Experimental and numerical study on Cu and Cd migration in different functional-area soils under simulated rainfall conditions

Natural rainfall exerts a significant influence on the migration of heavy metals in soil. However, the knowledge of migration characteristics and release kinetics of heavy metals in contaminated soils under different rainfall intensities still remains unclear. In this study, the simulated rainfall of different intensities was designed to experimentally and numerically investigate Cu and Cd movements in different functional-area (agriculture, industrial, urban) soils. A HYDRUS-2D model was optimized to simulate the migration process of Cu and Cd in soil under different rainfall conditions. The hydraulic properties and solute transport parameters used in the model were estimated based on isothermal adsorption and chloride ion penetration experimental measurements and related model fitting. Furthermore, Cu and Cd BTCs (Breakthrough Curves) were fitted using the HYDRUS-2D inverse solution function with two-site model. The results showed that the order of the migration capacity of Cu and Cd in different functional-area soils was agriculture soil > industrial soil > urban soil, and Cd had a greater risk of groundwater pollution than Cu. With the increase of rainfall intensity, the high proportion of the exchangeable state of Cu and Cd in contaminated soil is easy to be released. Furthermore, the model was proved to describe the distribution of Cu and Cd in the soil profile very well. The present results can improve understanding of the environmental behavior of Cu and Cd in different functional-areas soils and can be used as a basis for risk assessment of Cu and Cd polluting groundwater.