Hydraulic conductivity reduction in surfactant-enhanced aquifer remediation due to emulsification

Failures of surfactant-enhanced aquifer remediation in the field are often attributed to the reduction of hydraulic conductivity. Conductivity may be altered due to changes in liquid properties, in porous medium properties or in both. This research focused on the relative impact of the various mechanisms by which hydraulic conductivity may be reduced. The results of this study show that the addition of surfactant increases the viscosity of the aqueous phase. The impact of temperature was also substantial, increasing the viscosity as the temperature decreased. The solubilization of tetrachloroethylene (PCE) and the subsequent formation of emulsion increased the viscosity and density only slightly. Overall, the changes in viscosity and density can cause a reduction in conductivity ranging from 21 to 26% for a 4% surfactant solution of Witconol 2722 or Witconol SN 120, depending upon the amount of PCE solubilized. However, column experiments involving the flushing of an ex situ generated emulsions of a 4% solution of Witconol 2722 or Witconol SN 120 with PCE showed conductivity reductions on the order of 75--85%. The droplet and pore size measurements suggest that deposition is the main mechanism for reducing the flow pathways. The zeta potential measurements and visual observation from micromodel studies corroborated these results, indicating that the emulsion droplets adhered to the pore walls, forming multilayer deposits which plugged the pores of the porous medium. The model proposed by Soo and Radke (1986) for transport of emulsion was modified to include the dynamics of multilayer deposition to simulate the experiment data. This model was subsequently coupled with the advection-dispersion equation to calculate the formation and transport of an emulsion formed in situ formed emulsion. As a surfactant solution was flushed through columns containing residual saturation of PCE, the experimental and modeling results of the in situ experiments showed that there was hydraulic reduction of 35% even though residual concentration was reduced by 95--98%. This was due primarily to an increase in viscosity, with in situ emulsion formation, only accounting for an 8% reduction in hydraulic conductivity.