Numerical simulation of biodegradation controlled by transverse mixing

Microbial activity in aquifers is controlled by the mixing between the reac ting substrates. Conventional modelling methods that are commonly used to a nalyze reactive transport of organics in heterogeneous systems may give err oneous results because mixing is often over-represented in the model. This effect will be strangest when the reaction is controlled by transverse disp ersion 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 t his problem can be controlled by formulating the problem in streamline-orie nted coordinates. In both model formulations, nonlinear high-resolution tec hniques minimizing the amount of artificial diffusion were applied, so that fictitious mixing is exclusively due to grid-orientation effects. Addition ally it is shown that applying dispersivity values based on the second spat ial moment transverse to the direction of flow leads to an overestimation o f 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 appropriat e dispersivity values as well as the control of artificial transverse diffu sion is crucial when modelling mixing-controlled reactive transport. (C) 19 99 Elsevier Science B.V. All rights reserved.