Per- and Polyfluoroalkyl Substances in Sediments from an Urban Riparian Floodplain: Distribution, Partitioning, and Fate

Floodplain and wetland sediments serve as critical interfaces between surface water and groundwater, where per- and polyfluoroalkyl substances (PFAS) in urban waterways can infiltrate, accumulate, and partition across solid, liquid, and air-water interfaces. We measured the spatial and vertical distributions of 40 PFAS in floodplain sediments in a Southern California urban watershed. We detected perfluorooctanesulfonic acid (PFOS) most frequently and at the highest concentrations. We performed desorption and sequential batch adsorption-desorption experiments to estimate PFOS in situ pore-water concentrations and to quantify hysteretic behavior during repeated cycles of adsorption and desorption. Estimated in situ PFOS pore-water concentrations in vadose zone sediments were more than twice (78 ng/L) those in recently measured surface water (<30 ng/L), which may reflect seasonal fluctuations in water-content-dependent partitioning and long-term trends in surface water. Our new method of employing mass-labeled PFOS in batch experiments presents direct comparisons of the adsorption of field-derived PFOS to that of PFOS introduced at discrete steps in the laboratory. Our results suggest two mechanisms governing hysteresis in PFOS solid-phase partitioning across distinct time scales: (1) rapid mass exchange at sediment surfaces influenced by the historical maximum aqueous concentration, and (2) rate-limited diffusion of PFOS in organic-rich sediments occurring over long time scales in the field but not in most laboratory batch experiments.