Coupled hydrological modelling for PFBS movement into the subsurface

Per- and polyfluoroalkyl substances (PFAS) are chemical compounds containing carbon-fluorine bonds of high toxicity related with several concerning effects to human health and to the environment. Since PFAS are widely used in everyday life with numerous industrial uses as well as in fire training sites, airports and military areas, these substances have already penetrated to the soil and the aquatic environment. PFAS movement in surface water, soil and groundwater is a field of high interest during the last few years among the scientific community with much recent research focusing on PFAS movement, sorption and travel time through modeling. The aim of the present work is to examine the movement of Perfluorobutanesulfonic acid (PFBS) in the subsurface through numerical modeling. Different hydraulic models were utilized to simulate water and solute movement in the unsaturated zone and the groundwater. More specifically, HYDRUS 1-D and PHREEQC codes for unsaturated zone flow and MODFLOW 6 code for groundwater flow were implemented. The proposed framework was applied in Kifissos basin, Athens, Greece. The unsaturated zone flow model was constructed for a pilot area where Kifissos riverbed is natural (not channelized). To conceptualize the unsaturated zone column under the natural riverbed in the pilot site, several lithostratigraphic data were employed. Sensor data of the river stage were utilized for model inflow. Mass transport within the unsaturated zone was simulated using Hydrus 1D code for the convection and dispersion of chemical species in the liquid phase of the unsaturated zone. PFBS initial concentration was obtained from a grab sampling campaign. Reactive sorption onto the solid phase and adsorption onto the air-Air-Water Interface (AWI) of the unsaturated zone is simulated with PHREEQC 3, given the initial concentrations from Hydrus-1D mass transport simulation. For the simulation of air-water interfacial adsorption the predefined mathematical rate expressions have been scripted into RATES data block of PHREEQC 3. A regional groundwater model was constructed for the case study. The model includes two convertible (phreatic) aquifer layers. The complex lithology of Athens was configured through hydraulic properties zonation. Groundwater flow model performance was validated with existing measurements. Further discretization was applied to model grid at the vicinity of the pilot area. The Groundwater Transport Process (GWT) of MODFLOW 6 was utilized to simulate advection and dispersion processes. Mobile Storage Transfer (MST) Package was utilized to simulate solute storage, sorption, and decay on the mobile domain. Finally, the coupling of the unsaturated flow and solute transport results was accomplished through Mass Source Loading package (SRC). The coupling of several subsurface hydrological models revealed that PFBS can be characterized as threatening substance for groundwater due to its mobility, the minimal sorption at the AWI and the competitive displacement from solid surfaces with the introduction of longer-chain PFAS. Groundwater numerical modeling suggested that PFBS plume movement and concentration is affected by regional groundwater flow and sorption processes.

Acknowledgements: This reasearch is part of the project UPWATER (Understanding groundwater Pollution to protect and enhance WATERquality) that has received funding from the European Union under grant agreement No 101081807