Development of radiographic and microscopic techniques for the characterization of bacterial transport in intact sediment cores from Oyster, Virginia

Abstract

The objective of this study was to ascertain the physical and mineralogical properties responsible for the retention of bacteria in subsurface sediments. The sediment core chosen for this study was a fine-grained, quartz-rich sand with minor amounts of Fe and Al hydroxides. A bacterial transport experiment was performed using an intact core collected from a recent excavation of the Butler’s Bluff member of the Nassawadox formation in the borrow pit at Oyster, VA. and a 14C-labeled bacterial strain OYS2-A was selected for its relatively low adhesion. After the bacterial breakthrough was observed in the effluent, the intact core was dissected to determine the internal distribution of the injected bacteria retained in the sediment. The sediment was dried, epoxy fixed, and thin sectioned. The distribution of 14C activity in the thin sections was mapped using a phosphor screen and X-ray film. The remainder of the core was subsampled and the 14C activity of the subsamples was determined by liquid scintillation counting. The phosphor imaging technique was capable of directly imaging the distribution of radiolabeled bacteria in thin sections, because of its high sensitivity and linear response over a large activity range. The phosphor imaging signal intensity was utilized as a measure of bacterial concentration. The distribution of bacteria at the millimeter scale in the thin sections was compared to the grain size, porosity, and mineralogy as measured by scanning electron microscopy (SEM) and energy dispersive spectrum (EDS) analyses. No apparent correlation was observed between the retention or collision efficiency of bacteria in the sediment and the amount of Fe and Al hydroxides. This apparent lack of correlation can be qualitatively explained by combination of several factors including a nearly neutral surface charge of the bacterial strain, and texture of the Fe and Al hydroxides in the sediment. The combination of phosphor imaging with SEM-EDS proved to be a robust method for relating the physical and mineralogical microscopic properties of poorly indurated sediment to the distribution of adsorbed bacteria, allowing bacterial retention mechanisms to be unambiguously unraveled.