NONAQUEOUS PHASE LIQUID DISSOLUTION CHARACTERISTICS AND LARGE-SCALE CONTAMINANT TRANSPORT IN SUBSURFACE

Groundwater contamination by organic chemicals in the form of nonaqueous phase liquids (NAPLs) is a widespread threat to groundwater resources. NAPLs in the subsurface can become a long-term source of groundwater contamination as a result of slow NAPL-aqueous phase mass transfer. Over the last several years some significant advances have been made in determining NAPL-aqueous phase mass transfer (dissolution) characteristics. However, most of these studies have been conducted in simple one-dimensional, relatively homogeneous columns at small laboratory scales. The main objective of this study is to answer at least partially to the questions such as: How significant are differences among existing mass transfer rate correlations for large-scale problems even though many of these models showed very similar predictions over mass transfer rate at laboratory scales? A numerical model based on the 2-phase method of characteristics is developed to incorporate rate-limited dissolution processes and to examine the groundwater and soil gas contamination resulting from an immobile NAPL residual in water-saturated and variably unsaturated large-scale systems. A few recently developed dissolution rate coefficient correlation formulations are used to represent the rate-limited dissolution processes. One-dimensional aqueous phase simulations are performed to examine the differences between these correlation formulations by comparing effluent concentrations. Damkohler number analysis is used to determine degree of equilibrium for each correlation and extent of error introduced by assuming local equilibrium between the NAPL and aqueous phases. The numerical models are also used to simulate two-dimensional two-phase flow and contaminant transport to evaluate the potential of groundwater contamination resulting from residual NAPLs. While the simulation results illustrate the significance of the system scale on the rate of mass transfer between phases and the degree of equilibrium, the exact extent of the system scale effects remains to be answered. The modeling results from this study suggest more efforts to address the differences between laboratory and field-scale dissolution characteristics before a mature understanding of NAPL dissolution processes in subsurface can be achieved.