Roseanne Ford - InterPore2021 Invited Lecture - 1 June 2021

Transport of chemotactic bacteria in porous media with residual sources of chemical pollutants

Abstract:

Nonaqueous phase liquid (NAPL) contaminants are difficult to eliminate from natural aquifers due, in part, to the heterogeneous structure of the soil matrix. Residual NAPL ganglia remain trapped in regions where the hydraulic conductivity is relatively low and they are consequently less bioavailable. Bioremediation processes depend on adequate mixing of microbial populations and the groundwater contaminants that they degrade. The ability of chemotactic bacteria to sense a chemical gradient and swim preferentially up the gradient toward higher concentration can enhance the accumulation of bacteria near contaminant sources that may otherwise not be readily accessible by advection and dispersion alone.
In this work, we directly imaged Pseudomonas putida bacteria near naphthalene sources trapped within a pore network etched into a microfluidic device. We modeled bacterial transport at the pore scale using a convection-dispersion equation with the addition of chemotactic velocity to the convective term and first-order sorption-desorption kinetics for retention around NAPL ganglia. Previous simulations at the core scale in granular media showed that the heterogeneous hotspots of chemotactic bacteria around NAPL ganglia yielded overall greater retention of biomass in breakthrough curves compared to a nonchemotactic control. Our experimental observations at the pore scale confirmed the core scale simulation results by revealing greater accumulation of chemotactic bacteria (relative to a nonchemotactic control) near ganglia of naphthalene sources. Our pore scale simulation results showed that greater retention of bacteria was due to its chemotactic response to naphthalene gradients and sorption to NAPL ganglia. Our modeling predictions in combination with laboratory experiments at varying scales can be a useful tool to analyze the impact of chemotaxis on in situ bioremediation. This work is in collaboration with Xiaopu Wang, (China University of Petroleum-East China) and Beibei Gao (University of Virginia).