What Is an EPA Maximum Contaminant Level (MCL)?
MCLs are the foundation of US drinking water safety. Here's how they're set and what they mean for your tap.
TapSafeWater Editorial Team
February 18, 2026 · 5 min read
Photo by Naja Bertolt Jensen on Unsplash
Every time you check your city's water quality report, you'll see references to Maximum Contaminant Levels, or MCLs. Understanding what these numbers mean — and how they're set — is essential to interpreting your drinking water data.
The Short Definition
A Maximum Contaminant Level (MCL) is the highest level of a contaminant that the EPA allows in public drinking water. If your utility's water exceeds an MCL, they've committed a federal violation and must notify the public.
How MCLs Are Set
The EPA sets MCLs through a three-step process:
1. Health-Based Goal (MCLG)
First, the EPA establishes a Maximum Contaminant Level Goal — the level at which no known or anticipated health effects would occur, with a margin of safety. For many contaminants like lead and known carcinogens, the MCLG is zero.
2. Treatment Feasibility
Next, the EPA evaluates whether utilities can realistically achieve the MCLG using best available technology, taking cost into account.
3. Final MCL
The MCL is set as close to the MCLG as feasible. This is why MCLs are sometimes higher than zero even for known carcinogens — because absolute removal isn't technologically or economically possible.
Examples
- Lead: MCLG = 0, but treated as an "action level" of 15 ppb (soon 10 ppb)
- Arsenic: MCL = 10 ppb, MCLG = 0 (known carcinogen)
- PFOA: MCL = 4 ppt, MCLG = 0 (likely human carcinogen)
- Total Coliform Bacteria: MCL based on positive sample frequency
What Happens When an MCL Is Exceeded?
Utilities must:
- Notify customers within 30 days (or 24 hours for acute violations)
- Take corrective action
- Submit a compliance plan to state regulators
- Continue sampling at increased frequency
Why TapSafeWater Tracks This
Our grading system (A through F) is based on the number of health-based violations — instances where a utility exceeded one or more MCLs. A city with zero violations earns an A. A city with frequent MCL exceedances earns an F. This is the cleanest, most regulatory-aligned way to communicate water quality risk.
