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

Abstract

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.