The Ins and Outs of Per- and Polyfluoroalkyl Substances in the Great Lakes: The Role of Atmospheric Deposition.

As part of the Integrated Atmospheric Deposition Network, precipitation ( n = 207) and air ( n = 60) from five sites and water samples ( n = 87) from all five Great Lakes were collected in 2021-2023 and analyzed for 41 per- and polyfluoroalkyl substances (PFAS). These measurements were combined with other available data to estimate the mass budget for four representative compounds, PFBA, PFBS, PFOS, and PFOA for the basin. The median Σ 41 PFAS concentrations in precipitation across the five sites ranged between 2.4 and 4.5 ng/L. The median Σ 41 PFAS concentration in lake water was highest in Lake Ontario (11 ng/L) and lowest in Lake Superior (1.3 ng/L). The median Σ 41 PFAS concentration in air samples was highest in Cleveland at 410 pg/m 3 and lowest at Sleeping Bear Dunes at 146 pg/m 3 . The net mass transfer flows were generally negative for Lakes Superior, Michigan, and Huron and positive for Lakes Erie and Ontario, indicating that the three most northern lakes are accumulating PFAS and the other two are eliminating PFAS. Atmospheric deposition is an important source of PFAS, particularly for Lake Superior.