Operator terms · Water
What is a maximum contaminant level (MCL)?
A maximum contaminant level, or MCL, is the legal limit set for how much of a contaminant is allowed in drinking water. It is an enforceable standard, and a system that exceeds an MCL must act and notify its customers. Operators monitor and treat the water to keep contaminants below their MCLs.
Last reviewed 2026-08-03
An enforceable limit
An MCL is not a guideline; it is a standard a public water system must meet. Exceeding one triggers required actions and public notification.
What the operator does
Operators sample on a schedule and run treatment so contaminants stay below their MCLs. Monitoring is how a system proves it is meeting the limits.
Common questions
- Is an MCL enforceable?
- Yes. It is a legal limit a public water system must meet.
- What happens if a system exceeds one?
- It must take action and notify its customers.
- Who has to meet MCLs?
- Public water systems, confirmed through required monitoring.