PER AND POLY-FLUOROALKYL TRANSPORT IN THE UNSATURATED ZONE: INSIGHTS FROM A LARGE-SCALE COLUMN EXPERIMENT

Recent environmental and public health concerns have been raised about per- and poly-fluoroalkyl substances (PFAS). These anthropogenic chemicals have been detected above permissible limits at many groundwater sources around the world. Yet the fate and transport mechanism of PFAS in environmental ecosystems remains unclear.

This study aimed to investigate the mechanisms controlling the downward migration of PFAS across the unsaturated zone, from land surface to the water table. In parallel to natural environment studies, we characterize PFAS transport in controlled column experiments. The experiment was designed to enable high-resolution measurement in a large column (3m long, 0.4 m diameter). Along the column, 10 units of custom-made special suction cups and 7 water content sensors were placed to characterize water flow and PFAS transport in the unsaturated/saturated zones. Bromide was first used to trace water flow during infiltration experiments and determine the residence times across the unsaturated regions. Water and tracer application were done as individual events mimicking percolation of rainwater. Later, four different PFAS compounds of equal concentration (PFOS, PFOA, PFHxA, and PFPeA) were injected as a mixture into the column. Breakthrough of the PFAS compounds along the column, with respect to the breakthrough of Bromide and variations in the water content were used to characterize the transport properties.

Infiltration and drainage cycles were recorded through variations in sediment water content and breakthrough of the various PFAS and Br across the column. Wetting front propagation velocities and fluxes ranging from 190 to 680 cm/day and 1560 to 6880 cm according to the infiltration and drainage phases. Breakthroughs of the different PFAS compounds across the column exhibit sequential mobility where PFPeA >PFHxA >PFOA >PFOS where shorter carbon-chain length migrated faster than the longer variants. As PFOS and PFOA share the same carbon chain length but different functional groups, we observe that the sulphonic group (PFOS) of PFAS is more retarded than the carboxylic group (PFOA), with PFOS having a high soil partitioning coefficient. Overall, water flow, PFAS chemical structure, soil properties and partitioning mechanisms control PFAS migration in the subsurface.