The influence of field-scale heterogeneity on the surfactant-enhanced remediation of entrapped nonaqueous phase liquids

Numerical simulation techniques are used to explore the surfactant-enhanced solubilization of a spill of tetrachloroethylene (PCE) entrapped in a heterogeneous, saturated sandy aquifer. A two-dimensional, multicomponent solubilization simulator is employed to simulate spill cleanup, utilizing ensembles of realizations of random, spatially correlated permeability fields. Flushing volumes required to remove the organic liquid and to reduce effluent concentrations to specified levels are used as measures of remediation effectiveness. Simulations illustrate the long-term persistence of nonaqueous phase liquid (NAPL) in low permeability regions and associated tailing of effluent organic concentrations. The tendency of the high concentration organic plumes to migrate downward under gravity forces is also demonstrated, and is shown to enhance migration of aqueous phase organic into regions of low permeability, causing substantially increased concentration tailing. Remediation efficiency is shown to be strongly dependent on spatial correlation structure and variance of the permeability distribution. Delivery of surfactant to low permeability regions is primarily controlled by transverse mixing processes. The influence of local mass transfer limitations on cleanup efficiency, although statistically significant, is found to be relatively minor for the contamination scenarios explored.