Experimental Analysis of Mass Exchange Across a Heterogeneity Interface: Role of Counter-Current Transport and Non-Linear Diffusion

Solute transport in heterogeneous and fractured systems is a complex process given the permeability contrasts and the time scales discrepancies of transport in high-permeability versus low-permeability regions. We studied this phenomenon by injecting a solute (dyed water) in a micromodel comprising a single channel in contact with a porous medium and evaluated the mass exchange across the interface between the channel and porous medium (resembling the interface between free flow and porous media regions). Two sets of transport experiments were performed at three injection rates of 0.01, 0.1, and 1 ml/hr. Injection of dyed water into a clean-water-filled micromodel (referred to as the loading process hereafter) and injection of clean water into a dyed-water-filled micromodel (referred to as the unloading process hereafter). The dynamics of solute transport was recorded using time-lapse optical imaging. Our experimental results demonstrated the change of the mass exchange rate coefficient with time and a much smaller transfer rate coefficient during the unloading compared to the loading process. It is proposed that concentration-dependent counter-current advection-diffusion cause slow-down and further delay in the transport. These results may provide further explanation for the observed slow release of contamination in aquifers.