Simultaneous Transport of Two Bacterial Strains in Intact Cores from Oyster, Virginia: Biological Effects and Numerical Modeling

Abstract

The transport characteristics of two adhesion-deficient, indigenous groundwater strains, Comamonas sp. strain DA001 and Erwinia herbicola OYS2-A, were studied by using intact sediment cores (7 by 50 cm) from Oyster, Va. Both strains are gram-negative rods (1.10 by 0.56 and 1.56 by 0.46 m, respectively) with strongly hydrophilic membranes and a slightly negative surface charge. The two strains exhibited markedly different behaviors when they were transported through granular porous sediment. To eliminate any effects of physical and chemical heterogeneity on bacterial transport and thus isolate the biological effect, the two strains were simultaneously injected into the same core. DA001 cells were metabolically labeled with 35 S and tagged with a vital fluorescent stain, while OYS2-A cells were metabolically labeled with 14 C. The fast decay of 35 S allowed deconvolution of the two isotopes (and therefore the two strains). Dramatic differences in the transport behaviors were observed. The breakthrough of DA001 and the breakthrough of OYS2-A both occurred before the breakthrough of a conservative tracer (termed differential advection), with effluent recoveries of 55 and 30%, respectively. The retained bacterial concentration of OYS2-A in the sediment was twofold higher than that of DA001. Among the cell properties analyzed, the statistically significant differences between the two strains were cell length and diameter. The shorter, larger-diameter DA001 cells displayed a higher effluent recovery than the longer, smaller-diameter OYS2-A cells. CXTFIT modeling results indicated that compared to the DA001 cells, the OYS2-A cells experienced lower pore velocity, higher porosity, a higher attachment rate, and a lower detachment rate. All these factors may contribute to the observed differences in transport.Understanding bacterial transport in porous media is of great importance for successful implementation of bioremediation strategies in subsurface environments. One vital requirement for successful implementation of bioaugmentation, the injection of bacteria with degradative capabilities into the subsurface to remediate a contaminated aquifer (30,36,43), is the delivery of selected bacteria to and through the contaminated zone of an aquifer. In most bacterial transport studies, bacterial attachment to mineral grains significantly impairs effective dispersion of the introduced bacteria throughout the aquifer (8,23,25). Bacterial attachment to mineral grain surfaces is influenced by biological factors associated with the bacterial cells and by physical and chemical factors associated with a granular aquifer (12,17,22,34).The physical, chemical, and biological effects are typically grouped into two probabilities, as described in filtration theory: the probability of bacterial collision with a sediment grain collector upon approach (collector efficiency) and the probability of bacterial attachment to the collector upon collision (collision efficiency). The collector efficiency accounts for the physical factors (interceptio...

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