Flow behavior and residual saturation formation of liquid carbon tetrachloride in unsaturated heterogeneous porous media

The formation of residual, discontinuous nonaqueous phase liquids (NAPLs) in the vadose zone is a process that is not well understood. To obtain data that can be used to study the development of a residual NAPL saturation in the vadose zone and to test current corresponding models, detailed transient experiments were conducted in intermediate-scale columns and flow cell. The column experiments were conducted to determine residual carbon tetrachloride (CCl(4)) saturations of two sands and to evaluate the effect of CCl(4) vapors on the water distribution. In the intermediate-scale flow cell experiment, a rectangular zone of the fine-grained sand was packed in an otherwise medium-grained matrix. A limited amount of CCl(4) was injected from a small source and allowed to redistribute until a pseudo steady state situation had developed. A dual-energy gamma radiation system was used to determine fluid saturations at numerous locations. The experiments clearly demonstrated the formation of residual CCl(4) saturations in both sands. Simulations with an established multifluid flow simulator show the shortcomings of current relative permeability-saturation-capillary pressure (k-S-P) models. The results indicate that nonspreading behavior of NAPLs should be implemented in simulators to account for the formation of residual saturations.