A key aspect of the contamination of groundwater by pathogenic organisms is their transport through both unsaturated and saturated soils. An important source of these pathogens is domestic wastewater, which is released to the soil environment from on-site wastewater treatment systems, such as septic tanks. Surfactants—a major component of consumer detergents—are also present in domestic wastewater and have the potential to significantly enhance the transport of microorganisms through both unsaturated and saturated soils. The goal of this study was to explore how the surfactants present in domestic wastewater effect the bacterial transport through unsaturated porous media.

To achieve this goal, the main objective of this study was to examine the effects of the nonionic surfactants Brij 30 and Brij 35 and the anionic surfactant linear alkylbenzene sulfonate (LAS) and their mixtures on the transport of Escherichia coli through laboratory columns with sand and maintained in unsaturated conditions. In order to interpret these effects, it is important to understand the sorption of surfactants and their mixtures at the sand-water, gas-water and bacteria-water interfaces present in the system. Thus, a second objective was to experimentally determine sorption isotherms for these surfactants on the interfaces and to use the understanding of surfactant sorption on interfaces, to interpret the bacterial transport results.

The results showed an increase in bacterial transport with increase in surfactant concentration as an increase in the effluent cell concentration and decrease in the cell breakthrough time was observed in the presence of surfactants. A comparative observation of the effects of the surfactants indicated that LAS had a much greater effect on the bacterial transport than either of the Brij surfactants. These results followed the surfactant sorption isotherms where LAS showed higher adsorption than nonionic surfactants. The increase in cell transport appears to be due to increased repulsion between the bacteria and the gas-water and sand-water interfaces. This repulsion both decreases bacterial adhesion to the interfaces and pushes the bacteria into the higher fluid velocity region in the center of the flow paths. Overall, this study provides key insight into the effects of surfactants and their mixtures on bacterial transport through unsaturated porous media.