PFAS MCL Compliance and Groundwater Reporting: What Comes After the April 2024 Final Rule

The April 2024 EPA final rule set drinking water MCLs for six PFAS compounds. Three years later, public water systems are deep into compliance schedules, and the question now landing on consultants’ desks isn’t “what are the limits?” — it’s “what does the sampling and reporting workflow look like, and how do I handle non-detects at these concentrations?” Working at 4 parts per trillion requires different lab coordination and QA procedures than the regulatory frameworks most environmental consultants built their workflows around.

The Six PFAS MCLs and the Two-Part Compliance Framework

The April 2024 final rule (89 FR 32532) established MCLs under the Safe Drinking Water Act for six PFAS compounds:

• PFOA (perfluorooctanoic acid): 4 ppt (ng/L)
• PFOS (perfluorooctane sulfonic acid): 4 ppt (ng/L)
• PFHxS (perfluorohexane sulfonic acid): 10 ppt (ng/L)
• PFNA (perfluorononanoic acid): 10 ppt (ng/L)
• HFPO-DA (hexafluoropropylene oxide dimer acid, “GenX chemicals”): 10 ppt (ng/L)
• PFBS + PFNA + PFHxS hazard index: hazard index ≤1 (mixture rule for these three combined)

Compliance has a two-part structure: initial monitoring (the required sampling that establishes baseline) and ongoing compliance monitoring once initial data is available. Public water systems that used groundwater sources had until April 2026 to complete initial monitoring; ongoing compliance monitoring requirements began after that, with the full treatment compliance deadline in April 2029.

Consultants advising public water systems on PFAS: the April 2029 compliance deadline is when systems must be in full compliance with the MCLs (treatment installed and operating if needed). The initial monitoring and ongoing reporting obligations started earlier and are already in effect.

Analytical Method Requirements: EPA 533 vs. 537.1

Related: EPA NPDES eReporting requirements covers the NetDMR submission workflow for ongoing compliance reporting.

Both EPA Method 533 and EPA Method 537.1 are approved for PFAS in drinking water. The choice matters for detection limits and analyte coverage:

EPA Method 537.1: Covers 18 PFAS compounds. Minimum Reporting Level (MRL) for PFOA and PFOS is 4 ppt — exactly at the MCL. At 4 ppt, a result right at the MRL provides poor quantitative confidence; the lab is essentially reporting that the concentration is at the detection floor.

EPA Method 533: Covers 25 PFAS compounds with lower MRLs (1.7 ppt for PFOA and PFOS in many implementations). Better suited for confirming whether a result is truly below 4 ppt versus at or slightly above it. EPA recommends Method 533 when concentrations near the MCL are expected.

Sampling Procedures at ppt Concentrations

PFAS sampling requires procedural changes from standard VOC or metals protocols because fluoropolymer-containing materials are potential contamination sources:

• No Teflon-lined equipment. PTFE (Teflon) tubing, pump lines, and well caps can leach PFAS into samples. Use polyethylene or polypropylene equipment. This includes sampling pumps — verify that pump tubing is PFAS-free.
• No field blanks with PTFE-containing equipment. If you’re using equipment that has touched fluoropolymers, your field blanks will be contaminated before the sample water contacts them.
• No sunscreen, cosmetics, or stain-resistant clothing. PFAS-containing consumer products contaminate samples at ppt levels. Field crews should follow the project-specific sampler health and safety plan regarding personal care products on PFAS sampling days.
• Sample volume: 250 mL polyethylene containers for drinking water samples. Preserve at <6°C; hold time is 14 days from collection to extraction.

These requirements should be specified in the sampling and analysis plan (SAP), not improvised in the field. A PFAS-contaminated field blank triggered by equipment failure invalidates the associated samples and requires a re-sample event, which at typical lab turnaround times (10–21 days) can push a reporting deadline significantly.

Reporting Non-Detects at 4 ppt

The standard compliance reporting question: if the lab reports “<4 ppt” for PFOA, is the system in compliance? Yes — the system is in compliance for the reporting period. But the reporting format matters.

Under SDWA Section 1414 monitoring and reporting, water systems must report the analytical result as returned by the lab. A non-detect (“U-qualified” result) is reported with the associated MRL, not as zero. The Discharge Monitoring Report or equivalent state reporting form accepts the MRL with a qualifier code to indicate non-detect.

Where consultants get into trouble: substituting zero or the MRL/2 (a common approach in risk assessment contexts) in a regulatory compliance context where the actual lab result should be reported as-is. Compliance monitoring reports require the certified lab result, including the qualifier. Using substitute values without disclosure can create a compliance reporting discrepancy.

Groundwater Monitoring Near PFAS Sources: Different Framework

The April 2024 MCLs apply to public water systems under the SDWA. Private groundwater monitoring at remediation sites (contaminated sites with PFAS as a compound of concern) operates under a different regulatory framework — typically CERCLA, RCRA corrective action, or state cleanup programs — and the applicable cleanup standards are state-specific risk-based screening levels rather than SDWA MCLs.

As of mid-2026, most state environmental agencies have adopted PFAS groundwater screening values derived from EPA’s Regional Screening Levels (RSLs) or state-specific risk calculations. These screening levels for PFOA and PFOS at residential groundwater (ingestion pathway) typically range from 0.7 to 4 ppt, depending on state. Several states have adopted the SDWA MCL (4 ppt) as their groundwater cleanup standard for drinking water pathway; others use lower values from independent risk calculations.

For remediation sites: verify the applicable state PFAS screening levels before preparing exceedance tables. Using federal RSLs when your state has adopted a more stringent value, or vice versa, produces an incorrect exceedance determination.

Initial Monitoring vs. Ongoing Compliance Monitoring: Reporting Differences

Initial monitoring under the PFAS rule establishes a baseline. For groundwater sources, systems sample once (or use historical data from within the lookback period). Ongoing compliance monitoring frequency for systems that complete initial monitoring depends on the initial result:

• Below MRL at all entries: system may qualify for reduced monitoring frequency (consult primacy agency)
• Above MRL but below MCL: annual monitoring at that entry point
• At or above MCL: quarterly monitoring and public notification requirements trigger

Public notification for PFAS MCL exceedances must be issued within 30 days of confirming the exceedance (two consecutive sampling events or one sample confirmed by a second analysis at the same lab). The notification content requirements are specified in 40 CFR Part 141 Subpart Q. If you’re advising a public water system on PFAS compliance, understanding the public notification timeline and content requirements is part of the scope — the 30-day clock starts at confirmation, not at the end of the reporting period.