Upscaling microbial chemotaxis in porous media

Biodegradation is an important mechanism for contaminant reduction in groundwater environments; in fact, in situ bioremediation and bioaugmentation methods represent alternatives to traditional methods such as pump-and-treat. Microbial chemotaxis has been shown to significantly increase contaminant degradation in subsurface environments. In this work, the method of volume averaging is used to upscale the microscale chemotactic microbial transport equations in order to obtain the corresponding effective medium models for the mass balance of bacteria and the chemical attractant to which they respond. As a first approach, cellular growth/death and consumption of the attractant by chemical reaction are assumed to be negligible with respect to convective and diffusive transport mechanisms. For microorganisms, two effective coefficients are introduced, namely a total motility tensor and a total velocity vector. Our results show that, under certain conditions, these coefficients can differ considerably from the values corresponding to non-chemotactic transport. These transport coefficients show strong dependence of the microstructure of the porous medium, the fluid flow fields and the distribution of the attractant.