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.