(17c) Chemotaxis Enhances Apparent Microbial Dispersion in Systems That Are Chemically and Physically Heterogeneous

A substantial research effort has been devoted to describe and predict bacterial migration in natural subsurface environments. Previous mathematical models, in which bacteria were usually considered as immobile colloids, have been used extensively to describe bacterial transport behaviors in both laboratory tests and field-scale studies. However, neglect of bacterial own swimming properties in the transport models, such as motility and chemotaxis, may wrongly estimate the productivity of microbial populations in conditions when those biological attributes are significant, such as bioremediation and microbial oil enhanced recovery.

A 2-dimensional mathematical model was developed to simulate transport phenomena of chemotactic bacteria in a sand-packed column designed with structured physical heterogeneity in the presence of a localized chemical source. First, the hydrodynamic conditions were quantified by fitting the model output to the experimental observations of a conservative tracer (nitrate). Next, model simulations fit to data for nonchemotactic bacteria provided the essential microbial transport parameters. Finally, parameter values were input into the mathematical model to predict bacterial concentration in the column effluent under the influence of a chemoattractant gradient.

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. Based on parameter sensitivity analysis, chemotactic parameters determined in bulk aqueous fluid were adequate to predict the microbial transport in our intermediate-scale porous media. Further analysis of bacterial transport over a range of flow rates revealed that bacterial chemotaxis might be impeded at high fluid velocity and high shear stress, as reported previously in the literature. Comparison of motile bacteria with passive microspheres revealed the importance of accounting for motility when evaluating the microbial filtration rate. By quantitatively describing bacterial fate and transport in a heterogeneous system, this mathematical model serves to advance our understanding of chemotaxis and motility in porous media. Moreover, it provides insights for improving the design of in-situ microbial processes.