John A. Christ

Coupling Aggressive Mass Removal with Microbial Reductive Dechlorination for Remediation of DNAPL Source Zones: A Review and Assessment

The infiltration of dense non-aqueous-phase liquids (DNAPLs) into the saturated subsurface typically produces a highly contaminated zone that serves as a long-term source of dissolved-phase groundwater contamination. Applications of aggressive physical–chemical technologies to such source zones may remove > 90% of the contaminant mass under favorable conditions. The remaining contaminant mass, however, can create a rebounding of aqueous-phase concentrations within the treated zone. Stimulation of microbial reductive dechlorination within the source zone after aggressive mass removal has recently been proposed as a promising staged-treatment remediation technology for transforming the remaining contaminant mass. This article reviews available laboratory and field evidence that supports the development of a treatment strategy that combines aggressive source-zone removal technologies with subsequent promotion of sustained microbial reductive dechlorination. Physical–chemical source-zone treatment technologies compatible with posttreatment stimulation of microbial activity are identified, and studies examining the requirements and controls (i.e., limits) of reductive dechlorination of chlorinated ethenes are investigated. Illustrative calculations are presented to explore the potential effects of source-zone management alternatives. Results suggest that, for the favorable conditions assumed in these calculations (i.e., statistical homogeneity of aquifer properties, known source-zone DNAPL distribution, and successful bioenhancement in the source zone), source longevity may be reduced by as much as an order of magnitude when physical–chemical source-zone treatment is coupled with reductive dechlorination.

Predicting DNAPL mass discharge from pool-dominated source zones.

Models that link simplified descriptions of dense non-aqueous phase liquid (DNAPL) source zone architecture with predictions of mass flux can be effective screening tools for evaluation of source zone management strategies. Recent efforts have focused on the development and implementation of upscaled models to approximate the relationship between mass removal and flux-averaged, down-gradient contaminant concentration (or mass flux) reduction. The efficacy of these methods has been demonstrated for ganglia-dominated source zones. This work extends these methods to source zones dominated by high-saturation DNAPL pools. An existing upscaled mass transfer model was modified to reproduce dissolution behavior in pool-dominated scenarios by employing a two-domain (ganglia and pools) representation of the source zone. The two-domain upscaled model is parameterized using the initial fraction of the source zone that exists as pool regions, the initial fraction of contaminant eluting from these pool regions, and the flux-averaged down-gradient contaminant concentration. Comparisons of model predictions with a series of three-dimensional source zone numerical simulations and data from two-dimensional aquifer cell experiments demonstrate the ability of the model to predict DNAPL dissolution from ganglia- and pool-dominated source zones for all levels of mass recovery.

Development and application of an analytical model to aid design and implementation of in situ remediation technologies

Innovative in situ remediation technologies using injection/extraction well pairs which recirculate partially treated groundwater as a method of increasing overall treatment efficiency have recently been demonstrated. An important parameter in determining overall treatment efficiency is the fraction of flow that recycles between injection and extraction points. Numerical and semi-analytical methods are currently available to determine this parameter and an analytical solution has been presented for a single injection/extraction well pair. In this work, the analytical solution for fraction of recycled flow for a single injection/extraction well pair is extended to multiple co-linear well pairs. In addition, a semi-analytical method is presented which permits direct calculation of fraction recycle for a system of arbitrarily placed injection/extraction wells pumping at different rates. The solution is used to investigate the behavior of multiple well pairs, looking at how well placement and pumping rates impact capture zone width and fraction recirculation (and, therefore, overall treatment efficiency). Results of the two-well solution are compared with data obtained at a recent field-scale demonstration of in situ aerobic cometabolic bioremediation, where a pair of dual-screened injection/extraction wells was evaluated. A numerical model is then used to evaluate the impact of hydraulic conductivity anisotropy on the overall treatment efficiency of the dual-screened injection/extraction well pair. The method presented here provides a fast and accurate technique for determining the efficacy of injection/extraction systems, and represents a tool that can be useful when designing such in situ treatment systems.

Hydraulic containment: analytical and semi-analytical models for capture zone curve delineation

We present an efficient semi-analytical algorithm that uses complex potential theory and superposition to delineate the capture zone curves of extraction wells. This algorithm is more flexible than previously published techniques and allows the user to determine the capture zone for a number of arbitrarily positioned extraction wells pumping at different rates. The algorithm is applied to determine the capture zones and optimal well spacing of two wells pumping at different flow rates and positioned at various orientations to the direction of regional groundwater flow. The algorithm is also applied to determine capture zones for non-colinear three-well configurations as well as to determine optimal well spacing for up to six wells pumping at the same rate. We show that the optimal well spacing is found by minimizing the difference in the stream function evaluated at the stagnation points.

Degradation product partitioning in source zones containing chlorinated ethene dense non-aqueous-phase liquid.

Abiotic and biotic reductive dechlorination with chlorinated ethene dense non-aqueous-phase liquid (DNAPL) source zones can lead to significant fluxes of complete and incomplete transformation products. Accurate assessment of in situ rates of transformation and the potential for product sequestration requires knowledge of the distribution of these products among the solid, aqueous, and organic liquid phases present within the source zone. Here we consider the fluid-fluid partitioning of two of the most common incomplete transformation products, cis-1,2-dichloroethene (cis-DCE) and vinyl chloride (VC). The distributions of cis-DCE and VC between the aqueous phase and tetrachloroethene (PCE) and trichloroethene (TCE) DNAPLs, respectively, were quantified at 22 °C for the environmentally relevant, dilute range. The results suggest that partition coefficients (concentration basis) for VC and cis-DCE are 70 ± 1 L(aq)/L(TCE DNAPL) and 105 ± 1 L(aq)/L(PCE DNAPL,) respectively. VC partitioning data (in the dilute region) were reasonably approximated using the Raoults law analogy for liquid-liquid equilibrium. In contrast, data for the partitioning of cis-DCE were well described only when well-parametrized models for the excess Gibbs free energy were employed. In addition, available vapor-liquid and liquid-liquid data were employed with our measurements to assess the temperature dependence of the cis-DCE and VC partition coefficients. Overall, the results suggest that there is a strong thermodynamic driving force for the reversible sequestration of cis-DC and VC within DNAPL source zones. Implications of this partitioning include retardation during transport and underestimation of the transformation rates observed through analysis of aqueous-phase samples.

Microbially enhanced dissolution and reductive dechlorination of PCE by a mixed culture: Model validation and sensitivity analysis

Reductive dechlorination catalyzed by organohalide-respiring bacteria is often considered for remediation of non-aqueous phase liquid (NAPL) source zones due to cost savings, ease of implementation, regulatory acceptance, and sustainability. Despite knowledge of the key dechlorinators, an understanding of the processes and factors that control NAPL dissolution rates and detoxification (i.e., ethene formation) is lacking. A recent column study demonstrated a 5-fold cumulative enhancement in tetrachloroethene (PCE) dissolution and ethene formation (Amos et al., 2009). Spatial and temporal monitoring of key geochemical and microbial (i.e., Geobacter lovleyi and Dehalococcoides mccartyi strains) parameters in the column generated a data set used herein as the basis for refinement and testing of a multiphase, compositional transport model. The refined model is capable of simulating the reactive transport of multiple chemical constituents produced and consumed by organohalide-respiring bacteria and accounts for substrate limitations and competitive inhibition. Parameter estimation techniques were used to optimize the values of sensitive microbial kinetic parameters, including maximum utilization rates, biomass yield coefficients, and endogenous decay rates. Comparison and calibration of model simulations with the experimental data demonstrate that the model is able to accurately reproduce measured effluent concentrations, while delineating trends in dechlorinator growth and reductive dechlorination kinetics along the column. Sensitivity analyses performed on the optimized model parameters indicate that the rates of PCE and cis-1,2-dichloroethene (cis-DCE) transformation and Dehalococcoides growth govern bioenhanced dissolution, as long as electron donor (i.e., hydrogen flux) is not limiting. Dissolution enhancements were shown to be independent of cis-DCE accumulation; however, accumulation of cis-DCE, as well as column length and flow rate (i.e., column residence time), strongly influenced the extent of reductive dechlorination. When cis-DCE inhibition was neglected, the model over-predicted ethene production ten-fold, while reductions in residence time (i.e., a two-fold decrease in column length or two-fold increase in flow rate) resulted in a more than 70% decline in ethene production. These results suggest that spatial and temporal variations in microbial community composition and activity must be understood to model, predict, and manage bioenhanced NAPL dissolution.

An assembly model for simulation of large-scale ground water flow and transport.

When managing large-scale ground water contamination problems, it is often necessary to model flow and transport using finely discretized domains--for instance (1) to simulate flow and transport near a contamination source area or in the area where a remediation technology is being implemented; (2) to account for small-scale heterogeneities; (3) to represent ground water-surface water interactions; or (4) some combination of these scenarios. A model with a large domain and fine-grid resolution will need extensive computing resources. In this work, a domain decomposition-based assembly model implemented in a parallel computing environment is developed, which will allow efficient simulation of large-scale ground water flow and transport problems using domain-wide grid refinement. The method employs common ground water flow (MODFLOW) and transport (RT3D) simulators, enabling the solution of almost all commonly encountered ground water flow and transport problems. The basic approach partitions a large model domain into any number of subdomains. Parallel processors are used to solve the model equations within each subdomain. Schwarz iteration is applied to match the flow solution at the subdomain boundaries. For the transport model, an extended numerical array is implemented to permit the exchange of dispersive and advective flux information across subdomain boundaries. The model is verified using a conventional single-domain model. Model simulations demonstrate that the proposed model operated in a parallel computing environment can result in considerable savings in computer run times (between 50% and 80%) compared with conventional modeling approaches and may be used to simulate grid discretizations that were formerly intractable.

Incorporating Sustainability and Green Engineering into a Constrained Civil Engineering Curriculum

AbstractRevised Bodies of Knowledge, constituent input, and student demand are motivating the incorporation of sustainability concepts in traditional engineering curricula. While a variety of methods are in development, ranging from introductory engineering courses to capstone experiences to graduate-level treatment, the addition of new topics in an already constrained curriculum is creating a dilemma for many engineering programs. This work presents an example of how sustainability concepts were incorporated in a traditional civil engineering program administered within a constrained four-year curriculum. Using a variety of the treatments recommended in the literature, United States Air Force Academy civil and environmental engineering students learn and apply sustainability concepts at multiple points and in various frameworks during their four-year programs. Student feedback coupled with learning assessments suggest this framework may provide a more generally applicable model for incorporating sustaina...

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. However, the effect of this distribution morphology on volatilization has not been thoroughly quantified. Because field quantification of NAPL volatilization is often infeasible, a controlled laboratory experiment was conducted in a two-dimensional tank (28 cm × 15.5 cm × 2.5 cm) with water-wet sandy media and an emplaced trichloroethylene (TCE) source. The source was emplaced in two configurations to represent morphologies encountered in field settings: (1) NAPL pools directly exposed to the air phase and (2) NAPLs trapped in water-saturated zones that were occluded from the air phase. Airflow was passed through the tank and effluent concentrations of TCE were quantified. Models were used to analyze results, which indicated that mass transfer from directly exposed NAPL was fast and controlled by advective-dispersive-diffusive transport in the gas phase. However, sources occluded by pore water showed strong rate limitations and slower effective mass transfer. This difference is explained by diffusional resistance within the aqueous phase. Results demonstrate that vapor generation rates from a NAPL source will be influenced by the soil water content distribution within the source. The implications of the NAPL morphology on volatilization in the context of a dynamic water table or climate are discussed.

Quantification of experimental subsurface fluid saturations from high-resolution source zone images

Laboratory-scale aquifer cells are commonly used to investigate processes governing contaminant fate and transport in heterogeneous subsurface systems. In recent years, dramatic improvements in image processing methods have led to increasingly refined image resolution in these experiments. With these enhanced imagining methodologies, system parameters, such as nonaqueous phase liquid (NAPL) saturation, can now be quantified at the submillimeter scale. This fine level of resolution, however, is generally inconsistent with the typical scale of a representative elementary volume (REV), the averaging volume associated with property evaluation in continuum-based flow and transport models. Such inconsistency of scales calls into question the practice of directly comparing laboratory observations with continuum-based model simulations. This work explores the application of alternative approaches to image data processing for characterization of NAPL source zone architecture in aquifer cells and examines the implications of data processing on the interpretation of experimental-model comparisons. The utility of two alternative upscaling methods, a continuity-equation based (CB) and discrete-block based (DBB) approach are considered. Examination of the stability of point averages of saturation demonstrates that a REV length scale can be defined as ∼30 × d50 for the thickness-averaged, two-dimensional aquifer cell experiments examined. Quantification of upscaled source zone metrics, including the average saturation, the second spatial moment in the vertical direction, and the ganglia-to-pool (GTP) mass ratio reveals that the GTP is strongly sensitive to observation scale. Use of GTP values computed from upscaled images is shown to improve mathematical model predictions of experimental effluent concentrations by nearly 50%.