MRL vs. MCL: What Those Numbers in Water Records Actually Mean
Key Takeaways
- 1
MRL (Minimum Reporting Level) is the lowest concentration a lab can reliably measure.
- 2
MCL (Maximum Contaminant Level) is the enforceable regulatory limit set by EPA.
- 3
A detection above the MRL does not mean the MCL was exceeded.
- 4
Records showing 'ND' (non-detect) mean the compound was below the MRL, not necessarily absent.
Water quality records are full of abbreviations. Two of the most commonly confused are MRL and MCL. They sound similar but measure very different things. Understanding the difference helps you read monitoring records accurately.
What is an MRL?
The Minimum Reporting Level (MRL) is the lowest concentration at which a certified laboratory can reliably detect and quantify a compound. Results at or above the MRL are reported as detections. Results below the MRL are reported as non-detects (ND) — this does not mean the compound is completely absent, only that it was below the measurement threshold.
What is an MCL?
The Maximum Contaminant Level (MCL) is an enforceable limit set by EPA under the Safe Drinking Water Act. If a utility's water exceeds an MCL and the compliance deadline has passed, it constitutes a violation. MCLs are set based on health effects research and treatment feasibility.
How they relate
For PFAS, EPA set MCLs of 4 parts per trillion for PFOA and PFOS. Some MRLs used in UCMR 5 testing are at or near 4 ppt — meaning a detection could be right at the edge of the regulatory limit. For other compounds with higher MCLs, a detection well above the MRL might still be below the MCL.
When reading records: look at both the reported concentration and the applicable MCL. A detection above the MRL is a data point — a violation only occurs when the concentration exceeds the MCL and the compliance period has ended. Water Utility Report labels record types clearly to help you distinguish them.
What this does not mean
- A non-detect does not guarantee a compound is absent — it means it was below the lab's measurement threshold.
- A detection above the MRL is not automatically a regulatory violation.
- Water Utility Report does not determine whether a water supply is safe or unsafe to drink.
What to check next
- Review your utility's monitoring records and compare reported values to applicable MCLs.
- Check the UCMR 5 data tables for MRL values by compound.
- Contact your utility or state drinking water program for context on specific results.
Frequently Asked Questions
Sources
Last updated: 2026-05-01 · Water Utility Report
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