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 t o decide whether to formulate them kinetically or to assume a local ch emical equilibrium state. This decision in the chemical model part det ermines the mathematical structure of the overall model. In a kinetic formulation, the linear partial differential equations of the transpor t have to be coupled with a nonlinear system of ordinary differential equations describing the kinetic development, whereas in an equilibriu m formulation, the equations of the transport are coupled to a nonline ar system of algebraic equations describing the equilibrium state. Bas ically, two kinds of methods for solving reactive transport systems ma y be distinguished, namely, one-step methods which simultaneously solv e the transport and the chemical model parts and two-step methods whic h solve these model parts separately. We here present a sequential two -step method for kinetic transport models and an iterative two-step me thod for equilibrium transport models. We conduct a timescale analysis to check whether the error of the sequential two-step method is toler able 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 app lication of the kinetic transport model to laboratory column experimen ts governed by biodegradation.