Impact of spatially distributed nonaqueous phase liquid saturation and water content on soil vapor extraction in heterogeneous porous media

Abstract

The spatial distribution of nonaqueous phase liquid (NAPL) saturation, water content, and soil permeability determines the pore-scale processes that control soil vapor extraction (SVE) and the time scales for cleanup. It is now understood that there are three forms of NAPL phase in the vadose zone: free NAPL that is mobile and in contact with gas, residual NAPL that is immobile and in contact with gas, and trapped NAPL that is immobile and surrounded by water. We use a new k-S-P constitutive model that considers all three NAPL forms. The multiphase flow simulator (STOMP), which includes this new constitutive model, is used to distribute NAPL in heterogeneous porous media. For soil vapor extraction, we describe a conceptual model that distinguishes rate-limited mass transfer of trapped NAPL from equilibrium partitioning of free NAPL. The current STOMP, which does not include this new conceptual model for SVE, is first used to distribute NAPL in a stratified system, and then to simulate NAPL removal during soil vapor extraction (SVE). Simulation results show that the amount of trapped NAPL depends on water saturation and soil permeability, and the formation of residual NAPL has an influence on the amount of NAPL retained in the unsaturated zone due to the reduced NAPL relative permeability. SVE results show that it is necessary to consider the change of water saturation and NAPL forms over time in order to simulate mass transfer mechanisms from various forms of NAPL.