Evaluating solute-Trapping Induced Non-Fickian Transport in Partially Saturated Porous Media

We study the upscaling of pore-scale solute transport in partially saturated porous media at different saturation degrees. The interaction between structural heterogeneity, phases distribution and small-scale flow dynamics induces complex flow patterns and broad probability distributions of flow. In turn, this spatial distribution of flow velocities, at the pore scale, induces irregular (non-Fickian) transport of dissolved substances (e.g., contaminants), causing an earlier arrival and longer tailing, which may have grave consequences in underestimating risk assessments and prolonged cleanup times of contaminated sites.

Here, we suggest an integrated continuous time random walk (CTRW) modeling framework, which accounts for also the entrapping of particles in zones of low flow velocities, to estimate the resident times of solutes in the media. Furthermore, comparing the results of the CTRW model to a well-established numerical simulation method allows a phenomenological evaluation of the model's physical parameters for different conditions (i.e., volume of entrapped air, mean water flow rate, or solute molecular diffusion coefficient).   

In this study we show that entrapped air promotes preferential solute transport and solute trapping in low flow regions. Moreover, we demonstrate that the trapping frequency and trapping time depend on the interaction between advection and diffusion (i.e., the Péclet number). An integrative CTRW model captures the effects of trapping in stagnant regions and preferential transport on non-Fickian dispersion of solutes.