Investigation of surfactant-enhanced dissolution of entrapped nonaqueous phase liquid chemicals in a two-dimensional groundwater flow field

Because of their low solubility, waste chemicals in the form of nonaqueous phase liquids (NAPLs) that are entrapped in subsurface formations act as lo ng-term sources of groundwater contamination. In the design of remediation schemes that use surfactants, it is necessary to estimate the mass transfer rate coefficients under multi-dimensional flow fields that exit at field s ites. In this study, we investigate mass transfer under a two-dimensional f low field to obtain an understanding of the basic mechanisms of surfactant- enhanced dissolution and to quantify the mass transfer rates. Enhanced diss olution experiments in a two-dimensional test cell were conducted to measur e rates of mass depletion from entrapped NAPLs to a flowing aqueous phase c ontaining a surfactant. In situ measurement of transient saturation changes using a gamma attenuation system revealed dissolution patterns that are af fected by the dimensionality of the groundwater flow field. Numerical model ing of local flow fields that changed with time, due to depletion of NAPL s ources, enabled the examination of the basic mechanisms of NAPL dissolution in complex groundwater systems. Through nonlinear regression analysis, mas s transfer rates were correlated to porous media properties, NAPL saturatio n and aqueous phase velocity. Results from the experiments and numerical an alyses were used to identify deficiencies in existing methods of analysis t hat uses assumptions of one-dimensional flow. homogeneity of aquifer proper ties, local equilibrium and idealized transient mass transfer. (C) 2001 Els evier Science B.V. All rights reserved.