Geoelectrical measurement and modeling of biogeochemical breakthrough behavior during microbial activity

We recorded bulk electrical conductivity (σ_b) along a soil column during microbially-mediated selenite oxyanion reduction. Effluent fluid electrical conductivity and early time σ_b were modeled according to classic advective-dispersive transport of the nutrient medium. However, σ_b along the column exhibited strongly bimodal breakthrough which cannot be explained by changes in the electrical conductivity of the pore fluid. We model the anomalous breakthrough by adding a conduction path in parallel with the fluid phase, with a time dependence described by a microbial population-dynamics model. We incorporate a delay time to show that breakthrough curves along the column satisfy the same growth model parameters and offer a possible explanation based on biomass-limited growth that is delayed with distance from influent of the nutrient medium. Although the mechanism causing conductivity enhancement in the presence of biomass is uncertain, our results strongly suggest that biogeochemical breakthrough curves have been captured in geoelectrical datasets.