Modeling investigations of PFAS transport through variably-saturated glacial sediments

Understanding the fate and transport of PFAS in the subsurface is essential for groundwater and contaminated site management. Most international studies focused on the transport in homogeneous sands, while other hydrogeological settings are less well studied. In this contribution, we investigate the impact of different glacial geological settings typically found in the Northern Hemisphere, possibly containing fractures and heterogeneities, on PFAS leaching through unsaturated glacial sediments.

We have implemented a vertical cross-section model that simulates transient groundwater flow and PFAS transport through the variably-saturated zone, accounting for sorption to the solid phase and to air-water interfaces. The model was tested on measured breakthrough curve data from saturated and unsaturated laboratory column experiments considering PFAS with different chain lengths. Model parameters were obtained from a comprehensive literature review, laboratory studies, and field investigations of contaminated sites.

The model was used to investigate the leaching of PFAS with different chain lengths through different setups with glacial sediment. We observed that the hydrogeological setting determines the magnitude of the air-water interfacial area and, thus, the retention of surface-active PFAS like PFOS and PFOA. Further, the model outcomes demonstrated a chromatographic separation of PFAS with different chain lengths due to different retention mechanisms. The longer-chained PFAS were retained more strongly in the unsaturated zone, while shorter-chained compounds were mobile.

Low-permeability clay-rich layers and inclusions generally provided less retention for surface-active PFAS due to a typically higher water saturation and, thus, smaller interfacial area compared to high-permeability media like sands. Fractures and heterogeneities may lead to the formation of preferential flow paths and thereby a potential bypassing of the unsaturated zone, where sorption to the air-water interface could occur. On the other hand, matrix diffusion may slow the rate of plume expansion by retaining PFAS in low-permeability layers. Over time, back diffusion from the matrix can result in long-term release to the groundwater.

The modeling investigations based on realistic data conducted in this study led to an improved understanding of the transport of short- and longer-chained PFAS in variably saturated glacial geological settings. Our findings allowed analyzing the influence of key parameters and processes on PFAS fate and transport.