Occurrence of per- and polyfluoroalkyl substances in Minnesota wastewater treatment facility influents: A Statewide Analysis

Introduction
Per- and polyfluoroalkyl substances (PFAS) are synthetic compounds found in various water sources due to extensive industrial and commercial use. Linked to health issues like reproductive and kidney toxicity, cancers, immune effects, and high cholesterol, PFAS were regulated by the US EPA in April 2024. Wastewater treatment facilities (WWTFs) are significant PFAS sources, with levels in effluents and biosolids sometimes nearing regulatory limits. PFAS sources include industrial inputs (e.g., AFFFs) and domestic inputs (e.g., textiles, cosmetics). The fate of PFAS in WWTFs depends on precursor transformation and sorption to solids. This study evaluated influent PFAS data from 83 Minnesota WWTFs, including industrial sources, to understand PFAS distribution, frequency, and concentration. It also assessed PFAS precursors using the TOP assay, aiming to identify PFAS sources and prioritize source reduction.

Methodology
As part of the Minnesota Pollution Control Agency's 2022 PFAS monitoring plan, samples were collected from 83 WWTFs and analyzed for PFAS using EPA Method 1633. The Municipal Wastewater program requested voluntary participation from WWTFs with significant industrial users. Facilities monitored PFAS levels for four quarters, with data available online. The first two quarters of 2023 were evaluated, with samples collected in February/March and April/May/June. In total, 181 samples from 83 WWTFs were analyzed, including 6 industrial streams. PFAS quantification was done via EPA Method 1633, evaluating 40 PFAS analytes. The TOP assay was applied to 54 WWTFs to estimate precursor concentrations, following the method by Houtz and Sedlak (2012). Each treatment facility was tagged with a random facility number, sampling number, and identified as municipal or industrial based on the sample location name.

Results and Discussion
Quantified PFAS in Influent
The cumulative probability distribution of 40 PFAS compounds in WWTF influents showed a median concentration of 32 ng/L, with 75% of facilities below 80 ng/L. Only six WWTFs had concentrations over 500 ng/L in at least one quarterly sample, and only one had both samples above 500 ng/L. PFAS levels varied widely, with PFCAs and PFSAs detected in 92-99% of samples. Median concentrations were 8.3 ng/L for PFSAs and 14.8 ng/L for PFCAs. The study highlights the need for large-scale, multi-quarter sampling to capture PFAS variability. Every influent WWTF sample contained at least one PFAS, with PFOA and PFHxS detected in all 83 WWTFs. PFBA, PFHpS, PFBS, PFOS, and PFPeA were found in over 86% of WWTFs, while others like PFUnA and ADONA were detected in fewer than 5 WWTFs. The precursors 5:3 FTCA, N-MeFOSAA, and N-EtFOSAA were detected in 22%, 24%, and 19% of WWTFs, respectively. PFOA and PFOS had median concentrations of 2.21 ng/L and 3.71 ng/L. PFBS, 6:2 FTS, PFPeA, and 5:3 FTCA had the highest concentrations, with PFBS reaching 101,000 ng/L.

Comparison Between Quarterly Samples
The study compared PFAS levels between quarters 1 and 2, finding no significant difference in the sums of quantitated compounds. Of 88 WWTF streams, 20 had consistent PFAS levels, 19 had higher sums in quarter 1, and 54 had higher sums in quarter 2. The median increase was 50 ng/L, and the median decrease was 27 ng/L. Sites with elevated concentrations showed differences over 200 ng/L. Multiple sampling events are crucial for accurate PFAS level capture. Industrial streams had more stable PFAS concentrations, indicating steady wastewater flow or background levels. PFBA levels increased in 52 streams, with a maximum rise of 204 ng/L, and PFHxA levels increased in 28 streams, with a maximum rise of 59 ng/L. Temperature, commercial activities, and residential contributions influence PFAS fluctuations. Rainwater dilution decreases PFAS levels. Szabo et al. (2023) found daily and hourly PFAS concentration changes, highlighting the need for further research into environmental factors affecting short-chain PFAS dynamics.

Unknown Precursors - TOP Assay Results
This study analyzed 62 stream samples from 54 WWTFs using the TOP assay to assess PFAS precursors. 24 samples were excluded due to inconsistencies, and 3 more due to negative PFCA sums. The remaining 35 samples showed PFCA and precursor levels ranging from 0.017 to 2.46 nM, with a median of 0.11 nM. Industrial streams had lower precursor formation than the median. The TOP assay, rarely applied to influents, revealed higher PFAS levels than typical effluents. PFBA, PFPeA, and PFHxA were the dominant PFCAs formed. The study confirmed that long-chain PFAS formation is unlikely, aligning with the phase-out of such compounds in the U.S. The TOP assay highlighted the significant role of unknown precursors, constituting 24-94% of total PFAS. The results emphasize the need for ongoing monitoring and analysis to manage PFAS pollution effectively.

Conclusion This study of 83 Minnesota WWTFs found regulated PFAS like PFOA and PFOS at lower levels, while precursors like N-MeFOSAA, N-EtFOSAA, and 5:3 FTCA were higher. Unknown precursors made up 24%-94highlighting their significance. The variability between sampling events underscores the need for multiple samples to capture temporal and seasonal fluctuations. Understanding both short-chain and long-chain PFAS dynamics is vital for effective regulation and management.