An experimental and numerical modeling investigation was conducted to study interactions between microbial dynamics and transport processes in variably saturated porous media. These interactions are important in a variety of applied problems such as water and wastewater treatment, bioremediation, and oil-field recovery operations. These processes and interactions also have great ecological significance, with global scale implications for carbon cycling in the environment and the related issue of climate change. Experiments were conducted under variably saturated flow conditions in columns and 2D light-transmission chambers packed with translucent quartz sand. A bioluminescent Pseudomonas fluorescens bacterium was utilized in the experiments and bioluminescence was used as a non-destructive measure of bacterial density and distribution. In the column experiments, pressure heads increased (became less negative) at all measured depths, but significant changes in apparent volumetric water contents were only observed in the upper 5 cm of the columns. Permeability was reduced by a factor of 40 within one week during growth on glucose. In the chamber experiments, aqueous-phase saturations decreased by 7-9% in the region of primary colonization and the capillary fringe dropped by 5 cm during the 6-day experiment. The colonized region expanded laterally by 15 cm and upward against the flow by about 7-8 cm. The desaturation phenomenon resulted in increased lateral spreading of solutes around the colonized region. A numerical model was developed and used to help interpret the experimental data. Water flow was modeled using the single-phase Richards equation. Solute and bacterial transport, cell growth, substrate consumption, and gas diffusion were modeled using advection-dispersion-reaction equations. Observed changes in saturations and pressure heads were reproduced approximately using fluid-media scaling to represent an apparent surface-tension lowering effect, which was assumed to be due to sorption of cells and/or biosurfactants at gas-liquid interfaces. Microbial dynamics, and substrate and oxygen consumption were represented using first-order reversible kinetics for cell attachment/detachment, and dual Monod-type kinetics for cell growth and substrate and oxygen consumption. Reasonably good matches were obtained between the observed and simulated results. / Graduation date: 2003