Numerical modelling of the impact of surfactant partitioning on surfactant-enhanced aquifer remediation

The partitioning of surfactants into non-aqueous phase liquids (NAPLs) during Surfactant-Enhanced Aquifer Remediation (SEAR) is potentially an important and non-negligible phenomenon that can strongly impact remediation efficiency. This paper numerically investigates the impact of surfactant partitioning on the enhanced NAPL dissolution and mobilization mechanisms and the overall NAPL removal from the subsurface. For demonstration, a multiphase model is used to simulate a hypothetical SEAR consisting of Triton X100 surfactant solution for the removal of perchloroethylene (PCE) entrapped in contaminated porous medium at the core/column scale. The simulations are conducted for two-dimensional homogenous and three-dimensional heterogeneous systems. By simultaneously incorporating spatial heterogeneity of porous media, injection rate, and endpoint mobility ratio into the model, we delineate the interplay of surfactant partitioning with flow and transport dynamics. Our results show that surfactant partitioning from the aqueous phase across the interface to the NAPL phase can undermine both efficiency of the enhanced dissolution and mobilization of NAPL species. This undermining is more pronounced for when aqueous phase mobility is less than the mobility of the NAPL phase. For such conditions interfacial tension between the two phases is reduced less for partitioning than non-partitioning cases (due to loss of surfactant into NAPL phase) and a secondary water front is formed due to partitioning that makes aqueous phase breaks through earlier.