AN IMPROVED SHARP-INTERFACE MODEL FOR ASSESSING NAPL CONTAMINATION AND REMEDIATION OF GROUNDWATER SYSTEMS

A numerical model is presented for areal analyses of the three-dimensional (3-D) flow behavior of non-aqueous-phase liquids (NAPL's) in groundwater systems. The model is designed for specific application to chemical or petroleum spills and leaks, and remedial design and evaluation of a NAPL-contaminated site. The mathematical formulation is based on vertical integration of the 3-D two-phase flow equations and incorporation of the modified concept of gravity-segregated vertical equilibrium (GSVE) which yields sharp interfaces separating the zones of mobile NAPL and groundwater. History-dependent pseudo constitutive relations are developed for LNAPL's and DNAPL's (light and dense NAPL's) scenarios taking into account the effects of residual saturations. Owing to the sharp-interface assumption, the soil capillary pressure and relative permeability curves are not needed in the evaluation of pseudo functions. Efficient and mass-conservative nonlinear numerical techniques are adopted for solving the governing equations and treating practical boundary conditions which include injection and recovery wells and trenches. Simulation and application examples are provided to demonstrate verification and utility of the model. Numerical results obtained using the sharp-interface modeling approach are compared with analytical solutions and rigorous multiphase numerical solutions that account for vertical flow components and capillary effects. The verification results show the validity of the GSVE modeling assumptions and accuracy of the proposed formulation and computational schemes in predicting the NAPL recovery. The numerical study also indicates that the present model is highly efficient and is thus suitable for preliminary analyses of site-specific problems with limited data and personal computer resources.