Relative dominance of physical versus chemical effects on the transport of adhesion-deficient bacteria in intact cores from South Oyster, Virginia

Bacterial transport experiments were conducted using intact sediment cores collected from sites on the Delmarva Peninsula near South Oyster, VA, to delineate the relative importance of physical and chemical heterogeneity in controlling transport of an adhesion-deficient bacterial strain. Electron microscopy revealed that the sediments consisted of quartz and feldspar with a variable amount of clay and iron and aluminum hydroxide coatings on the grains. A nonmotile, gram-negative indigenous groundwater strain, designated as Comamonas sp. DA001, was injected into the cores along with a conservative tracer bromide (Br). DA001 cells were 1.2 x 0.6 microm in size with a hydrophilic surface and a slightly negative surface charge. Bacterial breakthrough preceded that of Br. This differential advection phenomenon can be accounted for by reduction of the effective porosity for the bacteria relative to Br. The distribution of cells remaining in the core as determined by scintillation counting and phosphor imaging techniques was variable, ranging from nearly uniform concentrations throughout the core to exponentially decreasing concentrations with distance from the point of injection. The fraction of bacterial retention in the core was positively correlated with the abundance of the metal hydroxides and negatively correlated with grain size. Because grain size was inversely correlated with the abundance of the metal hydroxide coatings, it was necessary to separate the effects of grain size and mineralogy. The fraction of the bacterial retention accounting for the effect of grain size, the collision efficiency, exhibited no correlation with the abundance of the metal hydroxides, indicating that the bacterial retention was primarily controlled by grain size. Reasons for the lack of influence of mineralogy on bacterial transport include (i) the slightly negatively charged bacterial surfaces; (ii) an insufficient heterogeneity of sediment surface properties; and (iii) the masking of the positive charge of the metal hydroxide surfaces by adsorbed organic carbon (up to 1180 ppm). This study demonstrates that the laboratory-based bacterial transport experiments are effective in delineating physical versus chemical controlling factors and provide an important link to field-based bacterial transport studies.