Flow and Colloid-Facilitated Contaminant Transport in the Vadose Zone: Numerical Study

The present paper focuses on the numerical model development of the colloid-facilitated contaminant transport using an equilibrium approach in the vadose zone of subsurface porous media. Flow in the vadose zone of subsurface porous media plays a prominent role in predicting the transport of contaminants from the ground surface to the water table. Ignoring the fact that standard flow equations and colloids are ubiquitous in subsurface environments can lead to a severe misjudgment of the distances traveled by the contaminants. Although contaminant transport with colloids in saturated porous media has been studied, the equilibrium-based colloid-associated contaminant transport in the unsaturated zone has received inadequate attention. The present study encompasses the flow equations for a vadose zone with a colloid-facilitated contaminant transport model. The present study encompasses the first numerical model of colloid-associated contaminant transport with an equilibrium approach under unsaturated flow conditions. The mixed form of the Richards equation is solved using a fully implicit finite-difference method with Picard's iteration and coupled with the solution of the transport equation. The breakthrough profiles and sensitivity analyses culminate in indicating that colloids enhance the mobility of contaminants by reducing the retardation factor. However, an engrossing finding is that the mobility of contaminants also relies upon the degree of interaction of pollutants with stationary porous matrix and suspended colloids. As the degree of interaction of the contaminants with the stationary solid matrix increases, retardation is noticed in the contaminant movement, even in the presence of colloids. In contrast, contaminants move faster as the degree of interaction of the contaminants with the suspended colloids increases.