Non-Fickian transport processes accelerate the movement of PFOS in unsaturated media: An experimental and modelling study.

The transport of per- and polyfluoroalkyl substances (PFASs) through unsaturated source-zone soils is a critical yet poorly understood aspect of their environmental behavior. To date, most experimental studies have only focused on the equilibrium or non-equilibrium partitioning of PFASs to the air-water interface, or solid-phase based equilibrium or non-equilibrium transport. Currently, there are discrepancies between air-water interfacial partitioning (K) results measured using a drainage-based column method (which supports a Langmuir isotherm) when compared to measurements from alternative experimental methods (which support a Freundlich isotherm).

Predicting Concentration- and Ionic-Strength-Dependent Air-Water Interfacial Partitioning Parameters of PFASs Using Quantitative Structure-Property Relationships (QSPRs).

Air-water interfacial retention of poly- and perfluoroalkyl substances (PFASs) is increasingly recognized as an important environmental process. Herein, column transport experiments were used to measure air-water interfacial partitioning values for several perfluoroalkyl ethers and for PFASs derived from aqueous film-forming foam, while batch experiments were used to determine equilibrium data for compounds exhibiting evidence of rate-limited partitioning. Experimental results suggest a Freundlich isotherm best describes PFAS air-water partitioning at environmentally relevant concentrations (10-10 ng/L).

Pore-Scale Mechanisms of Solid Phase Emergence During DNAPL Remediation by Chemical Oxidation.

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.

An overlooked mechanism underlying the attenuated temperature response of soil heterotrophic respiration.

Biogeochemical reactions occurring in soil pore space underpin gaseous emissions measured at macroscopic scales but are difficult to quantify due to their complexity and heterogeneity. We develop a volumetric-average method to calculate aerobic respiration rates analytically from soil with microscopic soil structure represented explicitly. Soil water content in the model is the result of the volumetric-average of the microscopic processes, and it is nonlinearly coupled with temperature and other factors.

Estimation of Transport Parameters of Perfluoroalkyl Acids (PFAAs) in Unsaturated Porous Media: Critical Experimental and Modeling Improvements.

Predicting the transport of perfluoroalkyl acids (PFAAs) in the vadose zone is critically important for PFAA site cleanup and risk mitigation. PFAAs exhibit several unusual and poorly understood transport behaviors, including partitioning to the air-water interface, which is currently the subject of debate. This study develops a novel use of quasi-saturated (residual air saturation) column experiments to estimate chemical partitioning parameters of both linear and branched perfluorooctane sulfonate (PFOS) in unsaturated soils.

The Mass Transfer Index (MTI): A semi-empirical approach for quantifying transport of solutes in variably saturated porous media.

The processes impacting solute transport through unsaturated porous media have been receiving renewed attention due to their relevance to the transport of emerging contaminants. A set of well-monitored and highly controlled experiments in sand columns were conducted to determine the effect of partial saturation on conservative solute breakthrough in porous media. The results suggest traditional transport parameter estimation methods inadequately account for the pore-scale processes of mass transfer to the immobile zones and the effects of partial saturation on advective transport, even for conservative tracers.

Effects of large-scale heterogeneity and temporally varying hydrologic processes on estimating immobile pore space: A mesoscale-laboratory experimental and numerical modeling investigation.

The advection-dispersion equation (ADE) often fails to predict solute transport, in part due to incomplete mixing in the subsurface, which the development of non-local models has attempted to deal with. One such model is dual-domain mass transfer (DDMT); one parameter that exists within this model type is called immobile porosity. Here, we explore the complexity of estimating immobile porosity under varying flow rates and density dependencies in a large-scale heterogeneous system.

Role of co-occurring competition and facilitation in plant spacing hydrodynamics in water-limited environments.

Plant performance (i.e., fecundity, growth, survival) depends on an individual's access to space and resources.

Effect of NAPL Source Morphology on Mass Transfer in the Vadose Zone.

The generation of vapor-phase contaminant plumes within the vadose zone is of interest for contaminated site management. Therefore, it is important to understand vapor sources such as non-aqueous-phase liquids (NAPLs) and processes that govern their volatilization. The distribution of NAPL, gas, and water phases within a source zone is expected to influence the rate of volatilization.

Study of the effect of wind speed on evaporation from soil through integrated modeling of the atmospheric boundary layer and shallow subsurface.

In an effort to develop methods based on integrating the subsurface to the atmospheric boundary layer to estimate evaporation, we developed a model based on the coupling of Navier-Stokes free flow and Darcy flow in porous medium. The model was tested using experimental data to study the effect of wind speed on evaporation. The model consists of the coupled equations of mass conservation for two-phase flow in porous medium with single-phase flow in the free-flow domain under nonisothermal, nonequilibrium phase change conditions.

Long-term mass transfer and mixing-controlled reactions of a DNAPL plume from persistent residuals.

Understanding and being able to predict the long-term behavior of DNAPL (i.e., PCE and TCE) residuals after active remediation has ceased have become increasingly important as attention at many sites turns from aggressive remediation to monitored natural attenuation and long-term stewardship.

Dissolution of dense non-aqueous phase liquids in vertical fractures: effect of finger residuals and dead-end pools.

Understanding the dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in rock fractures under different entrapment conditions is important for remediation activities and any related predictive modeling. This study investigates DNAPL dissolution in variable aperture fractures under two important entrapment configurations, namely, entrapped residual blobs from gravity fingering and pooling in a dead-end fracture. We performed a physical dissolution experiment of residual DNAPL blobs in a vertical analog fracture using light transmission techniques.

Experimental study of the effects of DNAPL distribution on mass rebound.

The release of stored dissolved contaminants from low permeability zones contributes to plume persistence beyond the time when dense nonaqueous phase liquid (DNAPL) has completely dissolved. This is fundamental to successfully meeting acceptable low concentrations in groundwater that are driven by site-specific cleanup goals. The study goals were to assess the role of DNAPL entrapment morphology on mass storage and plume longevity.

Effects of single-fracture aperture statistics on entrapment, dissolution and source depletion behavior of dense non-aqueous phase liquids.

Understanding of the entrapment and dissolution behavior of dense non-aqueous phase liquids (DNAPLs) in single fractures is important for modeling contaminant flux generation from fractured sites. Here a systematic numerical study is presented to investigate the effect of fracture aperture statistics on DNAPL migration, entrapment and dissolution within individual, variable-aperture fractures. Both fractures with open and closed bottom boundaries were considered.

Polymer-modified Fe0 nanoparticles target entrapped NAPL in two dimensional porous media: effect of particle concentration, NAPL saturation, and injection strategy.

Polymer-modified nanoscale zerovalent iron (NZVI) particles are delivered into porous media for in situ remediation of nonaqueous phase liquid (NAPL) source zones. A systematic and quantitative evaluation of NAPL targeting by polymer-modified NZVI in two-dimensional (2-D) porous media under field-relevant conditions has not been reported. This work evaluated the importance of NZVI particle concentration, NAPL saturation, and injection strategy on the ability of polymer-modified NZVI (MRNIP2) to target the NAPL/water interface in situ in a 2-D porous media model.

Transport and deposition of polymer-modified Fe0 nanoparticles in 2-D heterogeneous porous media: effects of particle concentration, Fe0 content, and coatings.

Concentrated suspensions of polymer-modified Fe(0) nanoparticles (NZVI) are injected into heterogeneous porous media for groundwater remediation. This study evaluated the effect of porous media heterogeneity and the dispersion properties including particle concentration, Fe(0) content, and adsorbed polymer mass and layer thickness which are expected to affect the delivery and emplacement of NZVI in heterogeneous porous media in a two-dimensional (2-D) cell. Heterogeneity in hydraulic conductivity had a significant impact on the deposition of NZVI.

Empirical correlations to estimate agglomerate size and deposition during injection of a polyelectrolyte-modified Fe0 nanoparticle at high particle concentration in saturated sand.

Controlled emplacement of polyelectrolyte-modified nanoscale zerovalent iron (NZVI) particles at high particle concentration (1-10 g/L) is needed for effective in situ subsurface remediation using NZVI. Deep bed filtration theory cannot be used to estimate the transport and deposition of concentrated polyelectrolyte-modified NZVI dispersions (>0.03 g/L) because particles agglomerate during transport which violates a fundamental assumption of the theory.

On integrating groundwater transport models with wireless sensor networks.

The emerging technology of wireless sensor networks (WSNs) is an integrated, distributed, wireless network of sensing devices. It has the potential to monitor dynamic hydrological and environmental processes more effectively than traditional monitoring and data acquisition techniques by providing environmental information at greater spatial and temporal resolutions. Furthermore, due to continuing high-performance computing development, these data may be introduced into increasingly robust and complex numerical models; for instance, the parameters of subsurface transport simulators may be automatically updated.

Particle size distribution, concentration, and magnetic attraction affect transport of polymer-modified Fe(0) nanoparticles in sand columns.

The effect of particle concentration, size distribution (polydispersity) and magnetic attractive forces (Fe(0) content) on agglomeration and transport of poly(styrene sulfonate) (PSS) modified NZVI was studied in water-saturated sand (d(p) = 300 microm) columns. Particle concentrations ranged from 0.03 to 6 g/L in 5 mM NaCl/5 mM NaHCO3 at a pore water velocity of 3.

The significance of heterogeneity on mass flux from DNAPL source zones: an experimental investigation.

Understanding the process of mass transfer from source zones of aquifers contaminated with organic chemicals in the form of dense non-aqueous phase liquids (DNAPL) is of importance in site management and remediation. A series of intermediate-scale tank experiments was conducted to examine the influence of aquifer heterogeneity on DNAPL mass transfer contributing to dissolved mass emission from source zone into groundwater under natural flow before and after remediation. A Tetrachloroethylene (PCE) spill was performed into six source zone models of increasing heterogeneity, and both the spatial distribution of the dissolution behavior and the net effluent mass flux were examined.

Biologically enhanced mass transfer of tetrachloroethene from DNAPL in source zones: experimental evaluation and influence of pool morphology.

High-saturation pools of dense nonaqueous phase liquid (DNAPL) are long-term sources of groundwater contamination at many hazardous-waste sites. DNAPL pools consist of a high saturation zone with slow dissolution overlaid by a transition zone with lower saturations and more rapid dissolution. Effects of biological activity on pool dissolution must be understood to evaluate and implement bioremediation strategies.

Partitioning of dissolved chlorinated ethenes into vegetable oil.

Food-grade soybean oil (SoyOil) has been used to enhance in situ anaerobic bioremediation at sites contaminated with chlorinated ethenes (CEs). The abiotic interactions of SoyOil with the CEs may be significant and need to be better understood. The oil: water partition coefficients (Kp) of dissolved CEs into SoyOil were measured in batch tests and ranged from 22 to 1200 with increasing chlorination.

Effects of heterogeneity and experimental scale on the biodegradation of diesel.

Biodegradation of petroleum hydrocarbon contamination is a common method for remediating soils and groundwater. Due to complexities with field-scale studies, biodegradation rates are typically evaluated at the bench-scale in laboratory studies. However, important field conditions can be difficult to mimic in the laboratory.

The effect of entrapped nonaqueous phase liquids on tracer transport in heterogeneous porous media: laboratory experiments at the intermediate scale.

This work considers the applicability of conservative tracers for detecting high-saturation nonaqueous-phase liquid (NAPL) entrapment in heterogeneous systems. For this purpose, a series of experiments and simulations was performed using a two-dimensional heterogeneous system (10x1.2 m), which represents an intermediate scale between laboratory and field scales.

Convergent-flow tracer tests in heterogeneous media: combined experimental-numerical analysis for determination of equivalent transport parameters.

In modeling transport within naturally heterogeneous aquifers, it is usually assumed that the transport equations valid at local scales can also be applied at larger scales. At larger scales, the heterogeneous domain is represented by an equivalent homogeneous medium. Convergent-flow tracer tests constitute one of the most frequently used field tests to estimate effective input parameters of equivalent homogeneous aquifers.