Multiphase flow and transport in fractured clay/sand sequences

A numerical model (Queen's University Multi-Phase Flow Simulator, QUMPFS) w as used to assess the rate of trichloroethylene (TCE) dense, non-aqueous ph ase liquid (DNAPL) migration through fractured clay, with special attention focused on the influence of interbedded sand lenses. The presence of these sand lenses was found to increase the time required for the non-wetting ph ase to migrate through the full 30 m vertical extent of the clay sequence f rom a few days to several years. Applied vertical hydraulic gradients were found to be moderately influential in systems consisting solely of fracture d clays, yet one of the dominant factors controlling speed of vertical migr ation when sand lenses were present. Larger displacement pressure of the sa nds relative to that of the fractures leads to slower DNAPL migration rates , due to the delays that occur during build-up of capillary pressures. Diss olution of DNAPL and subsequent matrix diffusion of the aqueous phase has l ittle effect on the rate of DNAPL migration through systems consisting of f ractured clay only, yet slows the rate of migration in systems containing s and lenses. In all cases examined, the rate of DNAPL loading to the lower a quifer far exceeded the rate of aqueous phase mass loading, It was also fou nd that DNAPL reaches the lower aquifer at approximately the same time as t he aqueous phase plumes even for systems experiencing downward groundwater flow due to the attenuation of the aqueous phase through matrix diffusion. (C) 2001 Elsevier Science B.V. All rights reserved.