Relative Contribution of Back Diffusion to Contaminant Plume Persistence depending on Different Clay Types

Contaminated aquitards could serve as long-term sources due to the contaminant release from aquitards to transmissive zones by back diffusion. This transport mechanism causes long-term plume persistence of contaminants in the groundwater system even after primary sources are removed. There have been several studies about the critical parameters influencing on the contaminant plume persistence such as source history, aquitard distribution, the contact area between high- and low-permeability zone, biochemical reaction and other boundary conditions. In this study, we investigate how different clay types can affect plume behavior. Three flow chambers with different clays (kaolinite, montmorillonite, and bentonite) were prepared respectively for diffusion experiments. The chambers were filled with 6 cm clay layer as an aquitard underlain by 5 cm glass bead as an aquifer. Bromide (1 g/L) as non-reactive tracer solution was injected for approximately 4 PVs. Subsequently, DI water was flushed for approximately 10 PVs to evaluate the relative contribution of back diffusion to plume persistence . The measured concentration data was compared with the 1-D analytical solution to obtain the appropriate influential parameters. The experimental results suggested that plume tailing behavior was strongly affected by the clay particle arrangement. Under the saturated condition, clay particle became expanded depending on the different binding force between the layers. The lower the particle expansion, the more complex of the solute pathway. And this structural complexation appears to be one of the main reason for the contaminant persistence in the aquatic system.