DIRECT DETERMINATION OF CHANGE OF BACTERIAL COLLISION EFFICIENCY WITH TRANSPORT DISTANCE IN FIELD SCALE BACTERIAL TRANSPORT EXPERIMENTS

Previous laboratory column and field injection bacterial transport experiments have observed an apparent decrease in bacterial adhesion with transport distance. These observations have been hypothesized as resulting from variability in cell surface properties within a monoclonal population that give rise to preferential transport of a subpopulation. However, there lacks direct measurements of changes in bacterial adhesion properties as a function of transport distance. This study was undertaken to directly measure changes in collision efficiency as a function of transport distance at the South Oyster field site near Oyster, VA. Following injection of an adhesion deficient strain, Comamonas sp. DA001, into a up-gradient well, bacterial samples were taken from multi-level samplers at various distances along the flow path, and were injected into columns (40 cm in length and 7.5 cm in diameter) packed with homogenized sediment collected from the same site. Electrophoretic mobilities were measured for each bacterial suspension. Collision efficiencies were determined from the column and field breakthrough data. The collision efficiencies estimated from field breakthrough generally decreased (did not significantly increase) with transport distance, as expected based on previous studies, whereas the collision efficiencies estimated from column breakthrough greatly increased with increased transport distance. Bacterial cell surface charge became progressively more negative for cells collected at greater distance in the field. We hypothesize that the apparent contradiction between field and column collision efficiencies was caused by difference in the degree of exposure of transported cells to quartz and iron oxide surfaces in the field and column sediment systems, and that the character of field-scale heterogeneity (geochemical and physical) plays a significant role in controlling the transport of bacteria. In the heterogeneous field system, the injected cells may be able to bypass surface sites favorable for attachment, whereas in the column system, the cells may be forced to interact with those sites. The different transport mechanisms for field and column systems may in part explain the observed difference between the two systems in collision efficiency as a function of transport distance.