Experimental and numerical study on Zn and Pb migration in the farmland soil under wastewater irrigation conditions

Long-term wastewater irrigation may lead to the accumulation, transformation, and migration of heavy metals in the farmland soil, increasing the risk of groundwater pollution. However, it is currently uncertain whether using wastewater for irrigation would lead to the migration of heavy metals zinc (Zn) and lead (Pb) into deeper layers of soil, in the local undeveloped wastewater irrigation farmland. In this study, the migration characteristics of Zn and Pb from irrigation wastewater in local farmland soil were investigated through a series of experiments including adsorption experiments, tracer, and heavy metals breakthrough experiments, as well as numerical simulations using HYDRUS-2D software. The results revealed that the Langmuir adsorption model, CDE model, and TSM model were effective in fitting the required adsorption and solute transport parameters for the simulations. Furthermore, both the soil experiments and simulation results showed that in the test soil, Pb had a stronger affinity for adsorption sites than Zn, while Zn exhibited greater mobility than Pb. After 10 years of wastewater irrigation, it was found that Zn had migrated to a maximum depth of 32.69 cm underground and Pb had only migrated to 19.59 cm. Despite their migration, the two heavy metals have not yet reached the groundwater zone. Instead, they had accumulated to higher concentrations in the local farmland soil. Moreover, the proportion of active forms of Zn and Pb decreased after flooded incubation. The present results can improve understanding of the environmental behavior of Zn and Pb in the farmland soil and can be used as a basis for risk assessment of Zn and Pb polluting groundwater.