Laboratory Experiments on Dispersive Transport Across Interfaces: the Role of Flow Cell Edges and Corners

Contrary to solutions given by the advection dispersion equation, asymmetric break through curves dependent upon flow direction have been experimentally produced for the conceptual system of two equal length sections of different homogeneous porous media separated by a sharp, macroscopic interface perpendicular to flow. The work presented in this paper builds upon the methodologies and results of Berkowitz et al. incorporating digital imaging and light reflectometry as a non-invasive method to fully reconstruct tracer concentration fields throughout time during laboratory experiments (Berkowitz et al., 2009). These images reveal a complex flow field within the system created by variations in pore geometry at the edges and corners of the flow cell. Two distinct and unexpected features observed in the digital images are discontinuous tracer concentration at the interface and a pocket of tracer lagging behind the interface in a single flow direction. A second experiment was conducted featuring glass balls of reduced diameter while retaining the same diameter ratio across the interface. The concentration break through curves produced by this second experiment are symmetric, suggesting that asymmetric break through curves are the result of complex pore geometries occurring when the flow cell is packed with glass balls of 4 mm diameter.