The EPA regulates over 90 contaminants in public drinking water under the Safe Drinking Water Act (SDWA), enacted in 1974 and amended several times since. The regulatory framework uses specific terms (MCL, MCLG, action level, treatment technique) that can be confusing. Here's a plain-language guide to what they mean and why they matter.
Maximum Contaminant Level (MCL)
An MCL is the highest permissible level of a contaminant in drinking water delivered to any user of a public water system. It is legally enforceable. Utilities that exceed an MCL are in violation and must notify customers, take corrective action, and report to the state.
MCLs are set as close to the MCLG as "feasible," meaning the EPA weighs both health science and the practical ability to measure and remove the contaminant at reasonable cost.
Maximum Contaminant Level Goal (MCLG)
The MCLG is the level at which no known or anticipated adverse health effects occur, with an adequate margin of safety. MCLGs are health-only targets; they are not legally enforceable. For carcinogens like arsenic, chromium, and many PFAS compounds, the MCLG is 0 (meaning no level is considered safe), but the enforceable MCL is set above zero because complete removal is technically or economically infeasible.
Example: Lead has an MCLG of 0 (no safe level) but no MCL. Instead, the EPA uses an action level of 15 ppb. See lead data →
Action Level (AL)
Used for lead and copper under the Lead and Copper Rule (LCR). Instead of setting an MCL, the EPA defined an action level. If more than 10% of "first-draw" tap samples exceed the AL, the utility must take specific corrective actions (replacing lead service lines, adding corrosion control treatment, notifying customers).
- Lead Action Level: 15 ppb (0.015 mg/L) -- being phased down to 10 ppb under the 2024 LCRR
- Copper Action Level: 1.3 mg/L
Treatment Technique (TT)
For some contaminants that are impractical to test for directly, the EPA specifies a required treatment technique instead of an MCL. Examples include:
- Surface Water Treatment Rule: requires filtration and disinfection to reduce Giardia and Cryptosporidium
- Acrylamide and Epichlorohydrin: both are prohibited from exceeding certain levels in polymer additives; treatment technique specifies how they may be used
Secondary Standards
Secondary MCLs (SMCLs) regulate contaminants that affect taste, odor, or appearance, not health. They are not federally enforceable (states may choose to enforce them). Examples:
- Iron: 0.3 mg/L (staining, taste)
- Manganese: 0.05 mg/L (staining, taste)
- Total Dissolved Solids: 500 mg/L
- pH: 6.5–8.5
Health Advisories (HAs)
For contaminants without final MCLs, the EPA issues non-enforceable health advisories. These include lifetime HAs and shorter-term HAs. Before the April 2024 PFAS MCLs, the EPA issued health advisories for PFOA and PFOS as low as 0.004 ppt, well below any prior limit. Health advisories are important signals but don't trigger enforcement action.
How Often Are MCLs Updated?
Rarely. The MCL-setting process is slow: the EPA must publish a Contaminant Candidate List (CCL), monitor unregulated contaminants via UCMR rules, propose a regulation, and finalize it, a process that often takes a decade or more. The 2024 PFAS MCLs were the first new drinking water standards in over 20 years.
Check Your Water Against These Standards
SafeWater tracks violations of enforceable MCLs for every public water system in the U.S. Look up your water system → | See state rankings → | View PFAS contamination →