Assessing Multicomponent DNAPL Biostabilization.I: Coal Tar

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 first paper of this set (Part I). Unmixed coal tar biometers, characterized by slow mass transfer and low-level microbial activity, exhibited reduced, aqueous-phase contaminant concentrations and aggregate toxicity, as well as stable DNAPL composition, consistently indicating favorable potential for in situ biostabilization.     

Modeling Long-Term Transport of Contaminants Resulting from Dissolution of a Coal Tar Pool in Saturated Porous Media

In this study, a three-dimensional mathematical model for simulating the transport of dissolved contaminants originating from an elliptic-shaped coal tar pool in homogeneous saturated porous media is developed. The methodology of the solution is based on a semianalytical approach that assumes the contaminant source input can be represented by the superposition of a series of consecutive short dissolution pulses. The model accounts for possible solidification of polycyclic aromatic hydrocarbon compounds as the coal tar pool dissolves. A circular-shaped synthetic coal tar pool consisting of 22 components is used for model simulation. Simulations show that even after 130 years of dissolution, the coal tar pool remains a potential groundwater pollution threat despite a significant reduction in overall pool volume.     

Factors Affecting Effectiveness and Efficiency of DNAPL Destruction Using Potassium Permanganate and Catalyzed Hydrogen Peroxide

This paper describes laboratory studies conducted to evaluate the impact of varying environmental conditions (dense nonaqueous phase liquid (DNAPL) type and mass, and properties of the subsurface porous media) and design features (oxidant type and load) on the effectiveness and efficiency of in situ chemical oxidation (ISCO) for destruction of DNAPL contaminants. Porous media in 160?mL zero-headspace reactors were employed to examine the destruction of trichloroethylene and perchloroethylene by the oxidants potassium permanganate and catalyzed hydrogen peroxide. Measures of oxidation effectiveness and efficiency include (1) media demand (mg-oxidant/kg-porous media), (2) oxidant demand (mol-oxidant/mol-DNAPL), (3) reaction rate constants for oxidant and DNAPL depletion (min?1), (4) the percent (%) DNAPL destroyed, and (5) the relative treatment efficiency, i.e., the rate of oxidant depletion versus rate of DNAPL destruction. While an obvious goal of ISCO for DNAPL treatment is high effectiveness (i.e., extensive contaminant destruction), it is also important to focus on oxidation efficiency, or to what extent the oxidant is utilized for contaminant destruction instead of competing side reactions, for improved cost effectiveness and/or treatment times. Results indicate that DNAPL contaminants can be treated both effectively and efficiently under many environmental and design conditions. In some cases, DNAPL treatment was more effective and efficient than dissolved/sorbed phase treatment. In these experiments, permanganate was a more effective oxidant, however catalyzed hydrogen peroxide treated contaminants more efficiently (e.g., less oxidant required per mass contaminant treated). Results also indicate that oxidation treatment goals can be dictated by environmental conditions, and that specific treatment goals can dictate remediation design parameters (e.g., faster contaminant destruction was realized in catalyzed hydrogen peroxide systems, whereas greater contaminant destruction occurred in permanganate systems).     

Assessing Multicomponent DNAPL Biostabilization Potential.?II: Aroclor 1242

Aroclors are dense nonaqueous-phase liquids (DNAPLs) composed of polychlorinated biphenyls, which are common subsurface contaminants. Because complete remediation of Aroclor is very difficult, biostabilization may offer an alternative where risk reduction can be achieved without destruction of the DNAPL mass. The potential for aerobic in situ biostabilization of Aroclor 1242 was evaluated using laboratory protocols similar to those described in the companion paper. Total microbial concentrations increased and stabilized in both mixed and unmixed systems, while the respiring cells did not stabilize in either system. After 100 days, the DNAPL in mixed biometers was depleted in dichlorobiphenyls; the DNAPL composition in unmixed biometers did not change significantly. The total aqueous polychlorinated biphenyl concentration was lower in the unmixed than mixed biometers; both were below the predicted equilibrium concentration. After 100 days, the chronic toxicity of the aqueous phase to Cerodaphnia was greater in the biotreated systems than in the unbiotreated systems. The results indicate that aerobic microbiological activity may be insufficient to fully stabilize Aroclor in the subsurface, in contrast to the clear biostabilization potential of coal tar.     

Influence of Soil Texture on Rate-Limited Micellar Solubilization

Observations from 1D soil column experiments are used to explore the factors influencing surfactant-enhanced solubilization of entrapped non-aqueous-phase liquids in sandy porous media. These experiments were designed to quantify the influence of porous medium properties and flow system parameters on the rate of contaminant removal. A 1% flushing solution of polyoxyethylene sorbitan monooleate (Witconol 2722) was employed to recover residual phase decane from a range of Ottawa sands. Deviations from local equilibrium conditions were observed in all experiments. For solution phase Darcy velocities ranging from 0.83 to 8.3 cm∕h, maximum column effluent concentration levels were consistently <55% of the batch-measured equilibrium value. Column data were used to develop an empirical modified Sherwood correlation for the prediction of initial, pseudo steady-state, solubilization rates assuming a linear driving force mass transfer expression. This correlation incorporates the Reynolds number, the mean grain size, and the sand uniformity index. The adequacy of this correlation for the prediction of initial steady-state solubilization rates in other sandy media is demonstrated for a natural aquifer material. Results of this study suggest a similar dependence of mass transfer rates on system hydrodynamics and soil properties for both micellar solubilization and dissolution.     

Effects of Groundwater Velocity and Permanganate Concentration on DNAPL Mass Depletion Rates during In Situ Oxidation

In situ chemical oxidation (ISCO) using permanganate has been increasingly applied to deplete mass from dense nonaqueous-phase liquid (DNAPL) source zones. However, uncertainty in the performance of ISCO on DNAPL contaminants is partially attributable to a limited understanding of interactions between the oxidant, subsurface hydrology, and DNAPL mass transfer, resulting in failure to optimize ISCO applications. To investigate these interactions, a factorial design experiment was conducted using one-dimensional flow through tube reactors to determine how groundwater velocity, permanganate concentration, and DNAPL type affected DNAPL mass depletion rates. DNAPL mass depletion rates were found to increase with increasing groundwater velocity, or increasing oxidant concentration. An interaction occurred between the two factors, where high oxidant concentrations had little impact on mass depletion rates at high velocities. High oxidant concentration systems experienced gas generation. Mass depletion rates were fastest at high velocities, but required additional oxidant mass and pore volume addition to achieve complete mass depletion. Lower-velocity systems were more efficient with respect to oxidant mass and pore volume requirements, but mass depletion rates were reduced.     

Destruction of a Carbon Tetrachloride Dense Nonaqueous Phase Liquid by Modified Fenton’s Reagent

Destruction of a dense nonaqueous phase liquid (DNAPL) by soluble iron (III)-catalyzed and pyrolusite (β-MnO2)-catalyzed Fenton’s reactions (hydrogen peroxide and transition metal catalysts) was investigated using carbon tetrachloride (CT) as a model contaminant. In the system amended with 5 mM soluble iron (III), 24% of the CT DNAPL was destroyed after 3 h while CT dissolution in parallel fill-and-draw systems was minimal, indicating that CT was degraded more rapidly than it dissolved into the aqueous phase. Fenton’s reactions catalyzed by the naturally occurring manganese oxide pyrolusite were even more effective in destroying CT DNAPLs, with 53% degradation after 3 h. Although Fenton’s reactions are characterized by hydroxyl radical generation, carbon tetrachloride is unreactive with hydroxyl radicals; therefore, a transient oxygen species other than hydroxyl radicals formed through Fenton’s propagation reactions was likely responsible for CT destruction. These results demonstrate that Fenton-like reactions in which nonhydroxyl radical species are generated may provide an effective method for the in situ treatment of DNAPLs.     

Effectiveness of Source-Zone Remediation of DNAPL-Contaminated Subsurface Systems

Concerted efforts to remediate subsurface systems contaminated with dense nonaqueous-phase liquids (DNAPLs) have met with limited success when measured by comparing solute concentrations to drinking water quality standards. One-dimensional and three-dimensional laboratory experiments and a field-scale experiment are used to investigate the effectiveness of source-zone remediation and to assess factors that contribute to the observed results. The three-dimensional laboratory experiment and the field-scale experiment used a surfactant flush followed by vapor extraction to reduce the DNAPL saturation, while vertical DNAPL mobilization was controlled using a brine barrier. DNAPL mobilization and recovery in the field-scale experiment was relatively ineffective due in part to the low saturation levels of the DNAPL. The results show essentially that complete removal of a DNAPL is required to reach typical cleanup standards and that details of the morphology and topology of a DNAPL distribution, in addition to the saturation, play an important role in determining the rate of mass transfer. The results are interpreted in terms of guidance for remediation approaches, realistic expectations for source-zone remediation, and elements needed for improved models of such systems.     

Managing Nitrate Problems for Domestic Wells in Irrigated Alluvial Aquifers

Many irrigated areas in the United States and abroad overlie unconfined aquifers. The soils are coarse textured with low water-holding capacity, and irrigation water is frequently necessary for crop growth. Many of these areas experience elevated nitrate levels in water from shallow domestic wells. It is often observed that the nitrate plumes are stratified with depth with the highest concentration just near the surface. The irrigation wells are generally screened toward the bottom part of the aquifer where the material is coarse and the available drawdown is greater. Pumpage is typically cyclic but can be somewhat continuous during drought conditions. It is often believed that the floating nitrate plume is recycled by the irrigation wells. Simulations were carried out to show that slight deepening of domestic wells in both stratified and more homogeneous aquifers could eliminate nitrate problems for many domestic wells even in close proximity of irrigation wells.     

Nonaqueous Phase Liquid Pool Dissolution as a Function of Average Pore Water Velocity

Bench-scale reactor experiments were performed to study the dissolution of a binary naphthalene-in-nonane mixture nonaqueous phase liquid (NAPL) pool over a wide range of average pore water velocities, vx (≈0.1–60 m/day). Experimental NAPL pool dissolution flux values were determined using a steady-state mass balance approach. The experimental flux data were compared to model predictions made assuming either local equilibrium or mass-transfer limited conditions. The local equilibrium model could describe the trends in the average effluent concentration and dissolution flux with 0.110?m/day. Data determined to be under mass-transfer limited conditions were fit to the nonequilibrium model to estimate values for an overall mass-transfer coefficient. The calculated overall mass-transfer coefficients had an average value of 0.407 m/day and showed no correlation with vx, probably due to mass-transfer resistance becoming dominated by the diffusional resistance in the NAPL. These results suggest that the nonequilibrium approach is better suited for describing high velocity (vx>10?m/day) dissolution of multicomponent NAPL pools, and that flushing of groundwater at very high velocities may not be an effective approach for enhancing NAPL-pool dissolution flux.     

Technologies for in situ cleanup of contaminated sites

Groundwater contamination by non-aqueous phase liquids (NAPLs) and denser than water non-aqueous phase liquids (DNAPLs) poses one of the greatest remedial challenges in the field of environmental engineering. Due to low water solubilities and aqueous diffusivities, conventional pump-and-treat technologies have a poor record of success in remediation of DNAPL contaminated aquifers. Better success has been found with the removal of volatile LNAPLs due to higher gaseous diffusivities, propensity for aerobic biodegradation, and ease of pumping and handling large quantities of gas. An evaluation of in situ cleanup technologies on the basis of their applicability to in situ treatment of NAPL contaminated aquifers is presented. Emphasis is placed on treatment of the separate phase occurring in the saturated zone. Soil washing, air sparging, biodegradation, electro-osmosis, enhanced steam extraction, stabilization/solidification, treatment walls, radio frequency heating, and containment systems and barriers are among the in situ technologies reviewed. In the context of the governing contaminant fate and transport processes, the relative merits of each technology are assessed on the basis of its theoretical background, field implementability, level of demonstration and performance, waste, technical and site applicability/limitations, commercial availability, and cost and residuals management.     

Hydrodynamic Changes in Sand due to Biogrowth on Naphthalene and Decane

Biological activity in zones of chemical contamination changes the pore characteristics that control the flow of water and transport of dissolved chemicals in soils. To further the understanding of these processes, column experiments were performed to evaluate the effect of biomass growth on decane or naphthalene dissolved in simulated groundwater on the hydraulic conductivity and dispersivity of sand. The effect of grain size, groundwater flowrate, and nitrogen limitation were investigated. Given the low carbon loading resulting from the solubility of decane and naphthalene, sparse and discontinuous biomass growth reduced the hydraulic conductivity of the sand by 2 to 3 orders of magnitude after 35 to 63 days. This biogrowth initially increased dispersivity of the sand, but after longer periods of growth dispersivity, decreased to stable values near that of the clean sand. The results indicate that biogrowth can have significant effects in natural systems with low carbon loading and nitrogen availability, and should be taken into account when using models to predict contaminant transport in the field.     

Solubility and activity of oxygen in liquid germanium and germanium-copper alloys

The solubility of oxygen in liquid germanium in the temperature range 1233 to 1397 K, and in liquid germanium-copper alloys at 1373 K, in equilibrium with GeO₂ has been measured by the phase equilibration technique. The solubility of oxygen in pure germanium is given by the relation log(at. pct 0)=-6470/T + 4.24 (±0.07). The standard free energy of solution of oxygen in liquid germanium is calculated from the saturation solubility, and recently measured values for the free energy of formation of GeO₂, assuming that oxygen obeys Sievert’s law up to the saturation limit. For the reaction, 1/2 O₂(g)→O_Ge ΔG° = -39,000 + 3.21T (±500) cal = -163,200 + 13.43T (±2100) J. where the standard state for dissolved oxygen is that which makes the value of activity equal to the concentration (in at. pct), in the limit, as concentration approaches zero. The effect of copper on the activity of oxygen dissolved in liquid germanium is found to be in good agreement with that predicted by a quasichemical model in which each oxygen was assumed to be bonded to four metal atoms and the nearest neighbor metal atoms to an oxygen atom are assumed to lose approximately half of their metallic bonds. On leave at the Department of Metallurgy and Materials Science, University of Toronto, when this investigation was undertaken.     

Solubility and activity of oxygen in liquid germanium and germanium-copper alloys

The solubility of oxygen in liquid germanium in the temperature range 1233 to 1397 K, and in liquid germanium-copper alloys at 1373 K, in equilibrium with GeO2 has been measured by the phase equilibration technique. The solubility of oxygen in pure germanium is given by the relation R470 log(at. pct 0)=-6470/T+4.24 (±0.07). The standard free energy of solution of oxygen in liquid germanium is calculated from the saturation solubility, and recently measured values for the free energy of formation of GeO₂, assuming that oxygen obeys Sievert’s law up to the saturation limit. For the reaction, 1/2 O₂(g)→ OGe ΔG° =-39,000 + 3.21T (±500) cal = -163,200 + 13.43T (±2100) J. where the standard state for dissolved oxygen is that which makes the value of activity equal to the concentration (in at. pct), in the limit, as concentration approaches zero. The effect of copper on the activity of oxygen dissolved in liquid germanium is found to be in good agreement with that predicted by a quasichemical model in which each oxygen was assumed to be bonded to four metal atoms and the nearest neighbor metal atoms to an oxygen atom are assumed to lose approximately half of their metallic bonds. K. FITZNER was on leave at the Department of Metallurgy and Materials Science, University of Toronto, when this investigation was undertaken.     

Mathematical modelling of the release of drug from porous, nonswelling transdermal drug-delivery devices

A general model is presented for the release of drug from porous nonswelling, transdermal drug-delivery devices and it is shown to reduce to previously proposed models in suitable limits. The processes which govern the release of drug are considered to be diffusion of dissolved drug and dissolution of dispersed drug, both in the body of the device and in the device pores, and transfer of drug between the two domains. In the classical limit of large dissolution rates, the problem reduces to one of the moving-boundary type, and solution of this problem in the case where the initial drug loading is much greater than the drug solubility in the device yields expressions for the flux of drug to a perfect sink (modelling in vitro conditions). It is shown that behaviour greatly differing from the classical first-order drug delivery (alpha t 1/2) may be exhibited, depending upon the parameter regime. In some situations the dissolution rates may not be so large and solutions of the general model are derived in the case where the dispersed drug is considered to be undepleted and the diffusivity in the solvent-filled pores is much larger than in the body of the delivery device. Numerical studies are undertaken, and the coupling of delivery device and skin-diffusion models (in order to model the complete transdermal drug-delivery process) is also considered.     

Equilibrium Modeling of (Nb,Ti, V)(C,N) Precipitation in Austenite of Microalloyed Steels

A theoretical model was proposed to predict complex equilibrium precipitation in Nb–Ti–V bearing microalloyed steel. It is assumed that the complex precipitate with B1 type consists of six kinds of binary compound, namely, NbC, NbN, TiC, TiN, VC, and VN. The composition of multicomponent precipitates and solute concentration in the matrix can be computed by the developed model which is based on the solubility product. The equilibrium volume fraction of the second phase particles at a given temperature can also be predicted easily. Comparison with other approaches in previous research was made. Good match demonstrated the model's validation and reliability.     

On homogenization of a binary alloy after dissolution of planar and spherical precipitates

In the framework of a diffusion-limited model, the effect of homogenization treatment following precipitate dissolution for a spherical and a planar geometry was considered. It was shown that, for a planar precipitate, the time for complete homogenization is considerably larger than the time required for the precipitate dissolution by ∼two orders of magnitude. For a spherical geometry, it was found that the concentration in the center of the dissolved precipitate at the instant of its dissolution decreases to a value lower than the equilibrium solubility value at the matrix/precipitate interface. The model is applicable for spherical particles larger thatn ∼5 to 500 nm, when the influence of the particle curvature can be neglected.     

Breakthrough Curves and Simulation of Virus Transport through Fractured Porous Media

In this paper, an advective dispersive virus transport equation, including first-order adsorption and an inactivation constant, is used for simulating the movement of viruses in fractured porous media. The implicit finite-difference numerical technique is used to solve the governing equations for viruses in the fractured porous media. In this work, the focus is (1)?to investigate the transport processes of the movement of viruses in both fractured rock and porous rock without fracture and (2)?to simulate the experimental data of biocolloids through a fractured aquifer model. It is seen that movement of the contaminant is faster in the fractured rock than in the porous rock formation. Higher values of diffusion coefficient, matrix porosity, mass transfer constant, and inactivation rate reduce both temporal and spatial virus concentrations in the fracture. Also, experimental data of biocolloids in the fractured aquifer model with constant and time-dependent inactivation rates were simulated successfully.     

Improved Compartmental Modeling and Application to Three-Phase Contaminant Transport in Unsaturated Porous Media

The compartmental modeling approach has been widely used for simulating contaminant transport in porous media and surface waters. Yet a commonly used compartmental model that has only first-order accuracy may introduce considerable numerical errors under certain circumstances. Following a review of compartmental systems and compartmental modeling methodologies, performance and limitations of such a compartmental model are discussed. In particular, improvement approaches, including multipoint, high-order, linear, and nonlinear methods, are presented in detail. Finally, a number of testing problems are examined and various compartmental models that describe three-phase (dissolved, adsorbed, and vapor phases) contaminant transport in unsaturated porous media are compared with each other and also with standard numerical and analytical counterparts. The comparisons highlight the accuracy, applicability, and limitations of different compartmental models.     

Nitrate Dynamics under Cyclic Irrigation Pumpage

Large-scale nitrate contamination of ground water is often observed in irrigated areas where the soils are generally permeable, and nitrate plumes have been reported to occur at shallow depths in unconfined aquifers. It has been hypothesized that these nitrate plumes can be captured by the pumping action of irrigation wells and recycled at the field site. This paper presents results from a field and modeling study investigating the distribution of nitrate in an unconfined sand aquifer in an irrigated system. Results from tracer tests under both natural and forced gradients indicate that irrigation pumping has minor but measurable effects on solute transport. Hydrological and solute transport modeling using various pumping schedules suggests that, in most cases, the influence of irrigation pumping on solute transport will be minimal, even on local scales.