TRANSPORT OF BACTERIAL PROBES IN THE SUBSURFACE: PHYSICOCHEMICAL AND BIOLOGICAL FACTORS, OR "COLLOIDS WOULDN'T DO THAT!"

Bacteria can be used as probes in combination with dissolved tracers to elucidate transport-related aquifer properties. For example, physical heterogeneity can be inferred by observing differential advection of bacteria (or other colloids) relative to dissolved tracers, as illustrated by field experiments conducted at Kamas and Fry Canyon, Utah. Bacterial breakthrough-elution curves also provide information regarding the processes that control bacterial loss from, and re-entrainment into, groundwater. A high-resolution bacterial tracking technique was used to focus on the low concentration portions of bacterial breakthrough and elution histories (initial breakthrough and extended tailing) during DOE-sponsored field transport experiments conducted at Oyster, Virginia. The rates of bacterial loss from, and re-entrainment into, groundwater were shown to be controlled by both physicochemical and biological factors during these relatively long duration-large scale field experiments. Physicochemical factors that operated during the transport experiments included variations in hydrodynamic shear (pore water velocity), and potentially included hydrodynamic collision between mobile and attached cells. Biological factors that influenced transport included heterogeneity in cell surface properties, as well as predation. An additional biological factor that potentially impacted transport was cell growth. Much remains to be understood regarding the above, and other, controls on bacterial transport. However, understanding these controls is important and necessary for a variety of reasons, such as the potential use of bacterial probes to assay properties of the subsurface, as well as the need to deliver bacteria with novel metabolic properties to specific subsurface locales.