A numerical investigation into factors affecting gas and aqueous phase plumes in the subsurface

An investigation into the fate and transport of volatile organic compounds (VOCs) in the subsurface requires the consideration of contaminant mass in both the aqueous and soil gas phases. As a result of water/gas phase partitioning, contaminated soil gas can act as a source of ground water pollution. Conversely, soil gas can be contaminated by partitioning from underlying ground water VOC plumes. This soil gas and aqueous phase interaction has motivated the popularity of soil gas sampling technology as a method of characterizing ground water VOC contamination. A finite-element-based numerical model was developed to accurately simulate the interaction between the soil gas phase and the aqueous phase. This interaction is complicated since the saturation of the aqueous phase varies dramatically across the capillary fringe. The two-phase flow equations for gas and water are used to describe the flow regime, while the advective-dispersive transport of the VOC is considered in both phases. Dissolution and volatilization from a non-mobile non-aqueous phase liquid is included as a volatile organic contaminant source. A deforming mesh allows the model to accurately track the water table movement, and a Eularian-Lagrangian formulation is used to control some of the numerical difficulties associated with the numerical solution of the advection-dispersion equation. An investigation into diffusion of a VOC from below the water table demonstrated that both the frequency and the magnitude of water table fluctuations have a profound influence on the degree of soil gas contamination. Two-dimensional large-scale, long-term simulations were performed to estimate the aqueous and soil gas phase plumes resulting from an immobilized trichloroethylene residual located in the unsaturated zone. The simulation results indicate that these plumes are very sensitive to the vertical position of the contaminant source. In addition, it was determined that seasonal fluctuations in soil gas VOC concentrations are primarily controlled by temperature fluctuations, while ground water VOC concentration fluctuations are primarily a result of infiltration fluctuations. (C) 1997 Elsevier Science B.V. (C) 1997 Elsevier Science B.V.