WATER SATURATION AND SURFACTANT EFFECTS ON BACTERIAL TRANSPORT IN SAND COLUMNS

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Abstract
Bacterial breakthrough curves from clean sand columns were used in two related studies of the effects of unsaturated flow and added surfactant on cell transport. In the first study, step-inputs of bacteria were used to evaluate the effects of water saturation (saturated and unsaturated condition), surfactant concentration (0 and 20 μmol/L sodium dodecyl-benzene sulfonate (DDBS)), and bacterial strain (aquifer isolates E3W7 and W31) on the kinetics of cell deposition and entrainment. These surfactant concentrations and bacterial strains did not have significant effects on the rate coefficients. Unsaturated columns, however, always had delayed cell breakthrough compared with saturated columns, and the deposition rate coefficient was significantly greater in unsaturated conditions compared with saturated conditions. After the outflow concentration of E3W7 from the saturated columns reached the inflow concentration, the columns were partially desaturated. This resulted in a rapid drop in outflow concentration-a decline of 29% with surfactant and 78% without surfactant. A second study was conducted to investigate the mechanism of surfactant-enhancement of bacterial transport by applying 100 μmol/L DDBS before a pulse of W31, concurrent with the pulse, following the pulse, at all times, or at no time. Only the presence of surfactant at all times and concurrently with the cell pulse resulted in significantly greater cell recovery compared with the no-surfactant control. This suggests that 100 μmol/L DDBS interacted with the cells to reduce their adsorption to gas-water interfaces in unsaturated sand.