Pore-scale mechanism of enhanced non-Fickian transport through residual NAPL zone in porous media

The residual contaminated zone in porous media varies the pore structure due to the immiscible characteristics. The varied pore structure has significant influence on the miscible solute transport through the residual contaminated zone in porous media. In this work, pore-scale mechanisms for enhanced non-Fickian transport through residual contaminated zone in porous media were numerically studied. To simulate the immiscible two-phase transport and obtain the residual contaminant distribution in porous media, an immiscible two-phase transport model (ITTM) was implemented by coupling modified Navier-stokes (N-S) equation (includes a field-dependent surface tension force) with advective Cahn-Hilliard equation. The varied pore structures were obtained by considering the immiscible interface between contaminant, water and soils as a non-slip boundary. The pore-scale aqueous solute transport model was used to simulate the aqueous solute transport in varied porous media. The aqueous flow field was solved directly using the continuity equation and the N-S equation under isothermal, incompressible, Newtonian, and steady-state flow conditions. The results showed that the residual contaminant distribution has influence on the miscible transport. The residual contaminant could vary pore structure and decreased the void in porous media. The varied pore structure could enhance the magnitude of the non-Fickian characteristics.