Impact of surfactants for aquifer remediation on physical properties of the aqueous phase

Surfactant-enhanced aquifer remediation (SEAR) has shown great promise in laboratory studies as a technique for removing nonaqueous phase liquids (NAPLs) from soils. Failures of SEAR in the field are often attributed to the reduction of conductivity of the soil. Conductivity may be altered due to changes in liquid properties, in porous medium properties or in both. This study focuses on the changes in liquid properties, specifically viscosity and density, in systems comprising aqueous solutions of surfactants selected for their solubilization potential of NAPLs that are denser than water, such as tetrachloroethylene (PCE). Measurements showed that the viscosity of the surfactant solutions increased with surfactant concentration, with the increase being considerably greater with the addition of ethanol as a co-surfactant. The impact of temperature was also substantial, with the viscosity increasing as the temperature decreased. The solubilization of PCE and the subsequent formation of a dilute emulsion also increased the viscosity, but only slightly. This increase could be predicted using equations developed for calculating the viscosity of emulsions, even at organic liquid concentrations below the point of emulsion formation. The density increased somewhat with the solubilization of PCE in the surfactant solution. This increase could be estimated from a weighted-volume average of the densities of PCE and the surfactant solution. The resultant reductions in conductivity calculated based on the measured changes in viscosity and density ranged from about 26% to 73% at a surfactant concentration of 4% and a PCE concentration of 40–50 g/l, depending on the temperature, the type of surfactant and the ethanol concentration. Since some laboratory studies, particularly those employing fine-grained soils, have reported reductions in conductivity greater than those determined here, changes in porous medium properties must contribute to conductivity reductions as well.