How to Read a PFAS Lab Report as a Small System Operator

A PFAS lab report is a table of numbers, flags, and method notes. It isn't a violation letter, and it isn't a treatment design. Your job is to read what the lab measured, then map each result to the rule that applies this year.

The federal PFAS National Primary Drinking Water Regulation was finalized in April 2024. PFOA and PFOS each have an MCL of 4.0 ppt, which is the same as 4.0 ng/L. Initial monitoring is due April 26, 2027. This isn't legal advice. Confirm current status with your state primacy agency before you brief the board.

What the April 2024 rule set, and what is still proposed

As finalized in April 2024, community and non-transient non-community systems owe initial PFAS monitoring by April 26, 2027. Treatment and compliance for PFOA and PFOS were originally set for April 2029. On May 18, 2026, EPA proposed a two-year exemption option that would move PFOA and PFOS compliance to April 2031 for systems that qualify. That proposal is proposed, not final.

EPA also proposed rescinding the MCLs for PFHxS, PFNA, HFPO-DA (GenX), and the Hazard Index mixture. Until a final action publishes, don't delete those analytes from your planning, and don't tell the board the MCLs are gone. Read the report against the rule in force the day you open the PDF, then confirm with the state. For the deadline calendar, see PFAS compliance deadlines for small water systems.

ppt and ng/L are the same number

Labs report PFAS in nanograms per liter (ng/L). EPA writes the PFOA and PFOS MCL as 4.0 parts per trillion (ppt). For water, 1 ppt equals 1 ng/L. If the report says PFOA 3.2 ng/L, that's 3.2 ppt. If someone converts ng/L into ug/L or mg/L and then compares that figure to 4.0, they'll misread the result by a factor of a thousand or a million.

Write the unit on the whiteboard before the board meeting. When a council member asks whether 3.2 is over the limit, the honest answer is: 3.2 ng/L of PFOA is below the 4.0 ppt MCL. It's still a detect, and it still belongs in the monitoring file.

MRL is not the MCL

The minimum reporting level (MRL) is the lowest concentration the lab will report for that method and that analyte. It's a lab limit, not a legal limit. The MCL is the federal standard. They're easy to mix up because both appear as small numbers in the same column.

• MRL. Method and lab-specific. A result below the MRL is often reported as ND, or as less-than the MRL.
• MCL. The legal ceiling. For PFOA and PFOS it's 4.0 ppt (ng/L) under the April 2024 NPDWR.
• Detect below the MCL. The compound is present. You aren't in MCL violation on that sample alone. Keep the result and keep sampling on the state's schedule.
• Result at or above the MCL. That's the number you take to the state and the board. Whether it's a violation depends on the averaging and compliance rules in force, not on one highlighted cell.

If the MRL on the report is 4.0 ng/L, the lab can't tell you whether you're under the MCL. Ask whether they can report below 4.0 ppt with the method your state accepts for compliance monitoring. UCMR methods and compliance methods aren't always the same conversation.

Read the flags before you brief anyone. ND or U means not detected above the MRL. J is an estimated value. B means the compound also showed up in a blank. A rejected flag means the lab doesn't stand behind the number. Don't brief the board on a rejected result. Don't ignore a J-flagged value if it sits near 4.0 ppt. Call the lab and write the answer in the file.

UCMR 5 is not compliance monitoring

UCMR 5 was EPA's nationwide look at unregulated contaminants, including PFAS, in a defined set of systems. A UCMR 5 detect is useful. It isn't an automatic MCL violation. Compliance monitoring under the April 2024 NPDWR is a different program, with different methods, different schedules, and a hard initial-monitoring date of April 26, 2027.

If your only PFAS paper is a UCMR 5 report from a prior year, keep it. Don't treat a UCMR detect as a current MCL violation. Don't treat a UCMR non-detect as proof you can skip initial monitoring. Ask the state whether that data can count toward initial monitoring, and get the answer in writing.

How to walk a report in ten minutes

• Confirm the PWSID, sample point, sample date, and lab accreditation. A beautiful number on the wrong tap is the wrong number.
• Confirm the method. UCMR 5 and compliance methods can differ.
• Find PFOA and PFOS first. Convert ng/L to ppt if needed (they should already match). Compare each result to 4.0 ppt.
• Then read PFHxS, PFNA, HFPO-DA (GenX), and any Hazard Index line. EPA has proposed rescinding those MCLs. Proposed isn't final. Ask the state how to treat those results today.
• Read every qualifier. Call the lab about anything rejected, blank-contaminated, or estimated near 4.0 ppt.
• File the report with the chain of custody.

What you tell the board this week

Use three sentences and stop. Here is the analyte, the result in ppt, and the MCL in force today. Here is whether this sample is UCMR 5 or compliance monitoring, and the April 26, 2027 initial-monitoring date you still owe. Here is what is proposed (the 2031 exemption option, the possible rescission of some MCLs) and what isn't final.

Don't say the system is out of compliance on PFAS because a UCMR report showed a detect. Don't say PFAS went away because of the May 18, 2026 proposal. If the result is at or above 4.0 ppt for PFOA or PFOS, call the state, then call the board, in that order if your primacy contact expects to hear it first.

FAQ

Is 4.0 ppt the same as 4.0 ng/L?

Yes, for drinking water. The PFOA and PFOS MCL is 4.0 ppt, which is 4.0 ng/L. If a report uses ug/L or mg/L, stop and convert before anyone briefs the board.

Does a UCMR 5 detect mean we have an MCL violation?

No. Don't treat a UCMR detect as an automatic MCL violation. UCMR 5 is unregulated-contaminant monitoring. Compliance monitoring under the April 2024 NPDWR is a separate duty, with initial monitoring due April 26, 2027. Ask the state whether older UCMR data can count toward that duty.

Can we wait for the 2031 exemption before we sample?

No. The May 18, 2026 two-year exemption option to April 2031 is proposed, not final, and it's about compliance and treatment timing for PFOA and PFOS, not about skipping initial monitoring. April 26, 2027 still sits on the calendar unless a final rule changes it. Confirm with the state.

What if the lab MRL is at or above the MCL?

Then the report can't show you that you're under 4.0 ppt. Ask the lab for a method your state accepts that reports below the MCL. A non-detect at an MRL of 4.0 ng/L isn't the same as a result of 2.0 ppt.

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Orevant provides compliance information tools. This page isn't legal advice. Confirm current PFAS MCLs, methods, and deadlines with your state primacy agency.