A General Paradigm of Modeling Three-Dimensional Subsurface Water Quality

This paper presents the model development of reactive chemical transport in subsurface water systems. Through the decomposition of the system of species transport equations via Gauss-Jordan column reduction of the reaction network, fast reactions and slow reactions are decoupled, which enables robust numerical integrations. Species reactive transport equations are transformed into two sets: algebraic equations (either mass action equations or users¡_ specified) of equilibrium variables and reactive transport equations of kinetic variables. As a result, the model uses kinetic-variables instead of biogeochemical species as primary dependent variables, which reduces the number of transport equations and simplifies reaction terms in these equations. In order to improve the efficiency and robustness of the computation, five options are provided to solve the advection-dispersion transport equations. They are Finite Element Method (FEM) Applied to the Conservative Form of Transport Equations, FEM Applied to the Advective Form of Transport Equations, Modified Lagrangian-Eulerian (LE) approach, LE approach with FEM Applied to the Conservative Form of Transport Equations for Upstream Flux Boundary, and LE approach with FEM Applied to the Advective Form of Transport Equations for Upstream Flux Boundary. Three chemical strategies are employed to deal with the reaction terms. They are Fully-implicit scheme, Mixed Predictor-corrector and Operator-splitting method, and Operator-splitting approach. Three example problems are employed to demonstrate the robustness of the numerical simulations and the design capability of the model.