Evaluating Nonequilibrium Solute Transport through Four Soils of Pakistan using a HYDRUS Model and Nonparametric Indices

Core Ideas Soil structure with a large volume of small macropores has greater preferential transport. Nonparametric indices differentiated the soils with preferential transport. Chemical non‐equilibrium under unsaturated flow diminished fast in coarse blocky soil. Leaching experiments in soil under different saturations show inconsistent results regarding preferential flow, and therefore further investigations are needed. The comparative degree of non‐equilibrium flow of chloride (Cl) was evaluated using replicated large intact columns taken from two alluvial (Rawal, Typic Hapludalfs and Kotli, Entic Haplusterts) and two loess (Guliana, Udic Haplustalfs and Mansehra, Typic Hapludalfs) soils that differed in structure below 20 cm depth. Lithium chloride (3.5 mM) was pulsed with constant flows generated by heads 10, −10, −40 mm water; Cl breakthroughs were fit into a physical non‐equilibrium model with no sorption; and lithium (Li) breakthroughs (measured for the two loess soils) were fit into a physical and chemical non‐equilibrium model using HYDRUS‐1D. The Cl breakthrough curves were highly skewed, with immobile water contents in the range of 0.10 to 0.39 cm 3 cm –3 , indicating that physical nonequilibrium occurred under saturated flow as well as under unsaturated conditions. The nonparametric indicators (i.e., apparent dispersivity, the piston flow to Cl velocity ratio, the holdback factor, and the arrival time of the first 5% of Cl) indicated a greater strength of preferential flow in the Rawal and Guliana soils, which had more well‐connected pores than the Kotli and Mansehra soils, which had fewer macropores and dense aggregates. Lithium breakthrough occurred under chemical non‐equilibrium, which diminished with desaturation, especially in the Mansehra soil. There were two sources of preferential flow: (i) macropore flow under saturated conditions and (ii) bypass flow in the matrix due to regions of disconnected water under unsaturated conditions. The study results will help understand leaching potential of agro‐chemicals in similar soils.