Chemotaxis impact on microbial transport in a packed column with structured physical heterogeneity

Chemotaxis, the directed migration of a bacterial population in response to a chemical concentration gradient, is believed to accelerate contaminant remediation rate in bioremediation processes by attracting bacteria toward the chemically favorable regions with low permeability, where the contaminant sources reside. The goal of this work is to investigate the effect of chemotaxis on microbial migration in heterogeneous systems.;A laboratory-scale column, with a coarse-grained sand core surrounded by a fine-grained annulus, was developed to simulate natural aquifers with strata of different hydraulic conductivities. A chemoattractant source was placed along the central axis to model contaminants trapped in the heterogeneous subsurface. Chemotactic bacterial strains, Escherichia coli HCB1 and Pseudomonas putida F1, were introduced into the column by a pulse injection. For E. coli HCB1, approximately 18% more of the total population relative to the control without attractant exited the column from the coarse sand layer under an average fluid velocity of 5.1 m/d. Although P. putida F1 demonstrated no observable changes in migration pathways with the model contaminant acetate under the same condition, when the flow rate was reduced to 1.9 m/d, approximately 6-10% more of the population relative to the control migrated through the coarse sand layer.;The analysis of chemotactic influences was further elucidated by a two-dimensional mathematical model, which incorporated a convective-like chemotaxis term to represent chemotactic migration. Consistency between experimental observation and model prediction supported the assertions that (1) dispersion-induced microbial transfer between adjacent conductive zones occurred at the interface and had little influence on bacterial transport in the bulk flow of the permeable layers and (2) the enhanced transverse bacterial migration in chemotactic experiments relative to nonchemotactic controls were mainly due to directed migration toward the chemical source zone. Additionally, the analysis of adsorption coefficient values supported the observation of a previous study that microbial deposition to the surface of porous media might be decreased under the effect of attractant gradients. Further analysis of bacterial transport over a range of flow rates revealed that bacterial chemotaxis may be impeded at high fluid velocity and high shear stress, as reported previously in the literature.