Yunwei Sun

Natural attenuation of chlorinated ethene compounds: model development and field-scale application at the Dover site

Abstract A multi-dimensional and multi-species reactive transport model was developed to aid in the analysis of natural attenuation design at chlorinated solvent sites. The model can simulate several simultaneously occurring attenuation processes including aerobic and anaerobic biological degradation processes. The developed model was applied to analyze field-scale transport and biodegradation processes occurring at the Area-6 site in Dover Air Force Base, Delaware. The model was calibrated to field data collected at this site. The calibrated model reproduced the general groundwater flow patterns, and also, it successfully recreated the observed distribution of tetrachloroethene (PCE), trichloroethene (TCE), dichloroethylene (DCE), vinyl chloride (VC) and chloride plumes. Field-scale decay rates of these contaminant plumes were also estimated. The decay rates are within the range of values that were previously estimated based on lab-scale microcosm and field-scale transect analyses. Model simulation results indicated that the anaerobic degradation rate of TCE, source loading rate, and groundwater transport rate are the important model parameters. Sensitivity analysis of the model indicated that the shape and extent of the predicted TCE plume is most sensitive to transmissivity values. The total mass of the predicted TCE plume is most sensitive to TCE anaerobic degradation rates. The numerical model developed in this study is a useful engineering tool for integrating field-scale natural attenuation data within a rational modeling framework. The model results can be used for quantifying the relative importance of various simultaneously occurring natural attenuation processes.

Development of analytical solutions for multispecies transport with serial and parallel reactions

A direct method for transforming multiple solute transport equations, coupled by linear, series, and/or parallel first-order, irreversible reactions, into a series of simple transport equations having known solutions is developed. Using this method, previously published analytical solutions to single-species transport problems, in which the transported species reacts with first-order kinetics, can be used to derive analytical solutions to multispecies transport systems with parallel, serial, and combined reaction networks. This new method overcomes many of the limitations that were implicit in previously published methods. In particular, the number of species that can be described is unlimited, and the reaction stoichiometry does not have to be unimolar. To illustrate the method, an analytical solution is derived for a five-species serial-parallel reactive transport system. The analytical solution obtained for this problem is compared with a numerical solution obtained with a previously developed code. This analytical method is applicable to the verification of new numerical codes.

Optimization of pump-treat-inject (PTI) design for the remediation of a contaminated aquifer: multi-stage design with chance constraints

Abstract A large scale groundwater remediation project using pump-and-treat (PAT) or pump-treat-inject (PTI) cannot be designed as a single time-step operation, because of uncertainties associated with the system. The changes in concentrations in reality may differ significantly from predicted ones. Instead, a multi-stage decision process is formulated and solved as a two-level hierarchical optimization model. Cost serves as the objective function, while contaminant concentration and total cleanup time are constraints. The entire cleanup time is divided into several stages. The number of wells for both pumping and injection is treated as a decision variable in each design stage. At the basic level, well locations and pumping/injection rates are sought so as to maximize mass removal of contaminants. At the upper level, the number of wells for pumping and injection is optimized, so as to minimize the cost, taking maximum contaminant level (MCL) as a constraint. Indices for the equivalent centroid and areal extent of a contaminant plume are proposed and used to initiate well locations at each design stage.

Analytical solutions for reactive transport of N-member radionuclide chains in a single fracture

Several numerical codes have been used to simulate radionuclide transport in fractured rock systems. The validation of such numerical codes can be accomplished by comparison of numerical simulations against appropriate analytical solutions. In this paper, we present analytical solutions for the reactive transport of N-member radionuclide chains (i.e., multiple species of radionuclides and their daughter species) through a discrete fracture in a porous rock matrix applying a system decomposition approach. We consider the transport of N-member radionuclide chains in a single-fracture-matrix system as a starting point to simulate more realistic and complex systems. The processes considered are advection along the fracture, lateral diffusion in the matrix, radioactive decay of multiple radionuclides, and adsorption in both the fracture and matrix. Different retardation factors can be specified for the fracture and matrix. However, all species are assumed to share the same retardation factors for the fracture and matrix, respectively. Although a daughter species may penetrate farther along the fracture than its parent species when a constant-concentration boundary condition is applied, our results indicate that all species retain the same transport speed in the fracture if a pulse of the first species is released into the fracture. This solution scheme provides a way to validate numerical computer codes of radionuclide transport in fractured rock, such as those being used to assess the performance of a potential nuclear-waste repository at Yucca Mountain.

Validation of the Multiscale Thermohydrologic Model used for analysis of a proposed repository at Yucca Mountain

Performance assessment and design evaluation of the proposed repository at Yucca Mountain are facilitated by a thermohydrologic modeling tool that simultaneously accounts for processes occurring at a scale of a few tens of centimeters around individual waste packages and emplacement drifts, and accounts for processes at the multi-kilometer scale of the mountain. The most straightforward approach is to account for the 3-D drift- and mountain-scale dimensionality all within a single monolithic thermohydrologic model. This approach is too computationally expensive to be a viable simulation tool capable of addressing all waste-package locations in the repository. The Multiscale Thermohydrologic Model (MSTHM) is a computationally efficient alternative to addressing these modeling issues. In this paper, we describe the principal calculation stages to predict temperature, relative humidity, and liquid-saturation, as well as other thermohydrologic variables, in the drifts and in the host rock. Using a three-drift repository example (which is a scaled-down version of the proposed repository), we demonstrate the validity of the MSTHM approach against a nested monolithic thermohydrologic model.

An Analytical Solution of Tetrachloroethylene Transport and Biodegradation

In this manuscript, we consider a transport system with a dechlorination reaction network, in which tetrachloroethylene (PCE) reacts to produce trichloroethylene (TCE), TCE reacts to form three daughter products, cis-1,2-dichloroethylene (cis-1,2-DCE), trans-1,2-dichloroethylene (trans-1,2-DCE), and 1,1-dichloroethylene (1,1-DCE), three DCEs further react to produce vinyl chloride (VC), finally VC reacts to produce ethylene (ETH). Because the partial differential equation describing the reactive transport of VC, is coupled by three reactant concentrations, currently the problem must be solved numerically. Following Lu et al. (2003), we extend the analytical solution from five species to the entire PCE reaction network. Using the singular value decomposition (SVD) the system of transport equations with convergent reactions is decoupled into seven orthogonal subsystems. Previously published analytical solutions of single species transport become the basic solutions in the transformed domain for each independent subsystem. The solutions in real concentration domain are obtained using the inverse transform. The solution derived in this study can then be used instead of Sun et al. (1999) in BIOCHLOR for simulating more realistic systems of biodegradation and reactive transport.

Detection of Noble Gas Radionuclides from an Underground Nuclear Explosion During a CTBT On-Site Inspection

The development of a technically sound approach to detecting the subsurface release of noble gas radionuclides is a critical component of the on-site inspection (OSI) protocol under the Comprehensive Nuclear Test Ban Treaty. In this context, we are investigating a variety of technical challenges that have a significant bearing on policy development and technical guidance regarding the detection of noble gases and the creation of a technically justifiable OSI concept of operation. The work focuses on optimizing the ability to capture radioactive noble gases subject to the constraints of possible OSI scenarios. This focus results from recognizing the difficulty of detecting gas releases in geologic environments—a lesson we learned previously from the non-proliferation experiment (NPE). Most of our evaluations of a sampling or transport issue necessarily involve computer simulations. This is partly due to the lack of OSI-relevant field data, such as that provided by the NPE, and partly a result of the ability of computer-based models to test a range of geologic and atmospheric scenarios far beyond what could ever be studied by field experiments, making this approach very highly cost effective. We review some highlights of the transport and sampling issues we have investigated and complete the discussion of these issues with a description of a preliminary design for subsurface sampling that addresses some of the sampling challenges discussed here.

Effect of reaction kinetics on predicted concentration profiles during subsurface bioremediation

The lack of a complete mechanistic description of subsurface interactions between contaminants and microbes has limited the ability to predict the effectiveness of subsurface bioremediation processes. Important microbial processes which must be included in such mechanistic descriptions of bioremediation are microbial growth, death and transport, including microbial attachment to and detachment from the soil surfaces. Recent advances have provided a better understanding of the relationships between contaminant destruction rates and such microbial processes. In this manuscript, contaminant profiles predicted using more realistic descriptions are compared with those obtained using less complete, previously published mathematical models. The more comprehensive model described here includes mathematical descriptions for microbial growth and transport. Contaminant profiles predicted using this model are compared with those obtained when the contaminant destruction processes are described by assuming that (1) no reaction occurs, (2) the contaminant reacts instantaneously with any available oxygen, and (3) dual-substrate Monod kinetics without consideration of microbial growth and transport. This work demonstrates that if reaction kinetics are ignored, the size of the plume will be overpredicted at large times and if the reaction is assumed to occur instantaneously, the plume will be underpredicted at small times. Thus, if these processes are not included in the mathematical description of bioremediation, erroneous interpretations of site characterization data collected during this period would be likely.

Modeling Noble Gas Transport and Detection for The Comprehensive Nuclear-Test-Ban Treaty

Detonation gases released by an underground nuclear test include trace amounts of 133Xe and 37Ar. In the context of the Comprehensive Nuclear Test Ban Treaty, On Site Inspection Protocol, such gases released from or sampled at the soil surface could be used to indicate the occurrence of an explosion in violation of the treaty. To better estimate the levels of detectability from an underground nuclear test (UNE), we developed mathematical models to evaluate the processes of 133Xe and 37Ar transport in fractured rock. Two models are developed respectively for representing thermal and isothermal transport. When the thermal process becomes minor under the condition of low temperature and low liquid saturation, the subsurface system is described using an isothermal and single-gas-phase transport model and barometric pumping becomes the major driving force to deliver 133Xe and 37Ar to the ground surface. A thermal test is simulated using a nonisothermal and two-phase transport model. In the model, steam production and bubble expansion are the major processes driving noble gas components to ground surface. After the temperature in the chimney drops below boiling, barometric pumping takes over the role as the major transport process.

Analytical Solutions of TCE Transport with Convergent Reactions

In this paper, we present analytical solutions for reactive transport with convergent reactions using the singular value decomposition approach. We consider a reaction network in which a compound reacts to form multiple daughter products, which further react to a single granddaughter. This reaction scheme is illustrated by the dechlorination of trichloroethylene (TCE), which reacts to form three daughter products, cis-dichloroethylene (cis-DCE), trans-dichloroethylene (trans-DCE), and 1,1-dichloroethylene (1,1-DCE). All three of these daughter products will further react to produce the same granddaughter, vinyl chloride (VC). In achieving the analytical solution, all reactions are assumed to be first-order. Because the partial differential equation describing the final product (granddaughter) concentration, is coupled by the three daughter concentrations, previously published methods for decoupling partial differential equations are not applicable. Instead, we conduct the singular value decomposition process analytically and transform the system of transport equations with convergent reactions into orthogonal (independent) equations, for which analytical solutions are available. The solutions derived in this manuscript are then compared with numerical solutions and the performance of the reactive transport system is analyzed.