Experimental and numerical simulation of the reactive contaminant migration for non-Darcian flow in a filled-single fracture

Accurate prediction of flow and solute migration through the subsurface porous media is essential for the reclamation of the polluted aquifer and future contamination control. This study focuses on the dispersion process under non-Darcian flow conditions in the laboratory using a synthetic single fracture. A sand-packed single fracture of 1000 cm length and 0.3 cm fracture aperture was fabricated in the laboratory for conducting flow and contaminant transport experiments. Non-Darcian flow conditions prevailed in the filled-single fracture and were best simulated by the Forchheimer equation. Sodium Flouride (NaF) was used as a reactive contaminant in the experiments and was injected using Pulse-type boundary conditions. The resulting Breakthrough Curves (BTCs) were found to be non-Fickian with long tailings and early arrival. Solutions of the Convective-Dispersive equation (CDE) and Mobile Immobile (MIM) transport equations (for constant, linear, and exponential distance-dependent dispersion) were obtained through the Implicit finite difference technique. For different flow velocities, the MIM model was better at simulating the long tailings and early arrival of BTCs. Further, it is observed that constant (MIMC) and exponential distance-dependent (MIME) dispersion models are better at simulating observed BTCs compared to the linear-distance dependent (MIML) dispersion model. Through the statistical analysis and goodness of fit, the suitability of MIME and MIMC in describing contaminant transport through fracture was further confirmed.

Keywords:  non-Fickian; Filled-single fracture; non-Darcian; Breakthrough curves; Forchheimer equation; MIM model.