Multiscale Modeling and Experimental Approaches to Link Macroscopic and Microscopic Bacterial Chemotaxis

Contamination caused by nonaqueous phase liquids (or NAPLs) is of great concern in ground water as NAPLs are ubiquitous and persistent pollutants, remaining recalcitrant to bioremediation due to low solubility and limited bioavailability. Chemotaxis, the directed movement of bacteria upon sensing chemical gradients, may facilitate remediation of NAPLs by transporting hydrocarbon-degrading bacteria to residual contamination trapped within the soil matrix. This dissertation work aimed to investigate the role of chemotaxis on bacterial transport in polluted porous media by using a combination of microscopic imaging, microfluidics, mathematical modeling and computer simulation. Naphthalene, one of the sixteen High Priority Pollutants designated by the Environment Protection Agency, was used as a model pollutant that is a chemoattractant for naphthalene-degrading Pseudomonas putida G7.

Bacterial chemotactic response was studied across different length scales from a sand column (10e-1 m) to a single pore (10e-4 m). Bacterial transport in a sand column interspersed with discrete NAPL ganglia was modeled using a modified convection-dispersion equation. Computer simulation of the model equation revealed hotspots of bacteria near NAPL ganglia due to chemotactic response and sorption at oil surfaces. Then bacterial transport in pores near NAPL ganglia was observed directly in a dual-permeability microfluidic device and elevated densities of bacteria were found in micropockets near NAPL sources. Simulation results at the pore scale indicated that bacterial distributions were strongly regulated by chemical gradients, which was accounted for in the macroscale model with an increased the bacterial dispersion coefficient by 50%. The transport mechanism in chemotactic bacteria was explored further in a micromodel with similar design, but different pore sizes and bacteria suspensions were introduced in varying fluid velocities. Experiments showed that fluid flow promoted chemotaxis in longer and shorter micropockets at pore velocities lower than 15 m/d and 11 m/d, respectively. The optimal fluid velocity depended on pore dimension and bacterial response time scale, which was further validated by using experimental data from previous literature. Lastly, the influence of fluid flow on bacterial motility and chemotaxis in a single-pore microfluidic chip was examined. We found that bacterial motility and chemotaxis were hampered as fluid velocity increased, which resulted in an adjustment of chemotactic sensitivity coefficient to different extents at varying fluid velocity in continuum modeling.

Results of this dissertation work suggested that chemotactic bacteria were more efficient than nonchemotactic bacteria in navigating among pore network in the presence of NAPL contaminants and it was possible to create preferable conditions to aid chemotaxis. Our experiments indicated chemotaxis may overcome the limited bioavailability of NAPLs trapped in low hydraulic permeable strata, and our models provided insight into the impact of chemotaxis on bacterial transport in porous media with the presence of fluid flow.