Spatial distribution of deposited bacteria following miscible displacement experiments in intact cores

Miscible displacement experiments were performed on intact sand columns ranging from 15 to 60 cm in length to determine whether bacterial deposition varies at the centimeter scale within aquifer sediments. A 1-pore-volume pulse 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 bacteria. Concentrations of sediment-associated (deposited) bacteria varied by up to 2 orders of magnitude in the direction perpendicular to flow demonstrating that bacterial deposition cannot be described mechanistically by a single rate coefficient. Incorporation of a distribution of sediment size and porosity values into Monte Carlo simulations indicates that physical heterogeneities are only partially responsible for the observed variability in deposited bacteria. A simple first-order model (classic filtration theory) adequately described the average spatial distribution of bacteria with depth within the 15-cm column. For the longer columns, however, the average concentration 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 deposited bacteria with depth. A sensitivity analysis was performed with a first-order dual-alpha model to understand the effects of an influent suspension with two subpopulations of bacteria on the decrease of deposited bacteria with flow path length. Numerical simulations show that even for small fractions (0.01) of nonsticky bacteria, the decrease in deposited bacteria may deviate substantially from the exponential decrease expected from colloid-filtration theory. Results from experimental as well as numerical studies demonstrate the importance of column dissections for understanding bacterial deposition in saturated porous media.