Transport of Pseudomonas fluorescens strain P17 through quartz sand columns as a function of water content

Porous media column experiments were used to investigate Pseudomonas fluorescens strain P17 transport as a function of water content and the influences of the solid-liquid and gas-liquid interfaces. Retention of radiolabeled P17 in washed quartz sand was evaluated at 100, 84, and 46% water saturation. At the completion of each experiment, the porous medium was extruded and sampled directly for cell retention on the basis of a radiolabel mass balance. Maximum cell retention occurred in the top centimeter of porous media at all three water contents and decreased with depth in the column. The total fraction of cells retained (R-t) was inversely proportional to water content, with nearly twice the cell retention at 46% saturation (R-t - 0.95) compared to retention in 100% water-saturated experiments (R-t = 0.50). Total retained cells were further divided into strongly and weakly attached fractions by settling a sample of the porous medium through groundwater to dislodge loosely adhering cells. Cells that became suspended in the solution represented the fraction retained at the gas-liquid interface or weakly attached to the solid-liquid interface (R-g). Those that remained attached to the porous medium were defined as cells strongly attached to the solid-liquid interface (R-s). Values of R-g/R-t were inversely related to water content, while R-s/R-t decreased with decreasing saturation. Bacteria thus preferentially accumulated at the gas-liquid interface with total cell removal inversely proportional to water content. The increased retention of bacteria at the gas-liquid interface indicates the presence of the interface is an important factor in limiting pathogen migration, evaluating biocolloid-facilitated transport of pollutants, and developing bioremediation strategies for unsaturated porous media. (C) 1999 Elsevier Science B.V. All rights reserved.