In the simplest case we model this situation as a one-dimensional multicomponent diffusion problem in which the connections to the mobile phase and the solid phase are described by Dirichlet and Neumann boundary conditions, respectively. Precipitation of secondary minerals (Al(OH)3) in the water column is linked to decreased diffusion coefficients of dissolved ions.

The resulting set of coupled non-linear partial differential equations has a nonstandard form, since a reaction-diffusion system is coupled to a condition of electric neutrality everywhere in space. Therefor, special care has to be taken when applying numerical techniques for parabolic systems.

Test calculations show that the model exhibits a new mechanism for SO24 accumulation in acid soils that interferes in a nocel way with variations of NO3 in the mobile water.