Glass micromodel study of bacterial dispersion in spatially periodic porous networks

Successful implementation of bioremediation clean-up strategies depends on accurate predictions of the transport of bacteria within the subsurface. In this study, etched flat-plate glass micromodels were used to examine bacterial transport in a homogenous network. These networks were created by acid-etching interconnected channels into a glass plate and then fusing it to an unetched plate forming semi-cylindrical pores. The transparent nature of the micromodel allows for both qualitative observations of the bacteria within the pores and quantitative measurements of their concentration. The micromodels are designed to allow establishment of a well-characterized step change in bacterial concentration (Escherichia coli NR50) within the network. During the experiments, bacteria are dispersed through the network by flow. Light scattering is used to detect the change in turbidity within the pores as the bacteria travel through the network. The change in turbidity is used to construct breakthrough curves and spatial concentration profiles of bacteria within the network. The breakthrough curves are fit to the one-dimensional advection/dispersion equation to determine dispersion coefficients at different interstitial fluid velocities. From the breakthrough curves, dispersion coefficients were reproducible for replicate experiments over a range of velocities in the advection-dominated regime. The dispersivity values for two network designs resembling an interconnecting capillary network and a spatially periodic network of cylinders were 0.28 and 0.33 cm respectively, which are slightly greater than the literature values found for other pore networks. Experiments were also conducted within the diffusion-dominated regime to examine the effects of bacterial motility on dispersion. The accumulation of bacteria on the pore walls became significant at the low flow rates and extended experimental times thereby rendering the use of light scattering to determine concentrations ineffective. Bacterial chemotaxis, created by a self-imposed oxygen gradient, was also observed in the micromodel under stagnant fluid conditions.