Numerical simulation of biodegradation controlled by transverse mixing

Microbial activity in aquifers is controlled by the mixing between the reacting substrates. Conventional modelling methods that are commonly used to analyze reactive transport of organics in heterogeneous systems may give erroneous results because mixing is often over-represented in the model. This effect will be strongest when the reaction is controlled by transverse dispersion as in the case of aerobic degradation of waste-water introduced into an aquifer by an injection well. We show that fictitious transverse mixing can be created by a numerical model based on rectangular grids, and that this problem can be controlled by formulating the problem in streamline-oriented coordinates. In both model formulations, nonlinear high-resolution techniques minimizing the amount of artificial diffusion were applied, so that fictitious mixing is exclusively due to grid-orientation effects. Additionally it is shown that applying dispersivity values based on the second spatial moment transverse to the direction of flow leads to an overestimation of mixing. The fictitious degradation produced by model-dependent transverse dispersion caused the modelled plume to degrade much faster, and therefore appear much shorter, than the actual plume. Thus, the choice of appropriate dispersivity values as well as the control of artificial transverse diffusion is crucial when modelling mixing-controlled reactive transport.