NONAQUEOUS PHASE LIQUID TRANSPORT IN VARIABLY SATURATED HETEROGENEOUS POROUS FORMATIONS

Groundwater contamination by organic chemicals in the form of nonaqueous phase liquids (NAPLs) is a widespread problem which poses a serious threat to groundwater resources. We examined the influence of spatial variability of porous formation properties on the transport of volatile NAPLs in the subsurface environment. The flow and transport problem was simulated using a two-dimensional multiphase numerical model to solve the equations for aqueous and gas phase flow and transport. The input stochastic variables were the permeability, and the hydraulic parameters. The results illustrate the importance of the statistical characteristics of heterogeneous porous media on the mass transfer processes and the NAPL transport in the subsurface. First, by assuming equilibrium mass transfer processes, it has been demonstrated through numerical simulations of 25 realizations of spatially correlated and cross-correlated random permeability and the hydraulic parameter fields that like the case for homogeneous media most of the NAPL mass dissolved and volatilized diffuses to the atmosphere, but the amount of NAPL mass accumulating in the ground water is significantly larger compared to the homogeneous case with most of the NAPL mass accumulating under the water table. This results in greater spreading of the NAPL plumes under the water table. The simulation results also suggest that the spatial variability of the permeability field has greater influence on the NAPL plume spreading than that of the retention curves and the relative permeability-saturation relations. The results also show significant variations from realization to realization. The results of nonequilibrium simulations indicate that the influence of porous formation heterogeneity has a similar trend with equilibrium simulations but the removal rate of the residual NAPL is slower and the NAPL mass tends to remain in the domain for much longer time.