This project sought to protect groundwater resources by preventing the vertical migration of per- and polyfluoroalky subtances (PFAS) from soil into groundwater. The overall objective of the project was to demonstrate and validate PFAS removal from soil using a mobile-scale soil washing system as a cost-effective mass removal and volume reduction technology that can be applied to treat PFAS-impacted soil in source zones. Specific goals included 1) bench-scale testing to demonstrate site suitability and treatment process optimization, 2) field-scale trials to demonstrate performance, and 3) cost optimization and reduction of lifecycle costs.
The technical approach focused on the treatment of coarse soil fractions (sand and gravel) and separation of fine soil fractions (silt and clay) for potential treatment by other means such as landfilling, thermal desorption, or stabilization. The treatment process was designed and optimized at the bench scale and demonstrated at the field scale by conducting a treatment trial at Eielson Air Force Base using a mobile soil washing plant. A modular pilot-scale soil washing system was designed to duplicate the unit treatment processes and sequence of treatment evaluated at the bench scale. The system consisted of two soil modules and a water treatment circuit.
The soil washing technology was validated by measuring PFAS mass removal efficiencies based on PFAS soil concentrations measured via the Department of Defense (DoD) Quality Systems Manual (QSM) and PFAS leachate concentrations via the synthetic precipitation leaching procedure (SPLP). PFAS concentrations in treated soils were compared to the Alaska Department of Environmental Conservation screening levels for soil migrating to groundwater criteria for perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and perfluorobutane sulfonate (PFBS). PFAS concentrations in the soil leachate were compared to United States Environmental Protection Agency (EPA) groundwater screening criteria for PFOA, PFOS, and PFBS. The results of the treatability testing and pilot tests confirmed that coarse grained sand and gravel were successfully treated to meet performance goals; however, the fine soil fractions were segregated for alternative treatment.
The chemical properties of PFAS make it suitable for treatment via volume reduction by soil washing. The volume reduction approach has demonstrated treatment of the sand and gravel soil fractions down to lower part per billion levels; however, as soil cleanup goals become more stringent, it is anticipated that more advanced soil washing processes will be needed to consistently treat the coarse-grained soil fractions to a PFAS concentration less than 1 part per billion. In anticipation of these new cleanup goals, continued research is required to demonstrate that optimization of the physical separation approach combined with chemical treatment can achieve these levels in coarse-grained soil as well as a portion of the fine-grained soil fraction. (Project Completion - 2023)