Influence of Tension-Driven Flow on the Transport of AFFF in Unsaturated Media

Per- and polyfluoroalkyl substances, and other chemicals in aqueous fire-fighting foam (AFFF), accumulate at air–water interfaces in the vadose zone, reducing interfacial tension and potentially causing tension-driven flow (TDF). This study investigates the importance of TDF on AFFF lateral spreading in the vadose zone using bench-scale column experiments and numerical modeling. Understanding AFFF spreading beyond an original spill footprint would enhance site conceptual models, wherein understanding the contaminant source zone is critical. Our experiments demonstrate that AFFF exhibits noteworthy TDF, characterized by substantial lateral water flow and AFFF spreading, causing variations in moisture content, which alters capillary pressure and hydraulic conductivity. The modified HYDRUS model accurately simulated experiments and was then used to simulate site-scale AFFF source zone dynamics. The numerical investigation demonstrated that TDF could lead to a significant increase in the surface footprint of an AFFF source zone, where wetter conditions yield a 6-fold increase compared to an initial spill, while drier soils exhibit a 4-fold expansion. The footprint expansion occurs over decades, which is highly relevant for our conceptual understanding of both historic and relatively new AFFF spills. This study highlights that the lateral flow of AFFF due to TDF is an important transport mechanism that controls the AFFF source zone.