Understanding PFAA Transport in Unsaturated Porous Media: Insights from Meter-Scale Column Experiments

Abstract

Per- and poly-fluoroalkyl substances (PFAS) are a group of contaminants that have gained significant attention due to their persistence, bioaccumulation potential, and adverse effects on human health and the environment. Understanding the transport behavior of PFAS in the vadose zone is crucial for effective remediation and risk assessment strategies. Although PFAS transport in the vadose zone is well-established for saturated geologic media, it is largely unexplored for unsaturated homogenous and heterogeneous systems. The primary objective of this study is to extend previous work by characterizing the transport of per- and poly-fluoroalkyl acids (PFAAs) in unsaturated porous media. Column experiments were conducted using 50-60 cm tall columns that were filled with sieved and fired sediments collected from a field site located in the Northern Highlands geologic region of Wisconsin. Short and long-chain PFAAs, including both carboxylic and sulfonic species, were introduced at the top of the column, followed by a steady-state flow of UltraPure MilliQ water. Results show delayed breakthrough when the saturation level is decreased, as retardation is enhanced when the air-water interfacial area is larger. As expected, PFAA species with longer carbon-fluorine chain lengths experienced greater degrees of retention compared to their short-chain counterparts. However, breakthrough timing between PFAA species was similar across all experiments, indicating sorption may be nonlinear. Heterogeneity resulted in spatially variable capillary pressure, yielding water saturation gradients that facilitated breakthrough timing that was delayed in comparison to the homogenous coarse sand experiment (850 µ𝑚 - 2 𝑚𝑚) and enhanced compared to the homogenous fine sand experiment (less than 850 µ𝑚). The presence of salinity measured by batch-type experiments showed the significance of salinity to increase PFAA sorption on sediments, as Kd was found to be higher than experiments using non-saline solutions. The last experiment conducted evaluated this observation and confirmed that salinity enhanced PFAA sorption processes, observed by delayed breakthrough with lower peak concentrations than observed in the non-saline homogeneous coarse experiment. Although breakthrough timing between these two experiments somewhat varies as expected, breakthrough trends between PFAA species across the two experiments differs. This highlights the importance of conducting large-scale column experiments rather than merely interporlating breakthrough from batch-type studies.By replicating vadose zone conditions and analyzing empirical data, this study assesses the retardation of PFAAs as influenced by heterogeneity and spatially variable saturation conditions. This study enhances understanding of PFAA transport in natural subsurface environments and aids the development of effective groundwater models and remediation/mitigation strategies.