Experimental study on retardation of a heavy NAPL vapor in partially saturated porous media

NAPL contaminants introduced into the unsaturated zone spread as a liquid phase; however, they can also vaporize and migrate in a gaseous state. Heavy vapors preferentially migrate downward due to their greater density and, thus, pose a potential threat to underlying aquifers. Large-scale column experiments were performed to quantify partitioning processes responsible for the retardation of carbon disulfide (CS2) vapor in partially saturated porous media. The results were compared with a theoretical approach taking into account the partitioning into the aqueous phase. The experiments were conducted in large, vertical columns (i.d. = 0.109 m) of 2 m length packed with different porous media. A slug of CS2 vapor and the conservative tracer argon was injected at the bottom of the column followed by a nitrogen chase. Different seepage velocities were applied to characterize the transport and to evaluate their impact on retardation. Concentrations of CS2 and argon were measured at the top outlet of the column using two gas chromatographs. The temporal-moment analysis for step input was employed to evaluate concentration breakthrough curves and to quantify diffusion/dispersion and retardation. The experiments conducted showed a pronounced retardation of CS2 in moist porous media as a function of porous medium and water saturation. An increase in the retardation coefficient with increasing water saturation was observed. Thus, the novel vapor-retardation experiments demonstrated that migrating CS2 vapor is retarded as a result of partitioning into the aqueous phase. Moreover, CS2 which is dissolved in the pore water is amenable to biodegradation. First evidence of CS2 decay by biodegradation was found in the experiments. The findings contribute to the understanding of vapor plume transport in the unsaturated zone and provides valuable experimental data for the transfer to field like conditions.