Effect of Sand Incorporation on Hydraulic Properties and Solute Transport Processes in Moderately Saline Soils

[Objective] To investigate the effects of different sand blending rates on the hydraulic properties and solute transport processes of moderately saline soils. [Methods] Four sand mixture ratios were established using an indoor soil column test: CK (0 sand mixing), B1 (10% sand mixing), B2 (15% sand mixing), and B3 (20% sand mixing), to investigate the solute transport characteristics of moderately saline soils under different sand mixing conditions and to perform model simulations. [Results] (1) In the range of 0~20%, sand mixing in moderately saline soil will obviously affect the change of soil moisture characteristic curve, and the larger the proportion of sand mixing, the closer the curve is to the Y-axis. Under any suction condition, the soil water content was ranked as CK>B1>B2>B3, and the larger the proportion of sand mixing, the more small pores in the soil, the reduction of small pores, and the weakening of soil water-holding properties. The saturated hydraulic conductivity of the soil increased with the increase of sand doping. (2) With the increase of sand doping, the solute penetration curve shifted significantly to the left, and the initial penetration time, complete penetration time, and total penetration time were reduced, and the total penetration time of B1, B2, and B3 treatments were reduced by 34.48%, 47.22%, and 69.71%, respectively, compared with that of CK. (3) Both the CED equation and the two-zone model can simulate the solute transport in soil under sand doping conditions well, but the fitting accuracy of the two-zone model is higher compared with the CDE equation. Analyses of the fitted parameters of the two-zone model showed that the soil pore flow rate, the water content ratio of the movable zone and the mass exchange coefficient gradually increased with the increase of sand doping, while the hydrodynamic dispersion coefficient and the dispersion degree showed a decreasing trend. [Conclusion] Therefore, sand blending in saline soils can effectively improve soil hydraulic properties, promote solute transport in the soil, and help alleviate the adverse effects of salinity on crop growth. The results of this study can provide reference for saline soil improvement and management work, and provide theoretical support and practical guidance for future research and practice.