Coupling of transport and chemical processes in numerical transport models

Abstract

As most groundwater pollutants do not behave like ideal tracers, the need arises to incorporate adsorption and chemical transformations into transport models for field problems. In the simplest case of a single pollutant undergoing a first-order reaction and linear equilibrium adsorption, the incorporation is trivial. In multicomponent transport with either non-linear kinetic or equilibrium reactions (including ion exchange), systems of nonlinear transport equations combined with algebraic equations result. Their solution is greatly simplified by using a two-step procedure in which each time step is separated into a transport step and chemical reaction step. The largest error of the two-step method curve occurs in heterogeneous reactions. The error can be interpreted as an additional artificial dispersion phenomenon which can be accepted as long as it remains smaller than the physical dispersion process. Discretization requirements can be loosened if iteration within one time step is introduced. This paper describes an iterative two-step procedure which incorporates multi-component interactions into one single two-dimensional finite difference transport code. The model is applied to equilibrium ion exchange during two-dimensional transport.