Practical PFAS Immobilization in the Vadose Zone by Extreme Soil Vapor Extraction: Conceptual Understanding, Modeling, and Cost Analysis

Practical and cost-effective technologies are needed for per- and polyfluoroalkyl substance (PFAS) sources in the vadose zone to prevent continued migration of these contaminants from soil to groundwater. Many PFAS are characterized by high air–water interfacial adsorption coefficient (K_aw) values, and therefore, the air–water interface exerts a strong control on their transport. As soil moisture decreases in the vadose zone, air–water interfacial area generally increases. As a result, the effective retention of some PFAS can be increased by 100-fold or more in some cases with relatively modest reductions in soil moisture content. Quantitative modeling and conceptual costing analysis confirm the viability of a two-pronged PFAS immobilization strategy where (1) a surface cap is installed which is intended to prevent water infiltration, and (2) extreme soil vapor extraction (XSVE) is applied to dry the soil, which reduces or eliminates downward water flux and increases PFAS retention. Modeling results show that water flux and PFAS mass discharge to groundwater can be essentially eliminated using this approach. Even if recharge is not completely prevented (due to a leaking cap and/or insufficient soil drying), simulations show PFAS mass discharge to groundwater will still be greatly reduced due to the significantly enhanced PFAS retention. The equipment required for this approach is commercially available, and installation costs are modest and predictable. Based on this analysis, future pilot testing and field demonstrations are warranted.