Modeling of biologically mediated redox processes in the subsurface

To model bacterially catalyzed redox processes a multicomponent transport reaction model is presented. The transport part of the model solves the transient convection dispersion differential equations. The pure chemical submodel is conceptually similar to conventional thermodynamic equilibrium models. The kinetic submodel describes the heterotrophic metabolisms of several groups of microorganisms. To model a complete redox sequence (aerobic carbonaceous oxidation, denitrification, Fe(III)-reduction, Mn(IV)-reduction, and sulfate reduction) four functional bacterial groups are defined. Their growth and metabolisms are formulated in terms of Monod equations. As in other biofilm models, diffusion-limited exchange between the different phases (mobile pore water, biophase, and aquifer material) is also considered in this approach. The submodels are coupled by the equations of the microbially mediated redox reactions. This numerical technique permits direct mechanistic modeling of the influence of microbially catalyzed redox reactions on the chemical milieu of an aquifer. A two-step method is applied to solve the coupled transport and biochemical reaction equations. The numerical model was applied to field data of a natural subsurface flow path.