239-6 Modeling PFAS Fate and Transport in the Vadose Zone: A Three-Dimensional Numerical Approach

Session: Federal PFAS Remediation: Successes and Challenges

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Per- and polyfluoroalkyl substances (PFAS) released from aqueous film-forming foam (AFFF) applications have been widely detected in the subsurface, posing persistent risks to groundwater quality. This study applies a three-dimensional numerical model using HYDRUS v5.04 with the PFAS module to investigate PFAS transport and retention mechanisms in the vadose zone, with a focus on historical contamination and long-term migration potential. The model solves Richards’ equation using van Genuchten soil hydraulic parameters and incorporates site-specific lithology, vadose zone thickness, and layering to construct a realistic subsurface domain.

Key PFAS transport mechanisms, such as sorption to soil particles and retention at the air-water interface, are incorporated into the model. Sorbed-phase PFAS concentrations from 3D interpolations were used as initial conditions, capturing the spatial extent of residual contamination. A transient model was run over a 40-year simulation period, with realistic precipitation and evapotranspiration inputs to establish dynamic soil moisture and pressure head conditions. Precipitation inputs were scaled to reflect enhanced recharge through fine-grained, low-permeability playa lake deposits, which act as focused infiltration zones.

Lysimeter data were used to calibrate model outputs, particularly for porewater PFAS concentrations, resulting in good agreement with measured values. Simulations predict an average water flux (recharge) of 8.5 × 10⁻⁴ ft/day and an average PFHxS mass flux across the lower model boundary of approximately 366 mg/day. These results indicate that the soil water flux, influenced by precipitation and evapotranspiration patterns, strongly governs PFAS migration, while retention at the air-water interface and within the solid phase continues to limit vertical transport.

The model provides insights for scoping vadose zone remediation strategies, including the potential use of evapotranspiration covers to reduce recharge. In addition, the output is being integrated with a groundwater model to evaluate the long-term impacts of vadose zone PFAS leaching on the underlying plume. Findings contribute to improved prediction and management of PFAS fate in unsaturated soils under complex environmental conditions.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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