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What an MCL violation means for your drinking water

Updated · Written from EPA, NSF, CDC and maker documents, each linked at the end · How we decide what to say

Short answer

An MCL violation means a regulated contaminant exceeded its legal limit as measured under the rules for that contaminant. It is not the same as a monitoring or paperwork violation. Your utility must tell you, and the speed of the notice depends on the risk.

The vocabulary

  • MCL (maximum contaminant level): the highest level of a contaminant allowed in drinking water; legally enforceable.
  • MCLG (maximum contaminant level goal): a non-enforceable health goal based only on possible health risks. EPA sets lead’s MCLG at zero.
  • Treatment technique (TT): a required process, such as corrosion control for lead, used instead of an MCL for some contaminants. Violating a treatment technique can also be a health-based violation.
  • Action level: a trigger for lead and copper. Exceeding it requires the utility to take steps, such as corrosion control, public education and lead pipe replacement; it is not a pass or fail on its own tap.
  • Monitoring and reporting violation: the system failed to test or report on time. This is serious as a compliance failure but does not by itself show contamination.

What counts as an MCL violation

For most contaminants, the determination is based on a running annual average of the utility’s regulatory samples, not on one sample. A single high sample does not automatically create a violation. In our data, “health-based violations” means MCL and treatment technique violations recorded in EPA’s SDWIS; we count them over the last five years and do not count monitoring and reporting violations as health-based (methodology).

How you are told

EPA’s Public Notification Rule requires a notice when a system violates drinking water regulations or provides water that may pose a risk to health. There are three tiers:

  1. Tier 1, immediate notice: where human health could be immediately affected, the supplier has 24 hours to notify people using media, posted notices or delivery.
  2. Tier 2, as soon as possible: levels above EPA or state standards or untreated water that does not pose an immediate risk; notice within 30 days of the violation.
  3. Tier 3, annual notice: violations without a direct health impact, such as failing to take a required sample on time; up to a year, often combined with the annual CCR.

Each notice must describe the violation and contaminant level, when it occurred, potential health effects, the population at risk, whether you need an alternative water supply, what the system is doing, what you can do, and when it expects a fix.

What you can do

  1. Read the notice and follow any use restrictions in it.
  2. Check the utility page on this site for its five-year record (find your utility).
  3. Call the utility with the questions on the notice’s list.
  4. For a health question, talk to a clinician or health department. A violation record on this site is a prompt to ask, not a diagnosis.

Related guides

Sources

Not medical or legal advice. This guide summarizes public documents so you can ask better questions of your utility, your health department or a certified lab. For health concerns, talk to a clinician; for compliance questions, to your state drinking water program. Pages on this site that name products may earn a commission, as disclosed on the affiliate disclosure page.