Saturation dependence of mass transfer for solute transport through residual unsaturated porous media

Abstract

The residual non-wetting phase in unsaturated porous media varies the aqueous pore space due to the immiscible characteristics. The varied pore space in turn has significant influence on the mass transfer for aqueous tracer transport through residual unsaturated porous media. In this study, the mass transfer for solute transport through residual unsaturated porous media was investigated. A pore-scale numerical simulation (PSNS) model consisting of both immiscible two-phase flow transport and aqueous-miscible solute transport was developed at pore scale to capture the topology of residual saturation and the tempo-spatial evolution of aqueous tracer in residual unsaturated porous media. The results showed that as the immiscible fluid-fluid displacement pattern varied from compact displacement to capillary fingering, the residual saturation (Srw) increased and the velocity spatial variability in the residual aqueous flow field was enhanced. Inversion of the advection-dispersion-equation (ADE) model and the continuous-time-random-walk (CTRW) model indicated the typical characteristics of anomalous aqueous tracer transport (i.e., early arrivals and long tails) in breakthrough curves (BTCs) in a range of (Srw) from 0.098 to 0.542. A non-monotonic relationship was found between the dispersivity and the Srw. Further, analysis of the overall dispersivity of aqueous tracer in terms of the mobile-immobile-type transport equation showed the coexistence of two mass transfer mechanisms (i.e., the dispersivity of the mobile domain and the solute exchange between the mobile and immobile domains). We introduced a dimensionless dispersivity index to quantify the relative contribution of these two mass transfer mechanisms to the overall dispersivity. However, it was found that the mass transfer of aqueous tracer in residual unsaturated porous media was controlled by the dispersion of aqueous tracer in the mobile domain at low Srw, whereas it was dominated by the solute exchange between the mobile and immobile domains at high Srw. Our study not only highlighted the saturation dependence of mass transfer in residual unsaturated porous media, but also emphasized the importance of the influence of saturation topology on mass transfer.