Modeling of chemically reactive groundwater transport

A complete reactive groundwater transport model must account for both chemical and transport processes. For the chemical processes one has to decide whether to formulate them kinetically or to assume a local chemical equilibrium state. This decision in the chemical model part determines the mathematical structure of the overall model. In a kinetic formulation, the linear partial differential equations of the transport have to be coupled with a nonlinear system of ordinary differential equations describing the kinetic development, whereas in an equilibrium formulation, the equations of the transport are coupled to a nonlinear system of algebraic equations describing the equilibrium state. Basically, two kinds of methods for solving reactive transport systems may be distinguished, namely, one‐step methods which simultaneously solve the transport and the chemical model parts and two‐step methods which solve these model parts separately. We here present a sequential two‐step method for kinetic transport models and an iterative two‐step method for equilibrium transport models. We conduct a timescale analysis to check whether the error of the sequential two‐step method is tolerable and whether a given kinetic transport system can be reduced to an equilibrium one. The numerical methods and the timescale analysis are applied to two test cases. Zysset et al. (1994) present a further application of the kinetic transport model to laboratory column experiments governed by biodegradation.