A numerical simulation study of the effect of the vadose zone with lenses on LNAPL migration under the fluctuating water table

Light Non-Aqueous Phase Liquids (LNAPLs) in vadose zone is of multi-phase characteristics, while factors such as heterogeneity and groundwater fluctuation are expected to significantly increase the complexity of LNAPLs contamination in vadose zone. Previous studies have mostly focused on revealing the contamination process of free-phase LNAPLs, few have explored deeply the influence of the heterogeneous structure on the migration and phase distribution pattern of LNAPLs when the water table fluctuates. A numerical model of multiphase flow in vadose zone is established based on TOUGH2 to reveal the migration and phase distribution of LNAPLs under the joint effect of different lithological lenses and water table fluctuation. The results show that (1) the migration and distribution regularity of LNAPLs in vadose zone is predominantly controlled by the variation in water content, which is presented under the effect of heterogeneity and water fluctuation. (2) In the steady groundwater scenario, LNAPL migrates in an “accumulation-lateral expansion-flow bypass” pattern around the fine-sand lens, while the coarse-sand lens acts as the "preferential route" for the vertical movement of LNAPL. Flow around the fine-sand lens is significantly enhanced by the variation in water content induced by groundwater fluctuation, and the coarse-sand lens further exhibits the "preferential space" effect. (3) When the water table is steady, LNAPL are concentrated inside and below the lens body in the fine-sand and coarse-sand lens models, respectively. In the fluctuating groundwater scenario, a greater range of LNAPL is presented in the vicinity of the lens with the distribution area in each model is 51% and 63% larger than that in the steady scenario. (4) Volatile flux of LNAPL, affected by the LNAPL-gas exposure conditions and the distribution of LNAPL, shows a "decreasing-then increasing" pattern in both models. The three-phase equilibrium state is disrupted by groundwater fluctuation, which is manifested by the enhanced volatilization during the stage of groundwater elevation, when the average volatile flux in the two models is 124%～126% higher compared to the steady scenario.This research provides a theoretical basis for the understanding of LNAPL pollution process in heterogeneous contaminated sites.