Tracking PFAS in rain and snow around the Lake superior basin 
Alissia M. Milani1, Kaitlyn J. Gruber2, Martin Shafer3, and Christina K. Remucal1,2,4
1 University of Wisconsin - Madison, Environmental Chemistry and Technology Program, Madison, WI, 2 University of Wisconsin - Madison, Department of Chemistry, Madison, WI, 3 University of Wisconsin - Madison, Wisconsin State Laboratory of Hygiene, Madison, WI, 4 University of Wisconsin - Madison, Department of Civil and Environmental Engineering, Madison, WI
Atmospheric transport and transformation of per- and polyfluoroalkyl substances (PFAS) are major mechanisms driving the global distribution of these toxic chemicals. Both wet (i.e., rain and snow) and dry atmospheric deposition may represent major input pathways of PFAS to many aquatic and terrestrial environments, especially in otherwise pristine natural systems. Lake Superior, the largest of the Great Lakes, is likely one such system with limited PFAS point sources and large meteoric loadings. Lake Superior also serves as a drinking water source to more than 10 million people, thus it is critical to understand how rain or snow may contribute to PFAS accumulation in Lake Superior, a lake with a very long hydraulic residence time of nearly 200 years. However, as in many locations, the atmospheric sources and loading of PFAS to the Upper Great Lakes are poorly characterized and constrained. Therefore, this research seeks to interpret spatial trends of PFAS in wet deposition with potential sources using statistical analyses and back-trajectory modeling. In collaboration with the National Atmospheric Deposition Program, wet deposition samples were collected on a weekly basis from nine sites around the US side of Lake Superior between 2022 and 2024. These samples were analyzed for 33 PFAS compounds in five major classes by solid-phase extraction, isotope dilution, liquid chromatography-tandem mass spectrometry (LC-MS/MS). Perfluorocarboxylic acids (PFCAs) were the most frequently detected class and accounted for 75.8% to 96.6% of targeted PFAS by mass. The abundance of PFCAs may partially result from atmospheric transformation of volatile precursor PFAS. However, the overall distribution of PFAS classes varied between sites, which may be driven by their proximity to potential point sources. Chemistry data from the NADP’s National Trends Network will be leveraged to better understand potential pollutant sources impacting wet deposition. Advancing our understanding of sources, transformation, and deposition of PFAS in precipitation can inform future research, drive policy development, and ultimately mitigate human exposure to these contaminants.