MCL (maximum contaminant level)

An MCL is the legal ceiling for a contaminant in public drinking water. EPA defines it as the highest level of a contaminant that is allowed in drinking water, and says MCLs are enforceable standards.

EPA sets each MCL as close as feasible to a health goal called the MCLG, using the best available treatment technology and taking cost into consideration. The MCLG itself is only a non-enforceable goal.

For PFAS, EPA has set MCLs for PFOA and PFOS at 4.0 parts per trillion, plus limits for other PFAS. These apply to public water systems. They do not set a legal limit for a private well or for soil.

Property owners still use them as a yardstick. A well result near the MCL is a reason to call a professional. Keep in mind that the compliance timeline is under proposed changes as of May 2026.

To read a lab report, learn parts per trillion first. Then compare your result with the table on EPA's page.