COMPLEX TRANSPORT OF PERFLUOROOCTANESULFONIC ACID (PFOS) IN A DOLOMITE AQUIFER

Per- and polyfluoroalkyl substances (PFASs) represent a significant human health risk if present in groundwater used as a drinking water source. Proper assessment of PFAS exposure via groundwater requires a better understanding of field-scale PFAS transport in aquifers. Laboratory studies have documented PFAS sorption behavior for a range of site-specific soil and rock matrix compositions, but fewer studies have documented factors controlling field‑scale PFAS fate and transport in the subsurface. Firefighter training sites are common PFAS point sources, and can serve as case studies to identify important field-scale transport factors. In the 1970s, activities at a former firefighter training site in University Park, Pennsylvania introduced perfluorooctanesulfonic acid (PFOS) to the underlying dolomite aquifer. The PFOS plume currently has two concentration maxima – located ~20 and ~220 m downgradient of the training site – with an intervening zone of lower concentrations. Based on analytical and numerical models informed by field and laboratory data, we propose that heterogeneity in aquifer organic carbon content (OC) is most likely responsible for the separation of the plume. Dissolved PFOS readily partitions to organic solids, so even small amounts of OC in an aquifer material significantly limits PFOS transport, whereas near-zero OC allows longer distance – essentially conservative – transport. This highlights an important consideration for modeling the subsurface transport of PFOS and other readily adsorbed contaminants: in aquifers with discrete layers of varying OC, the incorporation of a uniform site‑averaged OC value in models could lead to underestimates of transport distance, thereby misrepresenting downgradient exposure risks.