PFAS Monitoring Requirements for Small Water Systems: What You Need to Do Before 2027

Every community water system and non-transient non-community water system in the United States must complete PFAS initial monitoring under the April 2024 National Primary Drinking Water Regulation. The date that drives the work is not the compliance deadline. It is April 26, 2027, when initial monitoring ends and compliance monitoring begins. For a small groundwater system, the obligation is smaller than most operators assume — two samples per entry point inside a twelve month window, taken five to seven months apart — and the cost of missing it is larger. This page explains who must sample, how many samples, on what schedule, and what a missed period costs a small system.

Who must monitor for PFAS

The PFAS rule (89 FR 32532, April 26, 2024) covers every community water system (CWS) and non-transient non-community water system (NTNCWS), regardless of size or source type. Transient non-community systems, the campgrounds and rest stops serving a changing public, are not covered. If your system is a CWS or NTNCWS, you are in scope, and size changes the sampling schedule, not the obligation. See PFAS compliance deadlines for small water systems for the full timeline.

The two initial monitoring schedules

Initial monitoring requirements are set in 40 CFR 141.902(b)(1), and the schedule depends on your source and size:

• Groundwater systems serving 10,000 or fewer people. Two consecutive samples per entry point within a 12 month period, taken five to seven months apart. For most small groundwater systems, that is two sampling events.
• All other systems. Surface water systems, groundwater under the direct influence of surface water, groundwater systems serving more than 10,000 people, and any entry point that blends source types, must collect four consecutive quarters of samples per entry point within a 12 month period, two to four calendar months apart.

Sampling happens at every entry point to the distribution system under normal operating conditions. Previously collected data can satisfy part or all of the requirement: if your system already holds PFAS results, including UCMR 5 data, you can supplement rather than restart, as long as every required calendar period and entry point is represented (40 CFR 141.902(b)(1)(viii)). The question is not whether you have ever tested. It is whether the data you hold covers the required periods at every entry point.

The dates that drive the work

• April 26, 2027. Initial monitoring ends and compliance monitoring begins.
• April 26, 2029. The deadline for compliance with the PFAS MCLs.
• Quarterly monitoring. If any entry point detects a regulated PFAS at or above its trigger level, the system must sample quarterly for all regulated PFAS beginning April 26, 2027 (40 CFR 141.902(a)(6)).

The MCLs are 4 parts per trillion (ppt) for PFOA, 4 ppt for PFOS, and 10 ppt each for PFHxS, PFNA, and HFPO-DA (GenX), with a Hazard Index of 1.0 for mixtures of PFHxS, PFNA, GenX, and PFBS. The trigger levels sit at half those numbers — PFOA and PFOS at 2.0 ppt, the others at 5 ppt, and a Hazard Index of 0.5 — so the rule pushes you into more frequent sampling before you approach a violation, not after. Systems that begin operation or add a new source after April 26, 2027 must demonstrate compliance with the MCLs on the schedule the state sets (40 CFR 141.902(a)(4)).

What changed in 2026, and what did not

In May 2026, EPA proposed two changes to the PFAS rule. The first proposes to rescind the regulatory determinations and MCLs for PFHxS, PFNA, GenX, and the Hazard Index. The second proposes to extend the PFOA and PFOS compliance deadline to April 26, 2031 for systems that submit a request. Both are proposals, not final rules. Until a final rule publishes, every MCL and every monitoring obligation in this article remains in effect, and states with primacy can and do enforce their own stricter PFAS limits regardless of the federal outcome. See the 2031 extension proposal and what the rescission proposal does not change. Operators who read a headline and skip sampling are betting a state's monitoring calendar on a rule that has not changed; initial monitoring still ends April 26, 2027 either way.

What a missed monitoring period costs

A missed PFAS sample does not appear in the record as contamination. It appears as a monitoring violation in SDWIS, and that is what the primacy agency and every downstream reader sees. The sequence is familiar to small systems: the period passes, the violation is recorded, the state sends the notice, and a routine calendar item becomes a corrective conversation. Monitoring violations sit on the public record that grant reviewers, insurers, and boards pull. An exceedance discovered late also does not move the MCL compliance date; the 2029 deadline does not slide because you started sampling late.

Know where your system stands before the state does

The only way to know whether your system has satisfied initial monitoring is to compare your sampling history against this schedule at every entry point. That check is what the Orevant scan does: it pulls your EPA SDWIS record, maps your PFAS monitoring status against the rule, and tells you which sampling period is open, which entry points are covered, and what is due next, alongside every other open compliance item on your system.

Get Your PFAS Compliance Roadmap — $199 and see your monitoring status and every open deadline on one screen. Prefer the public record first? Check your water system free at orevant.com.

FAQ

Do I need to monitor for PFAS if my system is very small?

Yes. Every community water system and non-transient non-community water system must complete initial monitoring regardless of size. Small groundwater systems serving 10,000 or fewer people take two samples per entry point, five to seven months apart, inside a 12 month period. Size changes the schedule, not the obligation.

Can I use my UCMR 5 results instead of sampling again?

Possibly. Previously collected PFAS data can satisfy part or all of the initial monitoring requirement if it meets the rule's conditions, including entry point coverage and calendar period representation. The check is whether your existing data covers every required period at every entry point. A compliance scan maps that against your PWSID record.

What happens if EPA finalizes the rescission or extension proposals?

If finalized, the rescission would remove the PFHxS, PFNA, GenX, and Hazard Index MCLs, and the extension would move the PFOA and PFOS compliance date to 2031 for systems that file a request. Monitoring obligations remain in force until a final rule publishes, and state PFAS limits continue regardless. Initial monitoring still ends April 26, 2027 under the rule as it stands today.