Change of Collision Efficiency with Distance in Bacterial Transport Experiements

Previous bacterial transport studies have shown decreased bacterial adhesion with transport distance, largely based on laboratory core experiments. An inferred effect of microbial population variability is invoked to interpret experimental data, but there lacks direct measurement at field-scale, especially in correlation of transport distance with change of bacterial surface properties. This study was undertaken to determine change of collision efficiency with transport distance, taking advantage of the bacterial transport experiment in Oyster, VA in the summer of 2001. Upon injection of an adhesion deficient strain, Comamonas sp. DA001 into a up-gradient well, bacterial samples were taken from multi-level samplers along the flow path, and were injected into cores of 40 cm in length and 7.5 cm in diameter packed with homogenized sediment from the same site, South Oyster focus area (SOFA). Bacterial suspension samples were also measured for bacterial electrophoretic mobility distribution. Using filtration theory, collision efficiency, the probability of bacterial attachment to the grain surfaces upon collision and a quantitative measure of bacterial adhesion, was determined using CXTFIT model fitted attachment rate, measured grain size (10th percentile), porosity, flow velocity, and collector efficiency. Collision efficiency was also determined based on the fraction of retention in the cores. Contrary to previous results and interpretation of field-scale breakthrough curves, our experimentally determined collision efficiency increases with transport distance in the core experiments, which correlates with increasingly negative surface charge of the injected bacteria. Therefore we conclude that the apparent decrease in adhesion with transport distance in the field is strongly controlled by field-scale heterogeneity in physical and chemical aquifer properties and not by microbial population heterogeneity.