Modelling the Transport of PFAS Precursors in a Saturated Porous Medium downstream of a Colloidal Activated Carbon Barrier

Per- and polyfluoroalkyl substances (PFAS) represent an emerging global problem due to the increasing contamination of water resources, especially in firefighting training areas used by emergency services. For this reason, the EU has introduced regulations on PFAS concentration limits in drinking water for all member states. PFAS have various significant properties, such as high thermal stability, high surface activity and hydrophobic and oleophobic properties, which have led to their widespread use in numerous industrial sectors since the 1950s. Many of these substances are so called PFAS precursors, which are capable of partial transformation, breaking down to form a perfluorinated terminal substance. This behavior is typical of PFAS precursors, which, by increasing their mobility, affect the rate at which PFAS disperses in the saturated and unsaturated zones. Different PFAS compounds are able to compete for adsorption sites, which increases the mobility of some PFAS, influencing the risks associated with PFAS transport. Groundwater modelling is a key factor in understanding the transport and fate of PFAS precursors. This study focuses on a 1D numerical model using MODFLOW/MT3DMS to simulate the transport and fate of PFAS precursors in saturated porous media, analyzing the parameters that most influence the transport of contaminants like adsorption. The model was calibrated based on a detailed dataset of PFAS concentrations measured in a PFAS plume in groundwater. The plume originates from aqueous film-forming foam (AFFF) used at a fire training site near the civil airport in Örnsköldsvik, northern Sweden. The model was calibrated through the concentrations’ sampling of three monitoring well (107, SB3.4, 101B) placed downstream at different distances from a colloidal activated carbon (CAC) barrier. Sampling for each well was performed before and after the use of TOP (Total Oxidable Precursor) analysis. For each monitoring well, the calibration of PFBA, considered as a conservative tracer, was useful to determine the upstream groundwater inflow. The results show that short-chain terminal PFAS exhibits a high peak concentration followed by a drastic decline due to the effectiveness of the CAC barrier. PFAS precursors, on the other hand, exhibit a less pronounced and delayed peak, followed by a very slow decrease in concentrations that generates a long tail, a characteristic typical of slow desorption from the solid matrix. This research was focused on modelling the migration of PFAS and PFAS precursors downstream the barrier. The results show that the transport of the PFAS precursors can be modelled using TOP data and that the transport of PFAS precursors is slower than their corresponding PFAS end products. In conclusion, this thesis aims to use TOP data as a basis for modelling the transport of PFAS precursors, which differs from that of terminal PFAS due to different sorption behavior. Therefore, although these considerations emerged through 1D modelling, it would be advisable to extend the 1D model to a 2D/3D model. The extension to a 2D/3D model is therefore essential due to the presence of a physical bypass of the plume around the barrier and the difference in water supply between the three wells, which suggests that the water flow is not uniform and unidirectional. This will make it possible to optimize the adsorption parameters and analyze the transport models of PFAS precursors in more detail.