Numerisk modellering av PFAS-transport i omättad zon : En utvärdering av långsiktig spridning under förändrade nederbördsmönster

Sammanfattning

In recent years, awareness of PFAS (per-and polyfluoroalkyl substances) contamination in soil and groundwater has increased due to findings of the chemicals in drinking water and humans as well as knowledge of their toxicity. It has been found that PFAS can be retained in the vadose zone due to sorption mechanisms, through which especially long-chain PFAS interact with surfaces in the soil matrix such as the air-water interface. Therefore, understanding transport through the vadose zone is critical for estimating the longevity of PFAS in nature and the potential mass flux to downstream recipients. In this study, PFAS transport through the vadose zone was studied through statistical analysis and numerical modeling. Data were provided from a pilot study site at Sundsvall Timrå Airport in Sundsvall, where the Swedish Geotechnical Institute (SGI) and the Geological Survey of Sweden (SGU) investigate PFAS remediation in soils. Using pore water and soil moisture data, a statistical analysis was conducted. Field data was also used to calculate and compare Kd-values from the beginning and the end of the measurement period. Furthermore, three models were created and evaluated with regards to soil and porewater concentrations of PFOS, PFHxS and 6:2 FTAB. Lastly, two of the three models were used to predict the fate of PFAS in the studied soil, using precipitation input corresponding to three climate scenarios. The study showed that the correlation between water content and PFAS concentration is significant for some PFAS at certain depths and that more data is needed to be able to obtain significant results. Partitioning between soil and water was shown to be consistent between the beginning and the end of the measurement period, which indicates stable conditions in the studied soil. Of the three models, two were accurate in estimating pore water concentrations and soil concentrations for the period September 2023 - October 2025 and these two were used to predict the fate of the three PFAS. PFOS and 6:2 FTAB were shown to still be prevalent in the soil after 100 years assuming no decay, while PFHxS was transported out of the soil profile within 10 years. The models also showed that increased precipitation and rainfall intensity leads to more rapid PFAS transport, thereby increasing the mass flux to groundwater. The findings in the study emphasize the complexity of PFAS modeling in the vadose zone and the importance of considering the effects of interactions with the air-water interface. Furthermore, it highlights the need for increased remediation practices and the importance of targeting the source zone in order to decrease mass transport to groundwater.