A Maximum Contaminant Level (MCL) is a hard legal ceiling on a contaminant in drinking water. Cross it once and you have a health-based violation with a public notice attached. An action level is different: it is a trigger for treatment technique and follow-up, most commonly under the Lead and Copper Rule. For a small system, the practical difference is simple — miss an MCL and you answer to the state; miss an action level and you answer to a corrosion-control plan. This page lists the numbers small and mid-sized systems trip over most often, in plain terms.

The MCLs that drive the most small-system violations

  • Nitrate — 10 mg/L (as nitrogen). A Tier 1 acute violation when exceeded; most common in rural and agricultural areas drawing shallow groundwater.
  • Arsenic — 10 ppb (0.010 mg/L). Naturally occurring in groundwater across the West and Southwest; a health-based violation when exceeded.
  • Total trihalomethanes (TTHM) — 80 ppb (0.080 mg/L). A disinfection byproduct, measured on a running annual average.
  • Haloacetic acids (HAA5) — 60 ppb (0.060 mg/L). Disinfection byproducts formed with TTHM.
  • Fluoride — 4.0 mg/L (MCL); 2.0 mg/L is the secondary standard for dental fluorosis.
  • Uranium — 30 µg/L (0.030 mg/L). Naturally occurring in groundwater.
  • Combined radium-226/228 — 5 pCi/L. Gross alpha — 15 pCi/L.
  • Selenium — 0.05 mg/L. Total chromium — 0.1 mg/L.
  • Total coliform / E. coli — under the Revised Total Coliform Rule, any E. coli positive is an acute MCL violation with a Tier 1 notice inside 24 hours; no more than 5% of routine samples may be total-coliform positive.
  • Chlorine (MRDL) — 4.0 mg/L as a running annual average of the maximum residual disinfectant level. A zero residual in the distribution system is a treatment technique violation of its own.

PFAS: the newest enforceable limits

The federal PFAS National Primary Drinking Water Regulation sets enforceable MCLs of 4 parts per trillion (ppt) for PFOA and PFOS, and 10 ppt each for PFHxS, PFNA, and HFPO-DA (GenX). These are the floor, not the ceiling — several states set tighter numbers — and community water systems face initial monitoring well before the 2027 compliance milestones. A system that has never sampled has an unknown status, and unknown is the most expensive answer when the state asks.

See exactly what your system is up against

Search by system name or PWSID and get your EPA violation history, open requirements, and upcoming deadlines in under a minute. Free, no account, no obligation.

Action levels (Lead and Copper Rule)

  • Lead — 15 ppb at the 90th percentile, dropping to 10 ppb under the Lead and Copper Rule Improvements (LCRI). An exceedance triggers corrosion-control treatment, public education, and possibly service-line replacement.
  • Copper — 1.3 mg/L at the 90th percentile. An exceedance is about your own distribution pipes, not your source, and triggers corrosion-control treatment.

What happens the day you exceed one

An MCL exceedance starts two clocks: a public notice clock (Tier 1 = 24 hours, Tier 2 = 30 days, Tier 3 = 12 months) and a correction clock. An action-level exceedance starts a treatment-technique and follow-up sampling plan. Every day the item stays open in SDWIS is a separate citable failure, and it is public record that grant reviewers and insurers pull. See SDWA public notice tiers and deadlines for the notice clocks.

Know your own number, not the generic list

A list of MCLs cannot tell you what is open on your system, what is due next, or which contaminant your source is even required to monitor. That lives in your EPA SDWIS profile — and it is the one place a generic list cannot reach. Check your system free to see your open violations first, then Get Your Compliance Roadmap — $199 for the full rule-by-rule plan, or start $87/month monitoring so every MCL and deadline is tracked before it arrives.

FAQ

What is the difference between an MCL and an action level?

An MCL is a legal ceiling — exceed it and you have a violation with public notice attached. An action level (Lead and Copper Rule) is a trigger for treatment technique and follow-up sampling, not a violation by itself. Exceed the lead action level and you start corrosion-control treatment; exceed a nitrate MCL and you are in violation.

Which MCL violation is most common for small systems?

Not contamination — a missed sample. A late or missed coliform monitoring period becomes a monitoring violation the moment it happens, even when the water is safe. It sits on the public SDWIS record and can affect grant eligibility. The fix is a calendar that never misses a deadline.

Do the PFAS limits apply to my small system?

Yes. The federal PFAS MCLs of 4 ppt for PFOA and PFOS apply to every community and non-transient non-community water system regardless of size, and several states enforce tighter limits. Initial monitoring is due well before the 2027 milestones, so a system that has not sampled is flying blind on a deadline it cannot move.

Where do I find my own system's numbers?

Your open violations, monitoring schedule, and next deadline live in your EPA SDWIS profile, indexed by your PWSID. A generic MCL list cannot show you what is open or due on your system. Look up your system free at orevant.com, or run the $199 Complete System Report for the full roadmap.