PFAS transport through quasi-saturated porous media: Laboratory experiments and mixture effects

Understanding how per- and poly-fluoroalkyl substances (PFAS) are transported is critical to site characterization, monitoring, risk assessment, and remediation planning.  This includes an understanding of PFAS retention and release at air-water interfaces.  These interfaces exist throughout the vadose zone, but can also exist as trapped air bubbles created by water table fluctuations, recharge, and biogenic gas production.  In addition to being directly applicable to transport through trapped gas zones at PFAS-impacted sites, laboratory experiments using emplaced trapped gas (quasi-saturated conditions) provide a controlled method to investigate PFAS behavior, including the effects of different PFAS, concentrations, and mixtures. 

In this study, a series of laboratory experiments was conducted using one-dimensional sand-packed columns (20 cm × 7 cm dia.).  Trapped air was emplaced by sequential drainage and imbibition to create quasi-saturated conditions.  Each experiment included separate injections of non-reactive tracer (NaCl) and PFAS solutions through both water-saturated and quasi-saturated columns.  A clean, low organic carbon sand was used to eliminate solid-phase sorption (verified through comparison of non-reactive tracer and PFAS breakthrough in the water-saturated columns) and to isolate the effect of air-water interfaces.  Experiments were conducted using single-component solutions of PFOA, PFOS and 6:2 FTS, as well as mixtures of those PFAS, at concentrations of 0.1-1 mg/L.  Experiments were also conducted using diluted aqueous film-forming foam (AFFF) solutions.  Measured retardation factors in triplicate experiments were used to estimate air-water partitioning coefficients.

The results showed that PFAS breakthrough was significantly delayed in the presence of trapped air bubbles.  Breakthrough delay was greater for PFOS than for PFOA or 6:2 FTS, and was greater for lower PFAS concentrations, for the range of concentrations used in these experiments.  For PFAS mixtures, differences in retention were sufficient to completely separate breakthrough (i.e., PFOA and 6:2 FTS achieved complete breakthrough prior to any PFOS arrival) even over short (20 cm) distances.  However, the behavior of each PFAS tested was altered by the presence of other PFAS, with PFOA and 6:2 FTS experiencing earlier breakthrough at higher concentrations (concentration overshoot) in the presence of PFOS.  Mixture effects were also observed for branched and linear PFOS isomers, and for AFFF solutions, which was further complicated by the presence of hydrocarbon surfactants.  The experimental results will be presented along with numerical simulations of PFAS transport subject to air-water partitioning, both to interpret the behavior of PFAS mixtures in experimental systems and to explore the implications of mixture transport in more complex field scenarios.