ON SAFE GROUND NGI.NO Transport behaviour of PFAS and immobilization approaches with regenerated waste products in large scale unsaturated lysimeters 1. Objective The transport of PFAS through the lysimeter column was heavily influenced by the weekly irrigation. Figure 2 a) summarizes the leaching of PFOS based on the exchanged pore volumes and b) the residual PFOS concentration in each lysimeter. With the start of the irrigation PFOS concentrations are lowered due to a dilution effect (Figure 2a). With the end of the irrigation PFOS concentrations in the leachate were increasing again. The total amount of PFOS which leached was increased by 14% up to 47% for the additional 1.6 and 3.5 L/week irrigation, respectively, compared to natural precipitation. Figure 2b presents a chromatographic like distribution of PFOS in the different lysimeter columns, and their different retentions at the end of the experiment. Michel Hubert*1,2, Björn Bonnet*3, Lutz Ahrens3, Hans Peter H. Arp1,2 1 Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway. 2 Norwegian Geotechnical Institute (NGI), NO-0806 Oslo, Norway. 3 Swedish University of Agricultural Sciences (SLU), P.O. Box 7050, SE-75007 Uppsala, Sweden. Figure 1 – Lysimeter experiment at SLU, Sweden. Immobilization technologies aim to stabilize per- and polyfluoroalkyl substances (PFAS) within the soil to reduce and manage PFAS leaching. Two hurdles for their successful implementation exist. The first is that a detailed knowledge of PFAS transportation in the unsaturated zone is needed, both with and without the technical immobilization, under different conditions. The second is that many binders may have their own risks, such as anthracitic carbon associated with a high carbon footprint. This study investigates hereby the following aims: 2. Methods and Materials Natural precipitation vs. enhanced precipitation Reducing leaching of PFAS from hot spots Testing of novel, carbon sequestering biochars for PFAS stabilization Transport behaviour of PFAS in the unsaturated zone with changing climate 10 outdoor lysimeters were set up at SLU, Sweden (Figure 1). All lysimeters were packed with sandy soil spiked with 18 PFAS at concentrations found at typical AFFF contaminated sites. All lysimeters received the natural rainfall over the experimental time of 1 year. In addition, two lysimeter were weekly irrigated (1.6 L = 35 mm/h and 3.0 L = 64 mm/h, respectively). Different lysimeters contained different amendments, including 1% powdered activated carbon (PAC), 1% and 4% waste timber biochar (WT-BC), 1 and 4% sewage sludge biochar (SL-BC) (Sørmo et al. 2023). Leachate samples were taken weekly when available and section wise soil samples were taken after the end of the experiment. Figure 2 – a) Leaching behaviour of PFOS, b) residual PFOS in the soil for non-amended lysimeters. 3. Transport behavior of PFAS 4. Stabilization using waste derived biochars Figure 3 compares the stabilization of the environmentally friendly, waste derived biochars with the PAC lysimeter. 5. Conclusion For all tested amendments, the retardation increased with increasing –CF2 chain length. Furthermore, there is a clear headgroup dependency with retention increasing in the order of FTS > PFSAs > PFCAs. Compared to 1% PAC the biochar derived from waste timber (WT-BC) showed 30 to 40% total retention for shorter chain PFAS and 50% for longer chain PFAS. The sludge derived biochar (SL-BC) performed comparable better, with total retention for shorter chain PFAS in the range of 60 to 70% and 70 to 90% for longer chain PFAS. The increase of the amount of biochar mixed into the soil from 1 to 4% led to 5 to 10% higher PFAS retention. Figure 3 –PFAS stabilization [%] compared to the lysimeter amended with 1% PAC. Green shade identifies shorter chain and red shade longer chain PFAS. • PFAS contaminated hot spots are vulnarable to a changing rainfall and irrigation events. • Sludge derived biochar showed prom