Enhancing bacterial transport for bioaugmentation of aquifers using low ionic strength solutions and surfactants

The transport of bacteria in contaminated aquifers over a distance of only one meter can require tens to thousands of unsuccessful collisions of bacteria with soil grains. Previous work has shown that low ionic strength (IS) solutions and the nonionic surfactant Tween 20 can reduce bacterial adhesion to ultraclean surfaces such as glass and quartz porous media. In this study, we examined whether these results could be generalized to soils and to other surfactants, by measuring the retention of two species of radiolabeled microbes over short (1 cm) distances in soil minicolumns. Calculations were also made, using the clean-bed filtration theory, to evaluate if bacterial transport distances are sufficient for bioagumentation to occur over a large region of the subsurface. Collision efficiencies were expressed using the filtration model in terms of the sticking coefficient, α, defined as the fraction of collisions that are successful. In glass bead columns, α's for monoclonal populations were reduced from α = 0.19 (Alcaligenes paradoxus) and α = 0.01 (CD1), to α < 0.008 for Tween 80-phosphate buffer solutions and α < 0.0054 for low ionic strength (0.01 mM) solutions (Darcy velocity, U = 10^-3 m s^-1; Hammaker constant = 10^-20 J; and fluid properties of water at 22°C). Low α's were also obtained using other nonionic surfactants (Tween 80, Triton 100 and 705, POE-10, Brij + 35) and an anionic biosurfactant, all added at concentrations above their critical micelle concentration (CMC). Although sticking coefficients were also reduced by an order-of-magnitude for natural soils, sticking coefficients remained too high to permit wide dispersal of cells over distances of > 1 m. For A. paradoxus, α was reduced using a low ionic strength solution from 0.72 to 0.083 for the Arizona soil and from 1.7 to 0.2 for the Ringold soil; for CD1, α was reduced from 0.57 to 0.09 for the Ringold soil. Based on the soil grain diameters of these soils (127 μm, Arizona soil; 224 μm, Ringold soil), α's in this range will permit transport distances (defined as 99.9% reduction in cell concentration) of ~ 1 m (U = 10 m/d, 10°C) which may be sufficient for creating small, bioactive zones. However, in order to increase bacterial transport over distances > 1 m, methods other than simple solution chemistry changes will be needed to enhance aquifer bioaugmentation operations.