Modelling of transport of per- and polyfluoroalkyl substances in natural soil downstream a pilot-scale colloidal activated carbon barrier

Per- and polyfluoroalkyl substances (PFAS) are a widespread group of pollutants. One of the more common sources of PFAS pollution in Swedish groundwater is related to firefighting and training activities using aqueous film forming foams (AFFFs). While the data available on PFAS is increasing, differences in transport characteristics between the large number of PFAS species and lack of data relating to transport of precursors give rise to significant challenges within environmental risk assessment. At a fire training site in connection to Örnsköldsvik airport, Sweden, a pilot-scale colloidal activated carbon (CAC) barrier was installed on the 23^rd of November 2023, intercepting the PFAS plume and effectively ceasing downstream PFAS transport in the groundwater. The pilot-scale study is part of an ongoing governmental mission assigned to the Swedish Geotechnical Institute, in collaboration with the Geological Survey of Sweden and the Swedish Environmental Protection Agency among other governmental institutes. The contaminated site has been monitored over the span of two years, and geological and transient hydrological models have been done as groundwork to account for heterogeneity and seasonal variability in the area. The detailed monitoring of PFAS concentrations in space and time, including monthly time series for more than 50 points, in combination with the installation of the CAC barrier, allows careful observation of PFAS migration downstream the barrier.

The aim of this study is to improve our knowledge of PFAS transport and determine governing field parameters in the natural soil downstream the barrier by numerical modelling in MODFLOW/MT3D in combination with field observations. Several different PFAS are found in the groundwater and included in the modelling, as well as some target precursors. Calibration of the model allows estimation of field sorption parameters (Kd), which are critical for PFAS transport in the soil-groundwater system. Detailed total oxidizable precursor (TOP) measurements further allow analysis also of the migration and transport behaviour of unknown precursors connected to different terminal PFCAs accounting for 20 to 30% of the PFAS in the groundwater plume. The model and extensive measurements have shown real-world differences in transport parameters and precursor retardation greater than for perfluorinated PFAS.