Maximum Contaminant Level (MCL)
The highest level of a contaminant allowed in drinking water, set by the EPA and enforceable by law. Exceeding the MCL triggers a health-based violation.
How It Works
MCLs are the legally enforceable standards for drinking water quality. The EPA sets MCLs for over 90 contaminants, balancing health protection against technical feasibility and cost. MCLs are set "as close to the MCLG as feasible" using the best available treatment technology. For example, the MCL for arsenic is 10 ppb, high enough to be technically achievable for most water systems but below the level considered safe by current science (the MCLG is 0). Water systems must regularly test for regulated contaminants and report any MCL exceedances to the state and their customers. Persistent MCL violations can result in enforcement actions, fines, and required treatment upgrades.
Related Terms
- Maximum Contaminant Level Goal (MCLG), The level of a contaminant in drinking water below which there is no known or expected health risk, a non-enforceable goal that is often stricter than the enforceable MCL.
- Health-Based Violation, The most serious type of drinking water violation, indicating that water quality has exceeded a maximum contaminant level or failed to meet a treatment requirement that directly protects health.
- Action Level, A regulatory threshold that triggers required treatment or other corrective action when exceeded, used for lead (15 ppb) and copper (1.3 mg/L) instead of a traditional MCL.
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About This Definition
This definition is part of the IsWaterSafe Drinking Water Safety Glossary, 22 terms explaining water contaminants, treatment methods, and safety standards. Written for homeowners, renters, journalists, and public health professionals.