Brent E. Sleep

Compositional simulation of groundwater contamination by organic compounds: 1. Model development and verification

A compositional simulator is developed for application to the analysis of contamination and remediation of groundwater systems. Simultaneous flow of three fluid phases (water, gas, and organic) is modeled. Interphase partitioning and transport of an arbitrary number of organic and inorganic components can be simulated. Phase densities are functions of pressure and phase composition. The model includes several numerical options, ranging from fully implicit with first-order upstream weighting to implicit in pressure, explicit in saturations and concentration with third-order upstream weighting. The model is verified to the extent possible with analytical solutions for simplified cases of multiphase flow and contaminant transport. The accuracy and efficiency of the various numerical options used in the model are illustrated.

Carbon and Hydrogen Isotopic Fractionation during Anaerobic Biodegradation of Benzene

ABSTRACT Compound-specific isotope analysis has the potential to distinguish physical from biological attenuation processes in the subsurface. In this study, carbon and hydrogen isotopic fractionation effects during biodegradation of benzene under anaerobic conditions with different terminal-electron-accepting processes are reported for the first time. Different enrichment factors (ε) for carbon (range of −1.9 to −3.6‰) and hydrogen (range of −29 to −79‰) fractionation were observed during biodegradation of benzene under nitrate-reducing, sulfate-reducing, and methanogenic conditions. These differences are not related to differences in initial biomass or in rates of biodegradation. Carbon isotopic enrichment factors for anaerobic benzene biodegradation in this study are comparable to those previously published for aerobic benzene biodegradation. In contrast, hydrogen enrichment factors determined for anaerobic benzene biodegradation are significantly larger than those previously published for benzene biodegradation under aerobic conditions. A fundamental difference in the previously proposed initial step of aerobic versus proposed anaerobic biodegradation pathways may account for these differences in hydrogen isotopic fractionation. Potentially, C-H bond breakage in the initial step of the anaerobic benzene biodegradation pathway may account for the large fractionation observed compared to that in aerobic benzene biodegradation. Despite some differences in reported enrichment factors between cultures with different terminal-electron-accepting processes, carbon and hydrogen isotope analysis has the potential to provide direct evidence of anaerobic biodegradation of benzene in the field.

Contrasting carbon isotope fractionation during biodegradation of trichloroethylene and toluene: Implications for intrinsic bioremediation

Abstract In experiments involving anaerobic biodegradation of trichloroethylene (TCE), δ 13 C values for residual TCE changed from −30.4‰ to values more enriched than −16‰. All data exhibit a consistent correlation between δ 13 C value of the residual TCE and the extent of biodegradation of TCE, described by a fractionation factor ( α ) of 0.9929. In contrast, during aerobic biodegradation of toluene by two separate mixed consortia, no change in δ 13 C value of the residual toluene was observed within analytical uncertainty (0.5‰). Stable carbon isotopes have the potential to be a useful indicator for identification and monitoring of intrinsic bioremediation of chlorinated hydrocarbons such as TCE. Conversely, for aromatic hydrocarbons such as toluene, more conservative isotopic values may instead be more applicable as a means of source differentiation at sites with a history of multiple spills.

Compositional simulation of groundwater contamination by organic compounds: 2. Model applications

The flow and transport of organic compounds in variably saturated porous media is investigated using a compositional simulator. The simulator incorporates a number of numerical options to maximize computational efficiency and accuracy. The effect of field scale heterogeneities on the movement of organic compounds is demonstrated. The influence of infiltrating wetting fronts on gas phase transport of volatile organic compounds is shown to be significant. The long-term fate of a subsurface spill of a three-component dense organic liquid is simulated. Three-dimensional simulations of soil vacuum extraction demonstrate the difficulty in removing dissolved organic compounds from the saturated zone with this process.

The effect of temperature on capillary pressure‐saturation relationships for air‐water and perchloroethylene‐water systems

The temperature dependence of capillary pressure-saturation relationships was measured for air-water and perchloroethylene-water systems in silica sand. Changes in capillary pressures, irreducible water phase saturations, and residual nonwetting saturations with temperature were determined. Relationships for temperature dependence of contact angle and interfacial tension were incorporated into the van Genuchten [1980] model and fitted to the data. Capillary pressures at constant degrees of saturation decreased as temperature increased. Hysteresis decreased, irreducible water saturations increased, and residual nonwetting saturations decreased as temperature increased. The magnitude of the change in capillary pressures could not be explained by the temperature dependence of wetting-nonwetting interfacial tensions alone. Derived parameters for the temperature dependence of the contact angle predicted an increase of contact angle of roughly 45°–50° for air-water and perchloroethylene systems with a temperature increase from 20° to 80°C, while literature studies suggest that contact angles should decrease with increasing temperature. It was concluded that the parametric relationship for temperature effects incorporated into the van Genuchten [1980] model fit the data well, but other effects in addition to changes in interfacial tension and contact angle played a role in the temperature dependence of capillary pressure-saturation relationships.

Impact of nZVI stability on mobility in porous media

Nano-scale zero valent iron (nZVI) has received significant attention because of its potential to rapidly reduce a number of priority source zone contaminants. In order to effectively deliver nZVI to the source zone the nZVI particles must be stable. Previous laboratory studies have demonstrated the mobility of polymer modified suspensions of low concentration nZVI. More recently studies have shown potential for higher concentration nZVI suspensions to be transmitted through porous media. However, with increasing nZVI concentration aggregation is accelerated, reducing the available time for injection before nZVI settles. In this study the colloidal stability and mobility of nZVI concurrently synthesized and stabilized in the presence of carboxy-methyl-cellulose (CMC) are evaluated in one-dimensional column experiments. Low pore water velocity nZVI injections (4, 2, and 0.25 m/day) conducted over periods as long as 80 h with no mixing of the influent reservoir were used to investigate the effects of prolonged aggregation and settling of colloids on transport. A numerical simulator, based on colloid filtration theory, but accounting for particle aggregation and settling was used to evaluate the contributions of aggregation and settling on nZVI mobility. Results suggest that the prediction of nZVI sticking efficiency in column experiments becomes increasingly influenced by aggregation and settling in the influent reservoir as the period of injection increases. Consideration of nZVI stability is required for the prediction of nZVI mobility at the field scale and for the design of successful nZVI remediation plans.

Characterization of nZVI Mobility in a Field Scale Test

Nanoscale zerovalent iron (nZVI) particles were injected into a contaminated sandy subsurface area in Sarnia, Ontario. The nZVI was synthesized on site, creating a slurry of 1 g/L nanoparticles using the chemical precipitation method with sodium borohydride (NaBH4) as the reductant in the presence of 0.8% wt. sodium carboxymethylcellulose (CMC) polymer to form a stable suspension. Individual nZVI particles formed during synthesis had a transmission electron microscopy (TEM) quantified particle size of 86.0 nm and dynamic light scattering (DLS) quantified hydrodynamic diameter for the CMC and nZVI of 624.8 nm. The nZVI was delivered to the subsurface via gravity injection. Peak normalized total Fe breakthrough of 71% was observed 1m from the injection well and remained above 50% for the 24 h injection period. Samples collected from a monitoring well 1 m from the injection contained nanoparticles with TEM-measured particle diameter of 80.2 nm and hydrodynamic diameter of 562.9 nm. No morphological changes were discernible between the injected nanoparticles and nanoparticles recovered from the monitoring well. Energy dispersive X-ray spectroscopy (EDS) was used to confirm the elemental composition of the iron nanoparticles sampled from the downstream monitoring well, verifying the successful transport of nZVI particles. This study suggests that CMC stabilized nZVI can be transported at least 1 m to the contaminated source zone at significant Fe(0) concentrations for reaction with target contaminants.

Modeling transient organic vapor transport in porous media with the dusty gas model

The dusty gas model constitutive relationships were incorporated into a numerical model for three-phase, multicomponent flow and transport in porous media. The dusty gas model properly accounts for interactions between all gas-phase species in multicomponent gas mixtures. The model also included Knudsen diffusion, which becomes important in very fine grained soils. The dusty gas model results were compared to predictions based on Ficks law. For the cases studied, Ficks law overpredicted flux rates of organic compounds, the effect becoming more pronounced as the permeability of the soil and the Knudsen coefficient were reduced. Increasing moisture content also appeared to increase the difference between predictions based on the dusty gas model and those based on Ficks law. Hypothetical field-scale simulations were performed to show the impact of multicomponent effects and Knudsen diffusion in a sandy soil and in a clay soil. Results showed that remediation times were significantly underpredicted if Ficks law was used for gas-phase diffusion.

A Field-Validated Model for In Situ Transport of Polymer-Stabilized nZVI and Implications for Subsurface Injection

Nanoscale zerovalent iron (nZVI) particles have significant potential to remediate contaminated source zones. However, the transport of these particles through porous media is not well understood, especially at the field scale. This paper describes the simulation of a field injection of carboxylmethyl cellulose (CMC) stabilized nZVI using a 3D compositional simulator, modified to include colloidal filtration theory (CFT). The model includes composition dependent viscosity and spatially and temporally variable velocity, appropriate for the simulation of push-pull tests (PPTs) with CMC stabilized nZVI. Using only attachment efficiency as a fitting parameter, model results were in good agreement with field observations when spatially variable viscosity effects on collision efficiency were included in the transport modeling. This implies that CFT-modified transport equations can be used to simulate stabilized nZVI field transport. Model results show that an increase in solution viscosity, resulting from injection of CMC stabilized nZVI suspension, affects nZVI mobility by decreasing attachment as well as changing the hydraulics of the system. This effect is especially noticeable with intermittent pumping during PPTs. Results from this study suggest that careful consideration of nZVI suspension formulation is important for optimal delivery of nZVI which can be facilitated with the use of a compositional simulator.

Effects of biofilm growth on flow and transport through a glass parallel plate fracture.

The effects of biofilm growth on flow and solute transport through a sandblasted glass parallel plate fracture was investigated. The fracture was inoculated using soil microorganisms. Glucose, oxygen and other nutrients were supplied to support growth. The biomass initially formed discrete clusters attached to the glass surfaces, but over time formed a continuous biofilm. From dye tracer tests conducted during biofilm growth, it was observed that channels and low-permeability zones dominated transport. The hydraulic conductivity of the fracture showed a sigmoidal decrease with time. The hydraulic conductivity was reduced by a factor of 0.033, from 18 to 0.6 cm/s, corresponding to a 72% decrease in the hydraulic aperture, from 500 to 140 microm. In contrast, the mass balance aperture, determined from fluoride tracer tests, remained relatively constant, indicating that the impact of biomass growth on effective fracture porosity was much less than the effect on hydraulic conductivity. Analyses of pre-biofilm tracer tests revealed that both Taylor dispersion and macrodispersion were influencing transport. During biofilm growth, only macrodispersion was dominant. The macrodispersion coefficient alpha(macro) was found to increase logarithmically with hydraulic conductivity reduction.