MCL vs Treatment Technique vs Action Level: What You Tell the Board

A high number on a lab report isn't yet a violation. The first job is to name the kind of limit you're looking at: a maximum contaminant level, a treatment technique, or an action level. Those three rules send you down three different clocks.

This is the decision tree, not a list of every federal number. It answers three operator questions: what do I tell the board, what notice do I owe, and is this a violation. This isn't legal advice. Confirm the finding with your state primacy agency.

The three buckets

• Maximum contaminant level (MCL). A legal ceiling on what is in the water. Cross it under the rule's averaging, and you have a health-based violation. Nitrate at 10 mg/L as N is the example most small systems know.
• Treatment technique (TT). A required process, not a single concentration. The Surface Water Treatment Rule is the example: filter and disinfect as the rule requires. Fail the process, and you have a TT violation even if nobody handed you an MCL.
• Action level (AL). A trigger, not a ceiling. Lead under the Lead and Copper Rule is the example. Exceed the 90th-percentile action level and you start follow-up and treatment work. That exceedance isn't, by itself, an MCL violation.

A fourth bucket sits next to all three: monitoring and reporting. A missed sample is a monitoring and reporting violation. It isn't an MCL. It still goes on the public record. It still needs a notice on its own tier.

Start here: which paper are you holding?

• A lab result for a contaminant that has an MCL (nitrate, arsenic, TTHM, E. coli) goes down the MCL branch.
• A missed filter report, a turbidity excursion under SWTR, or a failure to maintain a required residual goes down the treatment-technique branch.
• A 90th-percentile lead or copper calculation goes down the action-level branch.
• A sample you didn't take, took late, or took at the wrong tap goes down the monitoring-and-reporting branch.

If you can't name the branch in one sentence, don't brief the board yet. Call the state, then talk.

Branch 1: MCL. Example, nitrate 10 mg/L as N

Nitrate has a federal MCL of 10 mg/L as nitrogen. That's a hard ceiling. A confirmed exceedance is a health-based MCL violation. Nitrate is acute. The public notice is Tier 1, which means you start the 24-hour clock. Tell the board four facts:

• We have a nitrate result above 10 mg/L as N. That's an MCL, not a treatment technique and not an action level.
• This is a violation. It will show on the public record.
• We owe Tier 1 public notice, beginning now, in the form the state accepts.
• We're resampling and we're calling the primacy agency. We aren't waiting for the next board packet.

Other MCLs use different notice tiers. A TTHM running-annual-average exceedance isn't a 24-hour nitrate event. Name the MCL, say whether the averaging makes it a violation, name the tier, and say what you're doing this week. Don't call it an action level.

Branch 2: treatment technique. Example, the Surface Water Treatment Rule

The Surface Water Treatment Rule (40 CFR 141.70 through 141.75) doesn't give small surface-water systems a single virus MCL to hit with a lab slip. It requires a treatment technique: filtration (unless you have a filtered-avoidance approval) and disinfection that meets the rule's residual and contact-time tests. You can be in violation with clean-looking finished-water bacteria samples if the process failed.

Tell the board this is a treatment-technique problem under the SWTR, not an MCL number. If you missed a required filter, residual, or reporting step, that can be a TT violation even when the tap sample looks fine. The notice tier depends on the specific TT failure. The fix is restoring the process and documenting it, not arguing about one bacteria result.

Branch 3: action level. Example, lead

Lead in drinking water is regulated with an action level, not an MCL. Today the federal lead action level is 0.015 mg/L (15 ppb) at the 90th percentile. On November 1, 2027, the LCRI drops that action level to 0.010 mg/L (10 ppb). An exceedance starts corrosion-control steps, public education, and follow-up sampling. A lead action-level exceedance already requires 24-hour Tier 1 public notice. It's still not an MCL violation.

Tell the board the 90th percentile is above the lead action level. That's an action-level exceedance, not an MCL. You still owe notice and follow-up. Say the trigger was crossed and here is the work the rule starts. On November 1, 2027 the trigger drops to 10 ppb. A pass at 12 ppb today isn't a pass next winter. Copper uses an action level of 1.3 mg/L at the 90th percentile. Same branch, still not an MCL.

The miss that isn't an MCL

You were supposed to pull a nitrate sample in the quarter. Nobody pulled it. That's monitoring and reporting, not an MCL. The water might be fine. The file isn't. Tell the board you missed a required sample, you'll sample on the state's make-up instructions, and you'll send the notice the rule requires, often Tier 3. Missing the sample doesn't mean the water exceeded a limit. It also doesn't prove the water met it. The same speech works for a missed TTHM pair or a late CCR. Don't upgrade it to an MCL, and don't downgrade it to paperwork.

The decision tree

• Is there a required sample you didn't take, or took late, or took in the wrong place? If yes, it's monitoring and reporting. Issue the monitoring notice. Sample on the state's make-up schedule.
• Is this a process you failed (SWTR filtration, residual, contact time, a required TT step)? If yes, it's a treatment technique. Ask the state for the tier. Restore the process.
• Is this a 90th-percentile lead or copper number above the action level? If yes, it's an action-level exceedance. For lead, start the 24-hour notice.
• Is this a contaminant with an MCL, and did the result (after the rule's averaging) cross that MCL? If yes, it's an MCL violation. Start the right public-notice tier. Nitrate at 10 mg/L as N is the acute example.

If two answers are yes, you have two findings. Don't merge them into one speech. A missed nitrate sample and a later nitrate MCL are two records, two notices, and two corrections.

FAQ

Is a lead action-level exceedance a violation?

It's an action-level exceedance, not an MCL violation. You still owe follow-up and, for lead, 24-hour Tier 1 public notice. Tell the board the trigger was crossed and here is the work.

Is a missed sample an MCL violation?

No. A missed sample is monitoring and reporting. It can still be a violation. It can still require notice. It doesn't mean the water crossed an MCL. It means you can't show the state the result you were required to show.

What do I say when the board asks if we're in violation?

Answer with the bucket. MCL exceedance after averaging: yes, health-based violation. Treatment-technique failure: yes, TT violation. Lead action-level exceedance: the trigger was crossed; the finding is an ALE, not an MCL. Missed sample: monitoring and reporting violation. If you don't know yet, say so and confirm with the state.

Does the Surface Water Treatment Rule use an MCL for viruses and Giardia?

The SWTR holds surface-water systems to a treatment technique. You meet it by filtration and disinfection as 40 CFR 141.70 through 141.75 require. A process failure can be a TT violation even when a bacteria sample is negative.

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Orevant provides compliance information tools. This page isn't legal advice. Confirm the finding, the notice tier, and the citation with your state primacy agency.