Salt leaching efficiency of subsurface drainage systems with mass transfer limitations

Soil leaching through subsurface drainage systems is a traditional strategy for removing excessive salts from the saline soils. However, the presence of the immobile domain (e.g., low-permeability zones, dead-end pores) and associated kinetic mass transfer (i.e., mass exchange between mobile and relatively immobile domains) would affect the salt leaching efficiency of subsurface drainage systems. In this study, we for the first time quantify the effect of mass transfer limitations on the soil leaching efficiency using numerical simulations and developed empirical solutions. Compared to single-domain models, mass transfer extended the leaching process as the trapped salt diffuses slowly from immobile domains, leading to a larger amount of water volume being wasted during the soil leaching process. The sensitivity analysis revealed that a lower mass transfer rate coefficient, a larger capacity ratio, lower soil hydraulic conductivity, greater drain spacing, and shallower drain depth contribute to prolonging the leaching timeframe to different extents. Additionally, when the mass transfer rate coefficient drops below a certain threshold, the volume of applied freshwater per unit area initially decreases and then increases with increasing drain spacing, while it monotonically increases with increasing drain depth. These findings offer valuable insights into the design of subsurface drainage networks in dual-domain soil systems.