A NUMERICAL INVESTIGATION INTO FACTORS AFFECTING GAS AND AQUEOUS-PHASE PLUMES IN THE SUBSURFACE

An investigation into the fate and transport of volatile organic compo unds (VOCs) in the subsurface requires the consideration of contaminan t mass in both the aqueous and soil gas phases. As a result of water/g as phase partitioning, contaminated soil gas can act as a source of gr ound water pollution. Conversely, soil gas can be contaminated by part itioning from underlying ground water VOC plumes. This soil gas and aq ueous phase interaction has motivated the popularity of soil gas sampl ing technology as a method of characterizing ground water VOC contamin ation. A finite-element-based numerical model was developed to accurat ely simulate the interaction between the soil gas phase and the aqueou s phase. This interaction is complicated since the saturation of the a queous phase varies dramatically across the capillary fringe. The two- phase flow equations for gas and water are used to describe the flow r egime, while the advective-dispersive transport of the VOC is consider ed in both phases. Dissolution and volatilization from a non-mobile no n-aqueous phase liquid is included as a volatile organic contaminant s ource. A deforming mesh allows the model to accurately track the water table movement, and a Eularian-Lagrangian formulation is used to cont rol some of the numerical difficulties associated with the numerical s olution of the advection-dispersion equation. An investigation into di ffusion of a VOC from below the water table demonstrated that both the frequency and the magnitude of water table fluctuations have a profou nd influence on the degree of soil gas contamination. Two-dimensional large-scale, long-term simulations were performed to estimate the aque ous and soil gas phase plumes resulting from an immobilized trichloroe thylene residual located in the unsaturated zone. The simulation resul ts indicate that these plumes are very sensitive to the vertical posit ion of the contaminant source. In addition, it was determined that sea sonal fluctuations in soil gas VOC concentrations are primarily contro lled 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.