Issues of Upscaling from Laboratory Columns to Field-scale Retention and Transport of Poly- and Perfluoroalkyl Substances in the Unsaturated Zone

Abstract

Per- and polyfluorinated alkyl substances (PFASs) are a large class of anthropogenic compounds which are widespread emerging contaminants of concern in the environment. Their environmental recalcitrance, long-term stability, high mobility, and toxicity make these compounds a significant threat to groundwater sources. Many PFASs are surfactants, imparting properties that impact their mobility in the subsurface.  In the last several years, progress has been made in the understanding of how these chemicals are retained and mobilized in the unsaturated zone by focusing on the mechanisms of partitioning from the water to the water-air interfaces within the pores. The fundamental knowledge of the mechanisms contributing to retention has been primarily studied using packed, one-dimensional laboratory columns with sands under highly idealized conditions. Few studies where either field soils are used in columns or pilot-scale field tests are conducted been reported or are in progress. Even without the chemical and physical complexities associated with the behavior of PFASs, modeling water infiltration and chemical transport in the field remains a challenge. Several factors, including spatial variability of the soil characteristics from pore to macro-scale, contribute to this challenge.  The soil in natural field systems where contamination has occurred is a mixture of silt, clay, and sand. The structure of the soil in its natural form changes within the vertical profile due to different levels of compaction and disturbance resulting from vegetation growth, decay, soil fracturing, bioturbation, etc. In addition, the texture variability results in heterogeneity at different spatial scales from pores to soil pockets (lenses) and layering (stratification). The water infiltrating through the soil carries the PFAS from the surface soils, where the chemicals have been introduced and deeper into the soil profile before reaching the water table, contaminating the groundwater. The soil disturbances and heterogeneity result in non-uniform water pathways, thus affecting the retention, mobilization, and transport. Additional challenges to PFASs that behave as surfactants come from spatial variability of the soil-water interfaces resulting from immobile zones that change spatially and dynamically, depending on the infiltration rates.   This paper discusses the conceptual issues that need to be addressed in transferring the knowledge and parameters determined using laboratory column studies where breakthrough data (BTC) are analyzed using many simplifications. Based on our experience of other problems of transport of chemicals in the vadose zone, the feasibility, challenges, and limitations of using multi-scale testing and modeling approach are presented.