Effect of Transient Irrigation on Boron Transport in Soils

Abstract

Water redistribution and solute concentration changes caused by water evaporation can affect B transport in unsaturated soils under a subsurface transient water flow regime. Our objective was to assess the effect of nonequilibrium adsorption on B transport in unsaturated, homogeneous soil under periodic infiltration cycles using adsorption and transport parameters obtained from independent batch and column experiments. Column experiments using flow interruption were conducted at two pore‐water velocities: 0.25 and 2.5 cm h−1 Transport of B at low velocity was unaffected by flow interruption, whereas interruption of fast flow resulted in appreciable perturbation of the B concentration in outflows. The convection–dispersion equation (CDE) with a rate‐limited reaction term was fitted to the fast‐flow B breakthrough curves (BTCs) to determine the mass‐transfer rate coefficients controlling nonequilibrium B transport behavior in soils. This CDE, coupled with the Richards equation, was used to simulate B transport in unsaturated soils under irrigation cycles consisting of water infiltration, internal drainage (redistribution), and evaporation. The B transport under transient nonmonotonic flow depended strongly on the rate‐limited step of B adsorption. At a small value of the mass‐transfer coefficient, the B BTCs had a characteristically jagged shape, showing increase–decrease cycles in solution B concentrations. The B adsorption tended to balance the solution B concentration increase caused by water evaporation. The final B concentrations, however, exceeded unity in the upper part of the BTCs that were obtained at different soil depths. The results revealed that deviation from adsorption equilibrium may lead to appreciable changes in solution B concentration, especially in the topsoil where the water flow is explicitly transient and nonmonotonic.