The influence of NAPL dissolution characteristics on field-scale contaminant transport in subsurface

A numerical model based on the two-phase method of characteristics (2PMOC) is developed to account for rate-limited dissolution processes and to examine the groundwater and soil gas contamination resulting from an immobile non-aqueous phase liquid (NAPL) residual in water-saturated and variably unsaturated field-scale systems. Recently developed dissolution rate coefficient correlation formulations are used to simulate the rate-limited dissolution processes. One-dimensional aqueous simulations are performed to examine differences between these correlations 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. Mathematical models are also used to simulate two-dimensional two-phase flow and contaminant transport to evaluate the potential of groundwater contamination resulting from a NAPL residual. 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.