaminant that was detected in your water alongside the applicable MCL. If a system exceeded an MCL during the reporting year, the CCR must say so and describe what the system is doing to return to compliance. When a contaminant has no measurable MCL because treatment performance is the controlling standard, the report instead references a treatment technique. You can look up MCLs for specific contaminants — organics, inorganic chemicals, microbial agents, disinfection byproducts, and radionuclides — in the contaminants index.
How it's measured
MCLs are expressed as a concentration in drinking water, typically in milligrams per liter (mg/L) or, for some contaminants such as PFAS, nanograms per liter (ng/L). The legal definitions that govern how violations are determined appear in 40 CFR § 141.2, and the specific numeric limits are set out across several subsections: organic contaminants (40 CFR § 141.61), inorganic contaminants (§ 141.62), microbial contaminants (§ 141.63), disinfection byproducts (§ 141.64), maximum residual disinfectant levels (§ 141.65), and radionuclides (§ 141.66). These are primary MCLs — health-based standards authorized under SDWA § 1412. Secondary MCLs, which address taste, odor, and aesthetic qualities rather than health effects, are set separately under 40 CFR Part 143 and are not required to appear on a CCR.
Related terms
- MCLG (Maximum Contaminant Level Goal) — the non-enforceable health goal that EPA uses as the target when setting an MCL
- Treatment Technique — an enforceable process requirement used when an MCL is not technically feasible to measure or enforce
- Consumer Confidence Report (CCR) — the annual water quality report where MCL compliance data is disclosed
- Contaminants index — MCL values and detection context for specific regulated substances
- Glossary index — all CCR terms defined
Citations
- 40 CFR § 141.2 — Definitions (MCL definition): law.cornell.edu/cfr/text/40/141.2
- 40 CFR §§ 141.61–141.66 — Specific MCL tables: law.cornell.edu/cfr/text/40/part-141
- 40 CFR Part 143 — Secondary MCLs: law.cornell.edu/cfr/text/40/part-143
- EPA Drinking Water Glossary: epa.gov/ground-water-and-drinking-water/drinking-water-glossary
- SDWA § 1412, 42 U.S.C. § 300g-1 — NPDWR rulemaking authority: law.cornell.edu/uscode/text/42/300g-1
Last reviewed: 2026-05-03.