Abstract Point sources of wastewater in the subsurface are a potential threat to groundwater quality. Modelling wastewater transport and transformation requires dealing with a complex system of processes and interactions between solutes as well as between the solutes and the solid phase. Therefore the process-based reactive transport model WSTM (Water and Solute Transport Model) was developed, based on the random walk approach and fulfilling the Fokker-Planck equation for the unsaturated water flow. The main focus of this paper is on the mathematical formulation and applications of the model.

Keywords random walk approach; reactive transport modelling; unsaturated zone