Multicomponent transport with coupled geochemical and microbiological reactions: model description and example simulations
Introduction
In order to assess the risk of groundwater contamination and to predict the success of remediation alternatives in the subsurface environment, it is necessary to accurately model the transport of contaminants in groundwater. The chemical composition of groundwater is often a complex mixture of inorganic and organic species. As the groundwater migrates through the porous media, the composition may be altered due to interactions with the various mineral phases present in the naturally heterogeneous solid matrix. Additionally, subsurface bacteria may be able to degrade some of the organic species in solution. The transport of a given contaminant species may be significantly affected by interactions with other dissolved species, mineral surfaces, and microorganisms. Therefore, a contaminant transport model must be able to simulate the important chemical and biological reaction processes, as well as the physical processes of advection and dispersion. The capability of modeling multiple components and multiple interacting processes is critical for groundwater pollution problems involving contaminant mixtures; an example of such a problem is the co-disposal of radionuclides and organics at DOE facilities (Riley and Zachara, 1992). Laboratory and/or field studies are necessary to examine the mechanism and kinetics of the fundamental reaction processes affecting the transport of contaminant species through porous media. The results of these studies can then be used in a numerical model, which provides a versatile, cost-effective tool to investigate the combined effects of a number of competing reaction processes.
Numerous chemical and microbiological reaction processes can affect the mobility of contaminants in the subsurface. These reaction processes can generally be classified as either homogeneous aqueous-phase reactions or heterogeneous interphase reactions involving mass transfer between the aqueous and immobile phases in the groundwater system (Rubin, 1983). With the exception of aqueous redox reactions, most aqueous speciation reactions (including complexation and acid–base reactions) are characterized by rapid rates, especially in comparison to the relatively slow rate of groundwater flow. Therefore, in groundwater modeling, it is usually valid to treat aqueous speciation reactions as if they proceed instantaneously to thermodynamic equilibrium (Steefel and Lasaga, 1994). In addition to interacting with one another in homogeneous reactions, the aqueous species may also take part in heterogeneous reaction processes such as adsorption, biodegradation, and precipitation/dissolution. Increasing evidence suggests that heterogeneous reaction processes may need to be described with a kinetic approach (Rifai and Bedient, 1990; Jardine et al., 1993aJardine et al., 1993b; Steefel and Lasaga, 1994; Szecsody et al., 1994).
This paper presents the development and application of FEREACT, a multicomponent reactive transport code developed to examine the geochemical and microbial processes that govern the behavior of contaminants in groundwater. This reactive transport code couples two-dimensional steady-state hydrologic transport with the effects of equilibrium aqueous speciation reactions and kinetically-controlled interphase reactions. The modular structure of FEREACT allows for the incorporation of any generic kinetic reaction rate terms, including highly nonlinear rate expressions. The model uses an iterative two-step solution algorithm to incorporate the effects of geochemical and microbial reaction processes in the governing transport equation. The two-step coupling approach results in a set of linear reactive transport equations that can be solved independently for each chemical component. FEREACT uses an efficient iterative matrix solver to evaluate the linearized reactive transport equations.
FEREACT is particularly suitable for two broad types of modeling applications. One is the design and interpretation of the results of one-dimensional column studies or two-dimensional intermediate-scale experiments concerning the transport of dissolved species through porous media. The other type of application involves formulation and testing of new hypotheses about transport behavior resulting from coupling among physical, chemical, and biological processes.
Section snippets
Reactive transport equations
In order to model the migration of a dissolved contaminant in the subsurface, an appropriate governing equation should include the effects of the transport processes of advection and dispersion and the effects of reactive processes such as adsorption and biodegradation. The application of mass balance principles leads to the well-known advection–dispersion–reaction equations. Following the nomenclature of Yeh and Tripathi (1989), FEREACT uses the total aqueous concentrations of a specified set
Application
The ability to model the effects of several coupled processes is essential for the remediation of sites that have been contaminated by complex mixtures of radioactive, inorganic and organic chemical wastes. This type of mixed-waste contamination has been observed at numerous DOE and defense-related sites. This application considers the migration of a radionuclide (cobalt, 60Co2+) and an organic ligand (ethylenediaminetetraacetate, EDTA4−) through a column packed with an iron oxide-coated sand.
Summary and conclusions
Many significant groundwater pollution problems involve complex reactive mixtures of inorganic and organic pollutants. One such example is discussed in this paper, namely the co-disposal of a radionuclide and an organic ligand in a metal oxide-coated sand. Mathematical models capable of simulating coupled geochemical and microbial processes are necessary for the assessment of these problems. In this paper, we present a model, FEREACT, that is capable of handling such complex interacting
Acknowledgements
This paper is based upon work supported by the Co-Contaminant Chemistry Subprogram within the Subsurface Science Program of the DOE Office of Health and Environmental Research. The support of Frank J. Wobber is gratefully appreciated. The authors would also like to acknowledge Carl Steefel, Ashok Chilakapati, and an anonymous reviewer for their valuable suggestions and comments.
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