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

The objective of this study was to ascertain the physical and mineralogical properties responsible for the retention of bacteria in subsurface sedimen ts. 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 exp eriment was performed using an intact core collected from a recent excavati on of the Butler's Bluff member of the Nassawadox formation in the borrow p it at Oyster, VA. and a C-14-labeled bacterial strain OYS2-A was selected f or its relatively low adhesion. After the bacterial breakthrough was observ ed in the effluent, the intact core was dissected to determine the internal distribution of the injected bacteria retained in the sediment. The sedime nt was dried, epoxy fixed, and thin sectioned. The distribution of C-14 act ivity in the thin sections was mapped using a phosphor screen and X-ray fil m. The remainder of the core was subsampled and the C-14 activity of the su bsamples was determined by Liquid scintillation counting. The phosphor imag ing technique was capable of directly imaging the distribution of radiolabe led bacteria in thin sections, because of its high sensitivity and linear r esponse over a large activity range. The phosphor imaging signal intensity was utilized as a measure of bacterial concentration. The distribution of b acteria at the millimeter scale in the thin sections was compared to the gr ain size, porosity, and mineralogy as measured by scanning electron microsc opy (SEM) and energy dispersive spectrum (EDS) analyses. No apparent correl ation was observed between the retention or collision efficiency of bacteri a in the sediment and the amount of Fe and Al hydroxides. This apparent lac k of correlation can be qualitatively explained by combination of several f actors including a nearly neutral surface charge of the bacterial strain, a nd 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 sedi ment to the distribution of adsorbed bacteria, allowing bacterial retention mechanisms to be unambiguously unraveled. (C) 1999 Elsevier Science B.V. A ll rights reserved.