Interactions Between Microbial Dynamics and Transport Processes in Variably Saturated Porous Media. 1. Experimental System and Results

An experimental and numerical investigation was conducted to study interactions between microbial growth, water flow, and solute transport in variably saturated porous media. This paper (Part 1) describes an experimental system that allowed continuous, noninvasive, observation of microbial activity while simultaneously monitoring hydrodynamics in a two-dimensional system under unsaturated flowing conditions. Induction of a bioluminescent phenotype provided for observation of the spatial and temporal development of microbial colonization by a Pseudomonas fluorescens bacterium. Water content distribution and solute flow paths were determined by measuring the transmission of light through the system. Results are presented for experiments conducted in homogeneous media, and for experiments that incorporated the inclusion of a textural heterogeneity in the system. Bacterial growth and accumulation over the course of the experiments had a significant impact on the hydrologic properties of the media. Observations of microbially-mediated changes included localized drying within a colonized region, diversion of flow around a colonized region, lowering of the capillary fringe height, and increase in the height of perched water above the interface of a coarse-textured inclusion upstream of a colonized region. A companion paper by Rockhold et al. (Part 2) describes the development and evaluation of a numerical model that simulates the processes observed in this system.