Air and LNAPL Entrapment in the Partially Saturated Fringe: Laboratory and Numerical Investigations

Laboratory experiments and numerical simulations were used to investigate air and LNAPL entrapment in the Partially Saturated Fringe (PSF) at the local scale of a heterogeneous groundwater system. Investigations led to the determination of several system properties that have a significant influence on entrapment in this region. Laboratory investigations were conducted to gather information and used in the verification of the numerical model TOUGH2/T2VOC in reference to air/LNAPL entrapment. This model was then used in conjunction with laboratory experiments to further investigations. Laboratory and numerical investigations focused on imbibition within simple heterogeneous systems. It was determined that several factors influence the degree of entrapment in the PSF. The rate of rise of the water table in the system has a significant influence on the amount of LNAPL entrapped in heterogeneities. Specifically, as the rate of water table rise increased, the amount of entrapment in heterogeneities decreased. It was also determined that connectivity of heterogeneities has a significant influence on the degree of air/LNAPL entrapment in the PSF. As the connectivity of coarse sand inclusions increased, the amount of air/LNAPL entrapped in these regions, and within the entire system, decreased. Specific sediment properties were also determined to have a significant influence on the degree of entrapment in the system. It was determined that in a system with coarse inclusions surrounded by finer sediment, as the sediment characteristics of the lens became more similar to those of the surrounding media, less air/LNAPL became entrapped. Specifically, the air entry pressure and degree of sorting of the lens were investigated. As the air entry pressure of the lens increased and/or the degree of sorting of the lens decreased, less air/LNAPL became entrapped in the system. To our knowledge, this is the first demonstration of entrapment of air or LNAPL in the PSF through active processes, and thus provides a solid foundation for future investigations into the PSF and possible application of remediation technology in this region.