Modeling of simultaneous processes of water flow, heat transfer, and multicomponent reactive solute transport in saturated-unsaturated porous media

A mathematical model has been developed to describe the simultaneous transport of water, heat, and multicomponent reactive chemicals in saturated-unsaturated subsurface soils. The water movement equation takes into account vapor transport in the unsaturated zone, as well as the fluxes caused by the gradients of temperature and solute concentration. The solute transport equations are formulated in terms of the total analytical concentration of each component species. Chemical reactions that can be dealt with in this model include complexation, acid base reaction, ion exchange, and precipitation-dissolution. The mathematical model has been solved by the Galerkin finite element method. The chemical transport equations are solved sequentially and separately from the chemical reaction equations, which are solved by the Newton-Raphson method. An iterative procedure is used among water flow equation, heat transfer equation, solute transport equations and chemical reaction equations. Three methods of solving the resulting systems of linear equations are implemented: the banded matrix method, the sparse matrix method, and the iterative methods. Two efficient and reliable iterative methods are provided: the conjugate gradient method for symmetric matrix equations and the bi-conjugate gradient squared method for general nonsymmetric matrix equations. The chemical system can be considered as either an open or a closed system. It has been demonstrated that the computer model can be used to solve one-, two-, and three dimensional problems. The model was used to simulate processes of salinization and leaching of soils under irrigation with realistic conditions. The soil solution included 48 chemical species in total. The computer model was used to simulate salinization of the soil if the irrigation rate was set equal to the crop uptake so that there was no leaching taking place. The simulation continued with increased rate and improved quality of irrigation water so that leaching of the saline soil took place. During both stages of the simulation, the computer model was shown to provide reason able results as how the dissolved concentrations changed in the soil solution during the salinization and leaching processes, how the composition on the exchange sites adjusted to new equilibrium, and how minerals dynamically precipitated and dissolved depending on degree of saturation of the soil solution with respect to each mineral.Graduate and Postdoctoral StudiesGraduat