A random-walk approach for simulating transport and transformations in the unsaturated zone

A new reactive transport modelling approach and its applications are presented, dealing with the impact of the transformation processes, e.g. redox-reactions, on the spreading of solutes, e.g. wastewater, in unsaturated zone. This method is based on the random walk approach, balancing the water and associated mass transport in the unsaturated zone. It allows the quantification of the mass transfer between different phases (i.e. mobile fluid phase, immobile solid phase) and of the reactive processes. It is applied for investigating the plume development from a point source, e.g. sewer leak, in unsaturated zone and its impact on groundwater. One advantage of the model is that the water and the solute transport are simultaneous calculated, without having to first run a flow model and then a transport model, saving simulation effort. Transport of dissolved solutes in the unsaturated soil zone depends on the unsaturated water flow, which is usually described by the Richard's equation. As the Richard's equation is reformulated as a Fokker-Planck equation for the water transport, the random walk approach can be used for balancing the water flow in the unsaturated zone by moving particles representing a defined water volume. Also, defining mass loadings for each particle represents the mass transport. Thus, the unsaturated water transport and mass transport are computed together. When a particle moves, it represents the mobile fluid phase and brings the solute mass from one location to another one. At the same time it exchanges mass with the immobile solid phase (i.e. the cell). This approach is implemented in the numerical model Water and Solute Transport Model (WSTM), where this exchange takes place between individual particles and the respective cell. The method was applied to simulate a sewer leak infiltration in unsaturated soils considering conservative and reactive solutes. The distributions of the water content and the solutes concentration are well represented. For the complete assessment of the wastewater impact on soils and groundwater detailed understanding of migration and transformation of solutes in the subsurface is required. The implementation of the relevant transformation processes (e.g. redox- reactions) into the numerical model enables detailed investigation of their interactions on transport. Simulation results considering transformation processes will be validated by the use of the experimental results in order to provide a tool with predictive capability.