Cadmium concentration and its typical input and output fluxes in agricultural soil downstream of a heavy metal sewage irrigation area.

Heavy metal pollution of agricultural soils in sewage irrigation areas is a serious environmental issue. Many prior studies have demonstrated that soil around the irrigation area is polluted with heavy metals, even though they had not been irrigated by wastewater. In this study, a paddy field downstream of the Zhangshi Irrigation Area was selected as the study area. The Cd concentrations and their representative input and output fluxes to and from the topsoil were systematically studied. The results showed that 95.5% of soil samples exceeded the screening value of Cd concentration. The Cd input fluxes via irrigation water and atmospheric deposition, accounting for 56.95% and 42.53% of the total input flux, respectively, were the main sources of Cd in soil. Crop harvesting was the main output pathway, accounting for 89.63% of the total output flux. An estimation of the annual mass balance showed that Cd in the studied area was in a state of accumulation, and the annual increase in Cd concentration in topsoil would be 2.46 µg kg-1 if the observed fluxes remain. These results will provide a reference for the development of strategies to control and reduce heavy metal contamination and diffusion in agricultural soils around irrigation areas.