Citation:

Yeh, G. T. AND H. P. Cheng. 3DHYDROGEOCHEM: A 3-DIMENSIONAL MODEL OF DENSITY-DEPENDENT SUBSURFACE FLOW AND THERMAL MULTISPECIES-MULTICOMPONENT HYDROGEOCHEMICAL TRANSPORT. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-98/159 (NTIS PB99-150419), 1999.

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Description:

This report presents a three-dimensional finite-element numerical model designed to simulate chemical transport in subsurface systems with temperature effect taken into account. The three-dimensional model is developed to provide (1) a tool of application, with which one is able to deal with a variety of real-world problems, (2) a tool of education, with which one can study how a factor would affect the whole system, and (3) a substructure, which one could modify to handle specific problems. The hydrological environment, to which the model can be applied to is a heterogeneous, anisotropic, saturated-unsaturated subsurface medium under either transient-state to steady-state flow conditions. In addition, the temperature within the system of interest can be both time- and location-dependent. For steady-state simulations, strong coupling among subsurface flow, chemical transport, and heat transfer is used in the model. For transient-state simulations, weak coupling is used, but a density effect is still considered in computations. Both the strong and the weak couplings are pictured and explained. The model employs chemical equilibrium to describe the relationship among chemicals. The chemical reactions included in the model are aqueous complexation, multi-site adsorption/desorption, multi-site ion-exchange, precipitation/dissolution, redox, and acid-base reactions. To discretize the domain of interest appropriately, the element used in the model can be a hexahedral, a triangular prism, or a tetrahedral element. To extend its applicability to a more real-world problems, two approaches are presented for the chemical transport module in this report. The first approach uses the pore velocity and dispersion coefficient to handle advection and dispersion, respectively, for aqueous components, whereas the second approach employs the retarded pore velocity and the retarded dispersion coefficient. Both approaches are designed to handle the problem with dominating precipitated species involved. The governing equations of subsurface flow, chemical transport, chemical equilibrium, and heat transfer are stated and/or derived. The numerical approaches with the finite element method to solve the governing equations are described. In this study, 25 designed examples have been used to verify the model. Four application examples, including a three-dimensional subsurface flow example, a three-dimensional reactive chemical transport, example, a three-dimensional heat transfer example, and a three-dimensional coupled flow-transport-transfer example, are presented to demonstrate the capability of the model. The modeling experience from this study is summarized in the last chapter of this report. Data input guide and program description are stated in Appendix A and Appendix B, respectively. In Appendix C, parameter specification for array dimensions in 3DHYDROGEOCHEM is given.

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