PNNL

Abstract

Bacterial transport experiments were conducted using intact sediment cores collected near South Oyster, VA to delineate the relative importance of physical and chemical heterogeneity in controlling transport of an adhesion-deficient bacterial strain. The sediments consisted of quartz and feldspar with a variable amount of clay and metal 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). Bacterial breakthrough preceded the arrival 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 was highly variable, ranging from nearly uniform concentrations to exponentially decreasing concentrations. 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 correlated with the abundance of the metal hydroxide coatings, it was difficult to separate the effects of grain size and mineralogy. The fraction of the bacterial retention accounted for by the effect of grain size 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 retention include 1) the slightly negatively charged bacterial surfaces; 2) insufficient heterogeneity of sediment surface properties; and 3) 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.