WATER SATURATION AND SURFACTANT EFFECTS ON BACTERIAL TRANSPORT IN SAND COLUMNS

Bacterial breakthrough curves from clean sand columns were used in two related studies of the effects of unsaturated now and added surfactan t on cell transport. In the first study, step-inputs of bacteria were used to evaluate the effects of water saturation (saturated and unsatu rated condition), surfactant concentration (0 and 20 mu mol/L sodium d odecyl-benzene sulfonate (DDBS)), and bacterial strain (aquifer isolat es E3W7 and W31) on the kinetics of cell deposition and entrainment. T hese surfactant concentrations and bacterial strains did not have sign ificant 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 uns aturated conditions compared with saturated conditions. After the outf low concentration of E3W7 from the saturated columns reached the inflo w concentration, the columns were partially desaturated. This resulted in a rapid drop in outflow concentration-a decline of 29% with surfac tant and 78% without surfactant. A second study was conducted to inves tigate the mechanism of surfactant-enhancement of bacterial transport by applying 100 mu mol/L DDBS before a pulse of W31, concurrent with t he pulse, following the pulse, at all times, or at no time. Only the p resence of surfactant at all times and concurrently with the cell puls e resulted in significantly greater cell recovery compared with the no -surfactant control. This suggests that 100 mu mol/L DDBS interacted w ith the cells to reduce their adsorption togas-mater interfaces in uns aturated sand.