Wetting–drying cycles enhance the release and transport of autochthonous colloidal particles in Chinese loess

Soil columns packed by Chinese loess were tested under stimulated wetting–drying cycles. The results of leaching tests showed that ACPs concentration, although changed with initial peak concentration and secondary peak concentration, eventually tended to a stable level at about 6.0 mg L⁻¹ within 48 h. The results of wetting–drying cycle tests showed that the initial peak concentration, cumulative mass and mean size of ACPs increased with the length of drying period with the drying period. The initial peak concentration of ACPs after 90-d drying period was about nine times of that after 1 d. In addition, the release and transport of ACPs increased with the wetting–drying cycles having the same drying duration. After ten wetting–drying cycles, the initial peak concentration of ACPs for 30-d drying period increased by 35.35% and that for 15 d increased by 18.33%. The preliminary mechanisms to enhance the release and transport of ACPs include physical mechanisms of the air–water interface scouring, the shear stress of preferential flow and the collapsing pore walls by capillary pressure, and chemical mechanism of the salts dissolving of loess matrix. These findings are helpful to evaluate the risk of soil and groundwater pollution.