UPSCALING OF MASS TRANSFER FROM ENTRAPPED DNAPL ZONES: AN EXPERIMENTAL AND A MODELING STUDY

The slow dissolution of organic components from entrapped nonaqueous phase liquids (NAPLs) causes widespread groundwater contamination. The quantification of the non-equilibrium mass transfer associated with natural dissolution helps to provide estimates of source longevity and the possible extent of groundwater contamination. Non-equilibrium mass transfer strongly depends on several factors such as properties of NAPL and soil physical and chemical heterogeneity. Estimates of mass transfer coefficient depend on the scale of measurement. Up-scaling refers to estimation of parameters at a scale that is larger than the measurement scale. In this paper, an up-scaling method for the determination of rate limited mass transfer coefficient for both natural and surfactant enhanced dissolution is developed. A numerical model that was validated using experimental data is used as a tool for this theoretical development. The model is capable of simulating non-equilibrium mass transfer for both natural and surfactant-enhanced conditions in heterogeneous formations. The mass transfer is represented using linear-driving force first-order kinetic model and the mass transfer coefficients are calculated using Gilland-Sherwood relationship based on a modified Sherwood number. Two candidate models are considered in this study: UTCHEM and modified MODFLOW/MT3D.

Carefully designed experiments of natural and enhanced dissolution of tetrachloroethene or PCE are conducted in a bench-scale, two-dimensional horizontal test tank to generate spatial and temporal PCE concentration profiles and breakthrough curves at various groundwater velocities, entrapment configurations, source sizes, and NAPL saturations. Expected outcome of this research is a validated numerical code that can be used to simulate up-scaled dissolution for both natural and surfactant-enhanced conditions.