Force analysis and visualization of NAPL removal during surfactant-related floods in a porous medium.

Governing mechanisms of dense non-aqueous phase liquid (DNAPL) removal during surfactant and surfactant-foam (SF) flooding were studied by porous-patterned glass model experiments. Physical forces, viscous forces and capillary forces, acting on trichloroethylene (TCE) blobs were quantified to understand DNAPL removal mechanisms during the floods, simultaneously visualizing the removal mechanisms. The viscous force of the remedial fluid was intimately related to TCE removal from the porous medium. The remedial fluid with a high viscous force displaced more TCE blobs. Displacement of residual TCE by the remedial fluid began as viscous pressure of flooding was closed to the capillary pressure of the porous medium. In the region of viscous pressure less than the capillary pressure, residual TCE was either retained or solubilized, not displaced, implying that TCE solubilization was the dominant TCE removal process. Glass porous model visualization validated a dominance of the capillary forces during a surfactant flush and a dominance of the viscous forces of the displacing fluid during a SF flood.

Plant aided bioremediation in the vadose zone: model development and applications.

Phytoremediation has the potential to enhance clean up of land contaminated by various pollutants. A mathematical model that includes a two-fluid phase flow model of water flow as well as a two-region soil model of contaminant reactions was developed and applied to various bioremediation scenarios in the unsaturated zone, especially to plant-aided bioremediation. To investigate model behavior and determine the main parameters and mechanisms that affect bioremediation in unplanted and planted soils, numerical simulations of theoretical scenarios were conducted before applying the model to field data. It is observed from the results that parameters affecting the contaminant concentration in the water phase, such as aqueous solubility, the octanol-water partition coefficient, and organic carbon content of the soil controlled the contaminant fate in the vadose zone. Simulation using the developed model also characterized the fate and transport of the contaminants both in planted and unplanted soils satisfactorily for field applications. Although phytoremediation has the potential for remediation of contaminated soils, results from both modeling and field studies suggested that plants may not always enhance the remediation efficiency when the soil already has a high microbial concentration, when the contaminant bioavailability is low, or when the overall reaction is mass transfer-limited. Therefore, other steps to increase contaminant bioavailability are needed in phytoremediation applications; natural purification mechanisms such as aging, volatilization, and natural bioremediation should be considered to maximize the plant effect and minimize the cost.

Solute transport and extraction by a single root in unsaturated soils: model development and experiment.

A contaminant transport model was developed to simulate the fate and transport of organic compounds such as TNT (2,4,6-trinitrotoluene), using the single-root system. Onions were planted for this system with 50-ml plastic tubes. Mass in the soil, soil solution, root and leaf was monitored using 14C-TNT. Model parameters were acquired from the experiments in the single-root system and were used to simulate total TNT concentration in soil, providing the average concentrations in the rhizosphere and bulk soil as well as root and leaf compartments. Because the existing RCF (root concentration factor) and TSCF (transpiration stream concentration factor) equations based on logKow (octanol-water partition coefficient) were not correlated to TNT uptake, a new term, root uptake rate (Rur), and a new Tscf equation, based on the experimental data, were introduced in the proposed model. The results from both modeling and experimental studies showed higher concentrations of TNT in the rhizosphere than in the bulk soil, because mass transported from the surrounding soil into the rhizosphere was higher than that by root uptake.

Uptake and phytotoxicity of TNT in onion plant.

The uptake of 14C-2,4,6-trinitrotoluene (TNT) in hydroponics was studied using onion plants. Of the total TNT mass (5 microM concentration), 75% was in the roots, 4.4% in the leaves, and 21% in the external solution at 2 days. The percent distribution in roots was lower with higher concentration in the external solution, but in leaves it was comparable at all concentrations (5-500 microM). Root concentration factor (RCF) in hydroponics was more than 85 in constant hydroponic experiment (CHE) at 5 microM and 150 in nonconstant hydroponic experiment (NHE) at 5 microM. The maximum RCF values in the hydroponic system were greater with lower solution concentration. Transpiration stream concentration factor (TSCF) values in the present study (NHE only: 0.31-0.56) were relatively similar to the values with predicted values (0.43-0.78), increasing with higher external TNT concentration. All predicted values for RCF and TSCF were derived in the literature from equations using logKow (log10 octanol-water partition coefficient). For phytotoxicity tested in hydroponics and wet paper method, 500 microM was toxic to onion plant, 50 microM was nontoxic for plant growth but limited the transpiration rate, and 5 microM was nontoxic as control.

Modeling dissolution and volatilization of LNAPL sources migrating on the groundwater table.

A vertically averaged two-dimensional model was developed to describe areal spreading and migration of light nonaqueous-phase liquids (LNAPLs) introduced into the subsurface by spills or leaks from underground storage tanks. The NAPL transport model was coupled with two-dimensional contaminant transport models to predict contamination of soil gas and groundwater resulting from a LNAPL migrating on the water table. Numerical solutions were obtained by using the finite-difference method. Simulations and sensitivity analyses were conducted with a LNAPL of pure benzene to study LNAPL migration and groundwater contamination. The model was applied to subsurface contamination by jet fuel. Results indicated that LNAPL migration were affected mostly by volatilization. The generation and movement of the dissolved plume was affected by the geology of the site and the free-product plume. Most of the spilled mass remained as a free LNAPL phase 20 years after the spill. The migration of LNAPL for such a long period resulted in the contamination of both groundwater and a large volume of soil.

The use of vegetation to remediate soil freshly contaminated by recalcitrant contaminants.

The use of vegetation to remediate soil contaminated by recalcitrant hydrocarbons was tested under field conditions. Specifically, an evaluation was made of the effectiveness of deep rooting grasses, Johnsongrass and Canadian wild rye in the dissipation of TNT and PBB's in the soils freshly contaminated to an initial concentration of 10.17+/-1.35 for TNT and 9.87+/-1.23 mg/kg for PBB. The experiment used 72 (1.5m long and 0.1m diameter) column lysimeters with four treatments: Johnsongrass; wild rye grass; a rotation of Johnsongrass and wild rye grass; and unplanted fallow conditions. In the laboratory, immunoassay test procedures determined the TNT and PBB concentrations in the soil, leachate, herbage and root samples. The root characteristics such as total root length, rooting density, and root surface area were quantified to a depth of 1.5m. Changes in microbial biomass were assessed for both rhizosphere soil and the bulk soil during the 2-year study. The largest and most rapid loss in soil chemical concentration was for TNT, which decreased to less than 250 microg/kg, the detection limit, by 93 days after germination. The PBB was at or near the detection limit of 500 microg/kg by 185 days after germination. There was no perceptible difference in contaminant concentration in the soil between the vegetation treatments and/or with depth.

A micromodel analysis of factors influencing NAPL removal by surfactant foam flooding.

A methodology to study the trichloroethylene (TCE) and dodecane removal in porous media by surfactant foams (SF) was presented by using etched-glass micromodels. The purpose of this work was to systematically evaluate the impact of various physicochemical factors such as gas fraction (GF), surfactant concentration, pore structure and nonaqueous phase liquid (NAPL) types on NAPL removal during SF flooding. The TCE displacement by SF was dependent on the gas fraction of SF. Low GFs (50% and 66%) were more efficient for TCE removal and sweep efficiencies than a high GF (85%). An increase in TCE removal was observed with increasing surfactant concentration at a fixed GF. TCE removal by SF flooding appeared to be dependent more to the value of Capillary number rather than to the concentration of surfactant solution. The effect of the pore heterogeneity was evaluated by employing two different types of micromodels. The Capillary number is an important parameter in the determination of sweep efficiency or gas saturation of SF in a nonhomogeneous porous medium. However, the TCE removal from a nonhomogeneous porous medium may not be associated with sweep efficiency. The initial configuration of residual TCE blobs in a nonhomogeneous porous medium would also be influential in displacing TCE. Sweep efficiencies and pressure responses of two NAPL systems (TCE and dodecane) were monitored to evaluate foam stability when the foam contacts the NAPLs. Stable foam contacting with TCE is implied, while it appears that dodecane cause the SF to collapse. All results indicate that the Capillary number (a ratio of viscous forces to capillary forces) is the most important parameter for TCE removal by SF flooding. Micromodel visualizations of water, surfactant and SF floods were showed and also discussed.

Contaminant transport in dual-porosity media with dissolved organic matter and bacteria present as mobile colloids.

In riverbank filtration, contaminant transport is affected by colloidal particles such as dissolved organic matter (DOM) and bacterial particles. In addition, the subsurface heterogeneity influences the behavior of contaminant transport in riverbank filtration. A mathematical model is developed to describe the contaminant transport in dual-porosity media in the presence of DOM and bacteria as mobile colloids. In the model development, a porous medium is divided into the mobile and immobile regions to consider the presence of ineffective micropores in physically heterogeneous riverbanks. We assume that the contaminant transport in the mobile region is controlled by the advection and dispersion while the contaminant transport in the immobile region occurs due to the molecular diffusion. The contaminant transfer between the mobile and immobile regions takes place by diffusive mass transfer. The mobile region is conceptualized as a four-phase system: two mobile colloidal phases, an aqueous phase, and a solid matrix. The complete set of governing equations is solved numerically with a fully implicit finite difference method. The model results show that in riverbank filtration, the contaminant can migrate further than expected due to the presence of DOM and bacteria. In addition, the contaminant mobility increases further in the presence of the immobile region in aquifers. A sensitivity analysis shows that in dual-porosity media, earlier breakthrough of the contaminant takes place as the volumetric fraction of the mobile region decreases. It is also demonstrated that as the contaminant mass transfer rate coefficient between the mobile and immobile regions increases, the contaminant concentration gradient between the two regions reverses at earlier pore volumes. The contaminant mass transfer coefficient between the mobile and immobile regions mainly controls the tailing effect of the contaminant breakthrough. The contaminant breakthrough curves are sensitive to changes in contaminant adsorption and desorption rate coefficients on DOM and bacteria. In situations where the contaminant is released in the presence of DOM and bacteria in dual-porosity media, the early breakthrough and tailing occur due to the colloidal facilitation and presence of immobile regions.

Gel barrier formation in unsaturated porous media.

The gel barrier formation by a gelling liquid (Colloidal Silica) injection in an unsaturated porous medium is investigated by developing a mathematical model and conducting numerical simulations. Gelation process is initiated by adding electrolytes such as NaCl, and the gel phase consisting of cross-linked colloidal silica particles grows as the gelation process proceeds. The mathematical model describing the transport and gelation of Colloidal Silica (CS) is based on coupled mass balance equations for the gel mixture (the sol phase plus the gel phase), gel phase (cross-linked colloidal silica particles plus water captured between cross-linked particles), and colloidal silica particles (discrete and cross-linked) and NaCl in the sol (suspension of discrete colloidal silica particles in water) and gel phases. The solutions in terms of volumetric fraction of the gel phase yield the gel mixture viscosity via the dependency on the volumetric fraction of gel phase. This dependency is determined from a kinetic gelation model with time-normalized viscosity curves. The proposed model is verified by comparing experimentally and numerically determined hydraulic conductivities of gel-treated soil columns at different CS injection volumes. The numerical experiments indicate that an impermeable gel layer is formed within the time period twice the gel-point in a one-dimensional flow system. At the same normalized time corresponding to twice the gel-point, the CS solutions with lower NaCl concentrations result in further migration and poor performance in plugging the pore space. The viscosity computation proposed in this study is compared with another method available in the literature. It is observed that the other method estimates the viscosity at the mixing zone higher than the one proposed by the authors. The proposed model can simulate realistic injection scenarios with various combinations of operating parameters such as NaCl concentration and NaCl mixing time, and thus providing guidelines in performing this technology on site.