Spatial distribution of deposited bacteria following miscible displacementexperiments in intact cores

Miscible displacement experiments were performed on intact sand columns ran ging from 15 to 60 cm in length to determine whether bacterial deposition v aries at the centimeter scale within aquifer sediments. A 1-pore-volume pul se of radiolabeled cell suspension was introduced into the columns followed by a 2-pore-volume flush of artificial groundwater. The columns were then drained and dissected along the axis of flow. At similar to 1-cm intervals, nine samples were removed for the enumeration of sediment-associated bacte ria. Concentrations of sediment-associated (deposited) bacteria varied by u p to 2 orders of magnitude in the direction perpendicular to flow demonstra ting that bacterial deposition cannot be described mechanistically by a sin gle rate coefficient. Incorporation of a distribution of sediment size and porosity values into Monte Carlo simulations indicates that physical hetero geneities are only partially responsible for the observed variability in de posited bacteria. A simple first-order model (classic filtration theory) ad equately described the average spatial distribution of bacteria with depth within the 15-cm column. For the longer columns, however, the average conce ntration of deposited bacteria did not decrease exponentially with depth. A second-order model, modified to include an influent suspension of bacteria consisting of two subpopulations with separate sticking efficiencies (dual -alpha population), was required to describe the observed decreases of depo sited bacteria with depth. A sensitivity analysis was performed with a firs t-order dual-alpha model to understand the effects of an influent suspensio n with two subpopulations of bacteria on the decrease of deposited bacteria with flow path length. Numerical simulations show that even for small frac tions (0.01) of nonsticky bacteria, the decrease in deposited bacteria may deviate substantially from the exponential decrease expected from colloid-f iltration theory. Results from experimental as well as numerical studies de monstrate the importance of column dissections for understanding bacterial deposition in saturated porous media.