Abstract:
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Objective】This study aimed to investigate the migration behavior of heavy metals cadmium (Cd) and lead (Pb) in farmland soil with compound pollution within soil and water environments, comprehensively analyze the pollution risk of Cd and Pb migration to groundwater environment, providing a reference for mitigating groundwater pollution risks in polluted farmland areas.【
Method】Taking the surface runoff of farmland soil with compound pollution of Cd and Pb as the research objects, slope runoff experiments were conducted under natural precipitation conditions. Taking maize and potato plantation areas polluted by heavy metals as monitored areas, field monitoring tests were carried out under natural precipitation conditions, and correlation analysis was performed between Cd and Pb contents in surface runoff and different monitoring parameters. Taking the subsurface flow of farmland soil with compound pollution of Cd and Pb as the research objects, soils in three different pollution levels (mild natural pollution, severe natural pollution, and severe anthropogenic pollution) were collected for indoor soil column leaching tests. Soil physiochemical properties and water sample indexes were determined to evaluate the pollution risk of Cd and Pb on underground water environment.【
Result】The results of slope runoff experiments indicated that the Cd and Pb contents in surface runoff exhibited a decreasing trend with the increase in rainfall events. Furthermore, the longer the migration distances, the lower the Cd and Pb contents. At the migration distance of 1.25 m, the highest values of Cd (0.1729 mg/L) and Pb (2.2809 mg/L) contents were both observed during the first precipitation event, significantly exceeding environmental standards, and the pollution risk of Cd migration via runoff to the groundwater environment was higher than that of Pb. The field monitoring test results showed that in maize and potato plantation areas, Cd contents reached their maximum and minimum values at the end of the second and fourth precipitation events, respectively; Pb contents reached their maximum and minimum values at the end of the third and fifth precipitation events, respectively. The migration of Cd and Pb in surface runoff in the maize plantation areas posed a heavy metal pollution risk to the surface water environment, whereas no such risk was observed in the potato plantation areas. Correlation analysis revealed that Cd and Pb contents in surface runoff had extremely significant (
P<0.01, the same below) or significant (
P<0.05, the same below) positive correlations with atmospheric deposition flux of Cd, atmospheric deposition flux of Pb, and precipitation amount, while had extremely significant or significant negative correlations with soil pH. The indoor soil column leaching test results showed that under different pollution levels, the Cd and Pb contents in soil leaching water generally exhibited a decreasing trend as leaching time increased. The Cd content in leaching water of shallow soil with severe natural pollution and severe anthropogenic pollution exceeded the standards, posing a pollution risk to the groundwater environment. The Cd content in soil with mild natural pollution was relatively low, posing no pollution risk to the groundwater environment. In maize plantation areas, the pollution risk of groundwater environment of Pb migration from surface soil to subsurface flow was higher than that of Cd. In contrast, in potato planting areas, the pollution risk of groundwater environment of Cd in both shallow and deep soil was greater than that from Pb.【
Conclusion】The migration behavior of Cd and Pb in farmland soil with compound pollution via surface runoff and subsurface flow poses pollution risks to the groundwater environment, with Cd exhibiting higher pollution risks than Pb. The Cd and Pb contents in surface runoff are correlated with the atmospheric deposition flux of Cd, atmospheric deposition flux of Pb, precipitation amount and soil pH.