Laboratory tests on effects of wetting–drying cycles and loess layer thickness on release and transport of loess colloidal particles in artificial loess columns

Although it is known that wetting–drying (W–D) cycles can enhance the release and transport of soil colloidal particles (SCPs), the effects of cycle times and soil layer thickness on the enhancement are open questions. Artificial loess column laboratory tests were conducted for these questions. The results showed that the concentration of SCPs has one initial peak and several secondary peak concentrations in leachate, and all the peak concentrations decreased with increasing loess layer thickness. The concentrations of 30-, 20- or 10-cm-thick loess layer finally changes into stable levels at about 10, 16 and 23 mg L⁻¹ within 48 h, respectively. Via ten W–D cycles, the initial peak concentrations of 30-, 20- or 10-cm-thick loess layer increased by 37.02%, 40.21% and 53.76%, respectively. After the same number of W–D cycles, the initial peak concentration from 10-cm-thick loess layer was about double that from 30 cm. In short, the release and transport of SCPs would increase with the increase of the times of W–D cycle and the decrease of loess layer thickness, which demonstrates a “memory effect” and a “leapfrogging mode” in time and space, respectively. The mobile SCPs have the potential to enhance the co-transport with contaminants in vadose zone, and even to groundwater. These findings can help to better understand the release and transport of SCPs and contaminants under seasonal rainfall climate conditions, and are helpful in evaluating the risk of soil and groundwater pollution.