MCLs, MCLGs, and Action Levels: Decoding Your Water Lab Report

Open any water lab report and you'll see a column called 'MCL' or 'Action Level' next to your result. These numbers are how you know whether your water is good or not — but they don't all mean the same thing. Here's what each acronym actually represents and how to use it when you're looking at your own results.

MCL — Maximum Contaminant Level

This is the legal limit a public water system is allowed to deliver. It's set by the EPA based on what's technically achievable at reasonable cost, balanced against health risk. If your result is at or above the MCL, your water is technically out of compliance. If you're on a private well, MCLs are the closest thing you have to a benchmark — but they're not enforced on you.

MCLG — Maximum Contaminant Level Goal

This is the level the EPA would prefer to see, based purely on health considerations and ignoring cost. For some contaminants — like lead and many disinfection byproducts — the MCLG is zero, even though the MCL is higher. The MCLG is your real health-based reference. If your result is between the MCLG and the MCL, you're legal but not ideal.

Action Level

Specific to lead and copper. If more than 10% of homes in a system test above the action level (15 ppb for lead, 1.3 ppm for copper), the utility is required to take corrective action. It's a fleet-wide trigger, not a personal safety threshold. For a single household, treat any detectable lead as worth investigating.

How to read your own report

1. Compare your result to the MCL — that's the legal line

2. Compare it to the MCLG — that's the health-ideal line

3. Look at the units carefully — ppm, ppb, and ppt are 1,000x apart

4. Note whether the test was first-draw or flushed — first-draw shows what's in your pipes, flushed shows what's in the supply

When to act

If you're above any MCL, treat it as urgent — switch to bottled or filtered water for drinking and cooking while you investigate. If you're below the MCL but above the MCLG for lead, install a certified filter and re-test. If everything is below MCLG, you're in great shape — keep your annual schedule.