Making Sense of UCMR 5 Results: Key PFAS Findings, Lithium Concerns, and UCMR 6 Signals
- By: Paul Jackson
- Tags: Drinking Water, PFAS
As UCMR 5 reporting wraps up, we finally have a near-complete picture of the individual concentrations of the 29 monitored PFAS plus lithium in drinking water systems across the country. In a recent webinar, I walked through what this UCMR 5 dataset reveals and some of the potential implications for water systems of all sizes.
Under UCMR 5, EPA required monitoring for 29 PFAS using Methods 537.1 and 533, plus lithium by EPA Method 200.7. Samples were collected at each entry point to the distribution system. Because PFAS are so ubiquitous in sampling materials and results needed to be reported at single-digit parts-per-trillion (ppt) levels, every sample also required a field reagent blank to verify that cross-contamination did not occur as a result of improper sampling.
EPA released the most recent UCMR 5 Data Summary in January, incorporating monitoring results from more than 10,000 public water systems and over 64,000 data sets submitted by UCMR‑approved laboratories. According to EPA, this release represents roughly 95% of the total results expected for UCMR 5, with only a small number of systems still completing sampling before the dataset is finalized later in 2026. The data summary report can be found online. EPA also offers a more detailed dive into the data through the UCMR 5 data finder tool.
Wondering whether your UCMR 5 sampling data can be used for NPDWR Initial Monitoring?
Reach out to one of our PFAS technical specialists.
What UCMR 5 Results Tell Us About PFAS
Exceedances for PFOA and PFOS, both with an MCL of 4 ppt, were not rare. Around 8% of all participating water systems reported PFOA results above the 4 ppt MCL, and 8.9% exceeded the 4 ppt MCL for PFOS. In contrast, GenX (HFPO-DA) with a 10 ppt MCL showed exceedances in just three out of more than 9,000 systems with results (0.03%).
PFHxS, also with a 10 ppt MCL, exceeded that level in roughly 0.7% of water systems, while PFNA at the same MCL was above the limit in only about 0.1% of systems—nine systems out of more than 9,000. When EPA evaluated the Hazard Index for the PFBS, HFPO-DA, PFHxS, and PFNA mixtures, only about 0.8% of systems exceeded the Hazard Index value of 1. The data summary also notes that large systems serving more than 10,000 consumers tend to have roughly twice as many exceedances in percentage terms as compared to smaller systems.
January 2026 Comparison of UCMR 5 Averages and the MCLs from the April 2024 NPDWR | PFAS in April 2024 NPDWR | MCL (μg/L)1 | UCMR 5 PWSs with full set(s) of results2 | UCMR 5 PWSs with average(s) greater than MCL | % UCMR 5 PWSs with average(s) greater than MCL3 | |||||
|---|---|---|---|---|---|---|---|---|---|---|
large PWS (>10,000) | medium PWS4 (3,300-10,000) | small PWS, (<3,000) | large | medium | small | large | medium | small | ||
PFOS | 0.0040 | 4,127 | 4,427 | 684 | 497 | 282 | 41 | 12.0% | 6.4% | 6.0% |
PFOA | 0.0040 | 4,128 | 4,427 | 684 | 452 | 253 | 30 | 10.9% | 5.7% | 4.4% |
HFPO-DA (GenX chemicals) | 0.01 | 4,129 | 4,427 | 684 | 2 | 1 | 0 | 0.05% | 0.02% | 0.0% |
PFHxS | 0.01 | 4,126 | 4,427 | 684 | 40 | 20 | 4 | 1.0% | 0.5% | 0.6% |
PFNA | 0,01 | 4,127 | 4,427 | 684 | 3 | 6 | 0 | 0.1% | 0.1% | 0.0% |
Hazard Index (HI) (HFPO-DA, PFHxS, PFNA, PFBS) | 1 (unitless) | 4,125 | 4,427 | 683 | 44 | 25 | 4 | 1.1% | 0.6% | 0.6% |
% UCMR 5 large PWSs (serving >10,000) with average(s) greater than MCL = 15.3% (632/4,129) | ||||||||||
% UCMR 5 medium PWSs⁴ (serving 3,300 – 10,000) with average(s) greater than MCL = 8.6% (382/4,427) | ||||||||||
% UCMR 5 small PWSs (serving <3,300) with average(s) greater than MCL = 7.3% (50/684) | ||||||||||
Estimated weighted % of PWSs nationwide with average(s) greater than MCL5 = 8.0% | ||||||||||
Source: The Fifth Unregulated Contaminant Monitoring Rule (UCMR 5) Data Summary: January 2026
Given the very low detection rates, I believe EPA’s plan to rescind the MCLs for GenX, PFHxS, PFNA and the PFAS Hazard Index is more likely to move forward. This would leave PFOA and PFOS as the only PFAS with enforceable federal MCLs in drinking water. At the same time, that outcome is far from certain as the proposal is still being challenged in the courts. For now, all six PFAS MCLs in the final PFAS NPDWR remain fully in force until EPA completes the rulemaking and the courts resolve the challenges.
UCMR 5 data also show that three unregulated PFAS (PFBA, PFHxA and PFPeA) were detected in roughly 18–20% of water systems. In this context, “detected” simply means results were above minimum reporting levels: 6 ppt for PFBA, 3 ppt for PFHxA and 3 ppt for PFPeA. EPA has not yet established a Health Reference Level (HRL) for these compounds, and there is still limited research to indicate whether concentrations at these levels pose a human health risk.
Unlike PFBA, PFHxA, and PFPeA, which were detected in close to one in five systems, many of the remaining PFAS were rarely reported above the minimum reporting limit, even in the large national dataset. That kind of sparse occurrence makes it seem unlikely that most of these lesser-seen PFAS will emerge as major regulatory drivers.
Lastly, it is important to remember that UCMR 5 reporting only covered 29 PFAS out of the thousands of compounds that meet the formal definition. In particular, it did not include several ultrashort-chain PFAS, such as TFA, that are increasingly drawing regulatory and scientific attention partly due to their extensive use globally. For more on this, refer to a recent post written by our PFAS Product Manager, Nick Nigro: Ultrashort-Chain PFAS: Why They Matter and How to Measure Them.
What UCMR 5 Results Tell Us About Lithium
Although I’ve focused mainly on PFAS in this post, the results for lithium, tested at the parts-per-billion (ppb) level with an HRL of 10 ppb, are well worth noting. Approximately 26.6% of all water systems exceeded the HRL for lithium—an unusually high rate for any contaminant in any round of UCMR. This high detection rate suggests lithium is a potential candidate for an EPA Health Advisory and, ultimately, a proposed MCL. We have seen this pattern before. Extensive UCMR occurrence data for PFOA and PFOS under earlier rounds of monitoring helped pave the way for today’s enforceable PFAS MCLs.
Looking Ahead: UCMR 6, Perchlorate, and What’s Next
In the webinar, I went into what we know so far about UCMR 6 timing and contaminant selection, including the potential role of PFAS, microplastics, pharmaceuticals, disinfection byproducts, and microbes. I also walked through the latest developments on perchlorate, including EPA’s proposed MCL and expected monitoring and compliance timelines that will affect tens of thousands of systems nationwide. Once the UCMR 6 proposal is released, I will certainly be sharing my perspectives.
We wrapped up the session with an extensive Q&A, in which I addressed practical questions about sampling, data interpretation, and planning for what comes next. The questions were very insightful, and it’s a good opportunity to hear how your peers are thinking about risk management and investment decisions in light of the evolving PFAS and lithium landscape.
If you want a deeper dive into the UCMR 5 data, how to interpret your own results, and what I believe the findings signal for future regulatory actions, I invite you to watch the full webinar on demand. In addition, please feel free to reach out to us with your questions or to discuss how these findings relate to your specific system.
