Single and Multicomponent Transport of PFOS and PFOA in Unsaturated Porous Media: The Effects of Trapped Gas Bubbles

ABSTRACT: The extensive use of per- and poly-fluoroalkyl substances (PFAS) over the last 80 years has resulted in their widespread presence in the environment. Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are two of the main PFAS of concern due to their toxicity, persistence and accumulation potential. Understanding how they are transported within groundwater systems is essential for conducting risk assessments and designing remediation strategies. Due to their surfactant-like nature, PFAS accumulate at air-water interfaces in the subsurface, which has been shown to contribute to 50-75% of total PFAS retention in unsaturated systems. In addition to air-water interfaces in the vadose zone, water table fluctuations, recharge and biogenic gas production form trapped gas zones which may affect PFAS transport. In this study, one-dimensional column experiments were conducted to quantify the effect of trapped gas zones on PFAS (PFOS and PFOA) transport. Columns were packed to produce a fully water-saturated and homogeneous sand pack. A non-reactive tracer (NRT) and PFAS were flushed continuously through the saturated pack and samples were collected and analyzed to determine the effluent concentrations over time. Each column was then sequentially drained and imbibed to simulate a water table fluctuation, and the NRT and PFAS experiments were repeated in the trapped gas pack. Results showed no retardation of PFAS under water-saturated conditions, demonstrating negligible sorption to the clean sands used in these experiments. However, significant retardation was observed in the trapped gas columns, indicating that trapped gas phase can retain PFAS in groundwater systems during infiltration, which was greater for PFOS than PFOA. In addition, data for mixtures of PFOS and PFOA will be presented. The amount of interfacial adsorption observed in these experiments has implications for PFAS-contaminated sites as trapped gas zones could serve as a buffer between the unsaturated and saturated zone and further limit PFAS transport from surface to groundwater.