| Permeability differences within groundwater aquifers can have a profound effect on the viability of bioremediation processes. Bioaugmentation is a bioremediation process in which motile bacteria are injected into chemically polluted groundwater to degrade chemical contamination. Permeability differences in geological media may alter the extent of bacterial migration. Previously, researchers have accounted for the impact of soil heterogeneity by assigning effective transport coefficients (Barton et al. 1995), (Sherwood et al. 2003). However, we have observed partitioning of the bacterial populations at permeability interfaces that cannot be explained using effective transport coefficients. Instead, we believe that bacteria are interacting with the solid surfaces of the porous media resulting in non-diffusive migration patterns. These interactions occur when motile bacteria encounter solid surfaces and lead to an increased residence time of the bacteria near the porous media surfaces. We propose to mathematically represent this surface association as an adsorption-like process.; To substantiate our hypothesis, we observed individual cells swimming through porous media environments and discovered different migration patterns depending upon the particle size of the porous media and the swimming characteristics of the bacterial strains. We also performed population-scale laboratory experiments, which introduced a modified light scattering assay capable of observing bacterial distributions near porous media interfaces. Using this assay we investigated the random motility response of a smooth-swimming Escherichia coli HCB437 mutant and the chemotactic responses of wild-type Escherichia coli HCB1 and wild-type Pseudomonas putida F1.; Results from the population-scale experiments were compared to mathematical models, which employed bacterial transport equations and the processes of effective transport and surface association. The models yielded similar profile shapes to the experimental profiles. In addition, Monte Carlo simulations produced a partitioning of the bacterial population at porous media interfaces in high surface area systems, when cells were assumed to non-elastically interact with the porous media surfaces. Thus, our experimental and simulated data supported the process of surface association and confirmed that bacterial migration through heterogeneous environments is influenced by the properties of the porous media and the swimming characteristics of the bacterial strains. These changes in bacterial migration could substantially impact the efficiency of bioremediation processes. |
