The governing factors controlling the leaching behaviour of PFAS-compounds from the unsaturated zone to groundwater: an analysis using a scenario-based modelling approach.

Abstract

Per- and polyfluoroalkyl substances (PFASs) have attracted attention for their widespread occurrence in the environment, combined with their high mobility and bioaccumulation potential. PFASs deposited on land surfaces are first transported through the unsaturated zone, where adsorption to both solid-water interfaces (SWIs) and air-water interfaces (AWIs) is expected to occur. In order to ensure that the concentration of PFAS in drinking water wells is sufficiently low to comply with strict regulations, a thorough understanding of these transport processes in the subsurface is necessary. This study uses an analytical solution to the advection-dispersion equation to perform numerical simulations of the transport of PFAS in the unsaturated zone in Dutch soils, under average Dutch climate conditions. The results show that the leaching of PFAS is fastest for short chained PFASs, and decreases in transport velocity with increasing chain length. For short chained PFASs the retardation factor is small (average per simulated soil for TFA: R = 1. 05 to 1. 34), and the transport velocity is mostly similar to the water velocity. For long chained PFASs the retardation factor is large (average per simulated soil for PFDA: R = 33. 3 to 645), and the transport velocity is mostly governed by sorption, especially to AWIs, but also to SWIs. The results also show that the leaching of long chained PFAS from the unsaturated zone is relatively faster in clayey soils, which have the highest water saturation, and therefore the smallest air-water interfacial area (Aaw). Most important uncertainties remain in the value of Aaw due to a lack of measurements in Dutch soils, but also due to the steady-state approach of this study which does not account for temporal variability in water saturation and consequently in the value of Aaw. Another source of uncertainty regarding the expected breakthrough and concentration in drinking water wells is the lack of data in the historical use and deposition of PFASs in the Netherlands.