Upscaling of microbially driven first-order reactions in heterogeneous porous media

Reactions mediated by microorganisms determine the fate of many chemicals in natural porous media. At the pore scale, the distribution of chemicals and microorganisms is not homogeneous, leading to heterogeneous distribution of microbial activities at the pore scale. We conducted pore scale reactive transport simulations to investigate the scaling of microbially mediated consumption reaction rates under a range of flow and reaction conditions. The results reveal that the scaling effects largely depended on Péclet and Damköhler numbers. Consumption rate estimates based on volume-averaged concentrations and reaction kinetics overestimated the true volumetric reaction rates, and large-sized biomass aggregates intensified these scaling errors. In contrast, the macroscopic rates estimated via flux-weighted concentrations underestimated the true volumetric reaction rates, with large microbial aggregates reducing scaling errors. This study also demonstrated that macroscopic rate estimates can be improved by combining information on the reaction kinetics with the flux-weighted concentrations.