Study of Scale and Time Dependent Parameters for Reactive Solute Transport through Stratified Porous Media

In this study, better simulation of observed experimental data for reactive solute transport through stratified porous media is intended. Effectiveness of distance and time dependent dispersion (TDD) to simulate fate of sorbing solute is compared to fulfill this objective. A physical and chemical-non-equilibrium (MPNE) model is used to capture this phenomenon to account for heterogeneity and site specific sorption in subsurface soil medium. Nonlinear least square optimization algorithm is used to estimate transport parameters. For a better resolution during breakthrough and tailing portion, semi-log plots are used to characterize the time scale simulation. Experiment data of reactive solute (fluoride) transport reported in Swami et al. (2016) is used for comparative analysis. It is observed that MPNE with TDD simulate experimental results with higher accuracy compared to distance dependent dispersion, especially in breakthrough and tailing portions. Parameters estimation algorithm resulted lesser values of effective dispersion and first order diffusive mass transfer for TDD. While higher values of effective dispersion and mass transfer coefficient for distance dependent dispersion identifies an overestimation in solute spreading for identical transport processes. To have a better idea of distance and time dependent dispersion in terms of mass recovery, temporal moment is calculated. Zeroth temporal moment obtained in the case of time dependent dispersion is less, which suggests that effective dispersion is less for TDD. Peclet and Damkohler number are computed to identify the dominance of dispersion and mass transfer over advection since porous medium is affected by physical heterogeneity. It is concluded that TDD gives a better description of contaminant transport through heterogeneous porous media, since it takes in to consideration the temporal variability of dispersion and diffusive mass transfer during the complete evolution of solute.