Mark L. Brusseau’s research while affiliated with University of Arizona and other places

Air–water interfacial adsorption represents a major source of retention for many per‐ and poly‐fluoroalkyl substances (PFAS). Therefore, transient hydrological fluxes that dynamically change the amount of air–water interfaces are expected to strongly influence PFAS retention in their source zones in the vadose zone. We employ mathematical modeling to study how seasonal groundwater table (GWT) fluctuations affect PFAS source‐zone leaching. The results suggest that, by periodically collapsing air–water interfaces, seasonal GWT fluctuations can lead to strong temporal variations in groundwater concentration and significantly enhance PFAS leaching in the vadose zone. The enhanced leaching is more pronounced for longer‐chain PFAS, coarser‐textured porous media, drier climates, and greater amplitudes of fluctuations. GWT fluctuations and lateral migration above the GWT introduce a downgradient persistent secondary source zone for longer‐chain PFAS. However, the enhanced leaching and the secondary source zone are greatly reduced when subsurface heterogeneity is present. In highly heterogeneous source zones, GWT fluctuations may even lead to overall slower leaching due to lateral flow (in the GWT fluctuation zone and above the GWT) moving PFAS into local regions with greater retention capacities. Model simplification analyses suggest that the enhanced source‐zone leaching due to GWT fluctuations may be approximated using a static but shallower GWT. Additionally, while vertical 1D models underestimate source‐zone leaching due to not representing lateral migration, they can be revised to account for lateral migration and provide lower‐ and upper‐bound estimates of PFAS source‐zone leaching under GWT fluctuations. Overall, our study suggests that representing GWT fluctuations is critical for quantifying source‐zone leaching of PFAS, especially the more interfacially active longer‐chain compounds.

Health risks related to gastrointestinal exposure to arsenic (As) are frequently evaluated using in vivo models. However, due to the expense, ethical issues, and technical difficulties of in vivo models, in vitro bioaccessibility (IVBA) approaches—which enable the quantification of the portion of a contaminant released from a solid matrix placed in contact with a biochemical fluid—are more feasible for routine analysis. Particulate matter from sulfide ore-derived mine tailings is a common source of As exposure to human populations. As these waste materials are deposited on the landscape, they undergo weathering-induced changes in As molecular speciation as a result of climatic forcing, with poorly understood impacts of bioaccessibility. In unweathered sulfidic mine tailings, As is found predominantly as As1- in the form of arsenopyrite. However, oxidative weathering in the surface layers shifts prevalence to AsV species adsorbed or co-precipitated with FeIII (oxyhydr)oxides and hydroxysulfates, including ferrihydrite, goethite, and jarosite. We postulated that the depth of oxidative weathering would correlate with mean annual precipitation – i.e., shallow in arid and deep in humid climates – for tailings subjected to similar weathering durations. Additionally, we hypothesized that As would exhibit lowest bioaccessibility in surface tailings where it is adsorbed to secondary FeIII (oxyhydr)oxide surfaces, and highest bioaccessibility in deep, unweathered tailings where it is incorporated into primary sulfides (as observed by X-ray absorption spectroscopy measurements). Samples were collected as a function of depth (0-200 cm) through the oxidative reaction front from 13 sites spanning a range in climate across the Western U.S. The lowest As bioaccessibility was found in the intermediate zone characterized by highest As accumulation in secondary mineral phases, whereas highest As bioaccessibility was mainly found in the least weathered parent material of aridic sites and in the highly altered surface layers of humid sites. This indicates that the distinct As speciation detected at varying depths and among the mine tailings sites significantly impacted As bioaccessibility results. The data also highlight the importance of the different gastrointestinal constituents (e.g., protons, pepsin, pancreatin) in influencing As release as a result of the acidic and reductive dissolution of host minerals in gastric and intestinal bioassays, respectively.

Per- and polyfluoroalkyl substances (PFAS) have become one of the most important contaminants due to their ubiquitous presence in the environment and potentially profound impacts on human health and the environment even at parts per trillion (ppt) concentration levels. A growing number of field investigations have revealed that soils act as PFAS reservoirs at many contaminated sites, with significant amounts of PFAS accumulating over several decades. Because PFAS accumulated in soils may migrate downward to contaminate groundwater resources, understanding the fate and transport of PFAS in soils is of paramount importance for characterizing, managing, and mitigating long-term groundwater contamination risks. Many PFAS are surfactants that adsorb at air–water and solid–water interfaces, which leads to complex transport behaviors of PFAS in soils. Concomitantly, PFAS present in porewater can modify surface tension and other interfacial properties, which in turn may impact variably saturated flow and PFAS transport. Furthermore, some PFAS are volatile (i.e., can migrate in the gas phase) and/or can transform under environmental conditions into persistent PFAS. These nonlinear and coupled processes are further complicated by complexities of the soil environment such as thin water films, spatial heterogeneity, and complex geochemical conditions. In this commentary, we present an overview of the current challenges in understanding the fate and transport of PFAS in the environment. Building upon that, we identify a few potential areas where porous media research may play an important role in addressing the problem of PFAS contamination in groundwater.

A framework for developing predictive models for PFAS physical–chemical properties and mass-partitioning parameters is presented. The framework is based on the objective of developing tools that are of sufficient simplicity to be used rapidly and routinely for initial site investigations and risk assessments. This is accomplished by the use of bespoke PFAS-specific QSPR models. The development of these models entails aggregation and curation of measured data sets for a target property or parameter, supplemented by estimates produced with quantum–chemical ab initio predictions. The application of bespoke QSPR models for PFAS is illustrated with several examples, including partitioning to different interfaces, uptake by several fish species, and partitioning to four different biological materials. Reasonable correlations to molar volume were observed for all systems. One notable observation is that the slopes of all of the regression functions are similar. This suggests that the partitioning processes in all of these systems are to some degree mediated by the same mechanism, namely hydrophobic interaction. Special factors and elements requiring consideration in the development of predictive models are discussed, including differences in bulk-phase versus interface partitioning processes.