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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.

In situ chemical oxidation (ISCO) has proven successful in the remediation of aquifers contaminated with dense nonaqueous phase liquids (DNAPLs). However, the treatment efficiency can often be hampered by the formation of solids or gas, reducing the contact between remediation agents and residual DNAPLs. To further improve the efficiency of ISCO, fundamental knowledge is needed about the complex multiphase flow and reactive transport processes as new solid and fluid phases emerge at the microscale. Here, via microfluidic experiments, we study the pore-scale dynamics of trichloroethylene degradation by permanganate. We visualize how the remediation evolves under the influence of solid phase emergence and explore the roles of injection rate, oxidant concentration, and stabilization supplement. Combining image processing, pressure analysis, and stoichiometry calculations, we provide comprehensive descriptions of the oxidant concentration-dependent growth patterns of the solid phase and their impact on the remediation efficiency. We further corroborate the stabilization mechanism provided by phosphate supplement, which is effective in inhibiting solid phase generation and thus highly beneficial for the oxidation remediation. This work elucidates the pore-scale mechanisms during remediation of chlorinated solvents with a particular context in the solid phase production and the associated effects, which is of general significance to understanding various processes in natural and engineered systems involving solid phase emergence or aggregation phenomena, such as groundwater and soil remediation.

Because of their environmental recalcitrance, high mobility, and toxicity, poly- and perfluoroalkyl substances (PFASs) are a growing and significant threat to groundwater throughout the world. There has been progress in the detection and treatment of PFASs, however their transport behavior in the subsurface are still not well understood. Despite the relative mobility in the subsurface, PFASs have been found in source-zone soils at significant levels even decades after their application ceased. This observation has largely been attributed to both the presence of highly sorptive polyfluorinated precursors to perfluoroalkyl acids (PFAAs) as well as the retention of PFASs at the air-water (A-W) interface.This paper discusses the general behavior of PFASs in the unsaturated zone and identifies several shortcomings related traditional contaminant transport experimentation and modeling methods when applied to PFASs. To address these issues, significant gaps in understanding related to the conceptualization, testing, and modeling of PFAS behavior in unsaturated zone have to be investigated. Our preliminary work confirms that PFASs are highly retarded and retained at the A-W interface. However, when PFAS-based aqueous film forming foams (AFFFs) are used, the mixture of PFASs introduced to the environment is significantly more complex: AFFF formulations can contain hundreds of PFASs in varying concentrations. Three key findings of our research with respect to PFAS transport in the unsaturated zone are:Current methods of breakthrough curve (BTC) analysis make simplifying assumptions which are likely insufficient for quantifying PFAS retention at the A-W interface. Both the residual air trapping and dead volume impacts are assumed to be negligible in traditional BTC methods. These assumptions are likely ill-suited for PFAS analysis. A more sophisticated understanding and analysis of equilibrium BTCs is proposed for PFASs. Most BTC analysis in past studies is often conducted using short (less than 100 cm) columns, on one compound at a time with gravimetric calculations of the degree of saturation of the whole soil sample. End effects create non-uniform saturation along the length of columns. Furthermore, conducting BTC analysis one compound at a time is time consuming. By combining non-targeted chemical analysis with high resolution mass spectrometry (HRMS), BTC analysis can be performed on field-relevant complex mixtures with high accuracy saturation measurements. Equilibrium partitioning is often assumed to describe retention of multi-component compounds. Because unsaturated transport is controlled by capillarity that depends on interfacial tension at A-W interfaces and PFASs are surfactants with unusual transport behavior, it is unlikely that equilibrium transport models will sufficiently describe PFASs transport. Conceptual models of physical and chemical non-equilibrium transport have been developed and are undergoing field and laboratory verification.

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Subsurface spills of high-molecular weight, multicomponent, dense nonaqueous-phase liquids (DNAPLs) are intractable for remediation by conventional techniques. This paper introduces the concept of biostabilization of the DNAPL source region as a means of achieving risk reduction at DNAPL-contaminated sites. Successful biostabilization depends upon the interplay among dissolution, degradability, and toxicity of various DNAPL constituents, difficult to predict a priori for the mixture. Bench-scale screening tests are proposed for identifying those DNAPLs that are amenable to biostabilization. The screening protocols compare four criteria: (1) microbial activity; (2) composition of the DNAPL residue; (3) aqueous phase contaminant concentrations; and (4) aggregate aqueous phase toxicity—across unbiotreated controls and in mixed versus unmixed biometers. The unmixed system represents slow dissolution from DNAPL pools in the quiescent subsurface. The protocols are developed and evaluated with DNAPL coal tar in ...

The study of the interaction between the land and atmosphere is paramount to our understanding of many emerging problems to include climate change, the movement of green house gases such as possible leaking of sequestered CO2 and the accurate detection of buried objects such as landmines. Soil moisture distribution in the shallow subsurface becomes a critical factor in all these problems. The heat and mass flux in the form of soil evaporation across the land surface couples the atmospheric boundary layer to the shallow subsurface. The coupling between land and the atmosphere leads to highly dynamic interactions between the porous media properties, transport processes and boundary conditions, resulting in dynamic evaporative behavior. However, the coupling at the land-atmospheric interface is rarely considered in most current models and their validation for practical applications. This is due to the complexity of the problem in field scenarios and the scarcity of field or laboratory data capable of testing and refining coupled energy and mass transfer theories. In most efforts to compute evaporation from soil, only indirect coupling is provided to characterize the interaction between non-isothermal multiphase flows under realistic atmospheric conditions even though heat and mass flux are controlled by the coupled dynamics of the land and the atmospheric boundary layer. In earlier drying modeling concepts, imposing evaporation flux (kinetic of relative humidity) and temperature as surface boundary condition is often needed. With the goal of improving our understanding of the land/atmospheric coupling, we developed a model based on the coupling of Navier-Stokes free flow and Darcy flow in porous medium. The model consists of the coupled equations of mass conservation for the liquid phase (water) and gas phase (water vapor and air) in porous medium with gas phase (water vapor and air) in free flow domain under non-isothermal, non-equilibrium conditions. The boundary conditions at the porous medium-free flow medium interface include dynamical, thermal and solutal equilibriums, and using the Beavers-Joseph slip boundary condition. What is unique about this model is that the evaporation rate and soil surface temperature conditions come directly from the model output. In order to experimentally validate the numerical results, we developed and used a unique two dimensional wind tunnel placed above a soil tank equipped with a network of different sensors. A series of experiments under varying boundary conditions, using a test sand for which the hydraulic and thermal properties were well characterized, were performed. Precision data for soil moisture, soil and air temperature and relative humidity, and also wind velocity under well-controlled transient heat and wind boundary conditions was generated.

This report summarizes the results of a workshop sponsored by the Department of Defense's (DoD) Strategic Environmental Research and Development Program (SERDP) and Environmental Security Technology Certification Program (ESTCP) that sought to determine the research, development, test, and evaluation (RDT&E) needs for reducing the uncertainty associated with dense nonaqueous phase liquid (DNAPL) source zone remediation at field sites.

This final report for ARO grant DAAH04-96-1-0033, a project jointly funded by the National Science Foundation under separate award, (NSF-9526S75), summarizes the research accomplished during the project at the Niwot Ridge research site in the Colorado Front Range established by the PI and upgraded during the course of this project by an ARO research instrumentation award. Near-infrared photographs of melting snow were analyzed using a moving windows procedure that can be used to define the correlation lengths in the snow reflectance surface. Liquid water contents of surface snow were measured with a dielectric sensor at 0.5 m intervals on two 100x100m grids. A circular array of 16 snow lysimeters, each with a catchment area of 0.2 cubic meters, was operated for two winter seasons and larger arrays of 36 and 106 snow lysimeters were operated over the winters of 1999 and 2000, respectively. The characteristics of ice columns and frozen rills present during spring snowmelt were investigated. Snow and ice temperature profiles in normal and melting snowpacks were deterinined. The results of the research suggest that ice colums and rills that develop in the snowpack during snowmelt provide preferential flow paths for meltwaters and that a positive feedback system develps such that certain areas of the snowpack receive more meltwater than others as snow melt advances in time. Four students were supported on the grant and seven publications resulted from the project.