REMOVAL OF CARBON TETRACHLORIDE FROM LAYERED POROUS MEDIA SYSTEMS IN INTERMEDIATE-SCALE SYSTEMS: EXPERIMENTS AND NUMERICAL SIMULATIONS

A series of intermediate-scale flow cell experiments has been conducted to identify the mechanisms that govern the removal of carbon tetrachloride (CT) during soil vapor extraction (SVE). Numerical model simulations have been compared with detailed data sets from intermediate-scale flow cell experiment. The flow cell was packed with configurations of fine-grained sand layers embedded in coarse-grained sand matrices. CT was injected at the top of the flow cell from single or multiple sources and allowed to redistribute in the variably saturated systems. A sloping water table, resulting in water movement in the saturated zone, was imposed near the bottom. A dual-energy gamma radiation system was used to determine CT saturations profiles and CT gaseous concentrations at the outlet of the flow cell and 15 sampling ports inside the flow cell were measured during subsequent CT removal using SVE. Several SVE schemes were used, including targeted removal near the water table to prevent CT vapors to migrate from the gas phase into the capillary fringe. Results show that CT mass was removed relatively quickly in the coarse-grained sand, followed by a slow removal from the fine-grained sands. The observed concentration tailing was mainly due to diffusion from the fine-grained sand layer to the coarse-grained sand zone. Comparison of experimental and numerical simulation results demonstrate that proper numerical modeling of CT removal through SVE can be achieved using equilibrium evaporation as long as detailed fine-scale knowledge of the CT distribution and physical heterogeneity is incorporated into the model. However, it was shown for one of the experiments that CT removal could also be accurately fitted by a first-order mass transfer analytical model. This potential ambiguous result might potentially lead to an erroneous conclusion that the long-term tailing in the experiment was kinetically controlled due to rate-limited NAPL transfer into the gaseous phase.