Fig. 2. Seasonal fluctuations in perfluoroalkyl acids (PFAA) at the surface-water/groundwater boundary. Vertical depth profiles of the sum of perfluoroalkyl sulfonates (PFSA), the sum of perfluoroalkyl carboxylates (PFCA), inferred precursors, pH, dissolved oxygen (DO), temperature, and nitrate at the surface-water/groundwater boundary at downwelling sites in September 2017, February 2018, and February 2019. *Site GWOUT-R-S sampled in September 2017 (see legend) was landward of the lake shore due to low water level in Ashumet Pond; the samples are from 15 and 100 cm below the water table, located 40 cm below the ground surface. See Fig. S4† for profiles of individual PFAA concentrations.