The influence of heterogeneity and spill conditions on NAPL dissolution fingering

The objective of this work is to develop upscaled models for NAPL dissolution in systems where NAPL dissolution fingering is important. Previous work has shown that preferential dissolution pathways develop in NAPL-contaminated regions, as evidenced in two-dimensional physical and numerical experiments. The primary cause of finger creation and growth is permeability feedback resulting from NAPL dissolution that alters the flow field. Although there are a myriad of ways that fingering can be induced in idealized laboratory or numerical experiments, there are open research questions regarding dissolution fingering in natural systems. An important feature of NAPL contamination in the subsurface is heterogeneity in the NAPL residual phase, which is highly dependent on NAPL spill conditions and porous media heterogeneity. The formation of NAPL ganglia and entrapment of NAPL pools in these systems is influenced by the tortuous paths taken by free-phase NAPL during infiltration and redistribution processes. The characterization of dissolution fingering under these conditions remains a subject of physical and numerical investigation. In this work, we explore the full series of processes that lead to dissolution fingering in heterogeneous porous media using two-dimensional numerical simulations. We model NAPL spills into different media under various spill conditions and the subsequent redistribution when the source is eliminated. In addition to the expected effect of heterogeneity on the residual NAPL distribution, we also find that the characteristics of the NAPL residual phase are more sensitive to the size and extent of the spill during infiltration than the dynamics of the redistribution process. Once the NAPL is no longer mobile, we simulate dissolution of the ganglia and pools with appropriate local mass transfer relationships. The resulting numerical data are used to relate features such as spatial and temporal changes in the structure of the dissolution front to porous media heterogeneity and spill characteristics. We find that processes occurring at two distinct spatial scales—the scale of the heterogeneity and the scale of the trapped NAPL ganglia—control dissolution front dynamics. These features can be used to explore new upscaling relationships that can be used in larger-scale modeling of NAPL dissolution in complex natural porous media systems.