Distribution of cadmium in soils along the altitude of riparian zone(Fengdu-Zhongxian section) in the Three Gorges Reservoir region

Abstract

Cadmium(Cd) is one of the major pollutants in the water-level fluctuation zone of the Three Gorges Reservoir region. The migration and fraction transformation of Cd directly threat the safety of water environment. At the riparian zones from Fengdu to Zhongxian in the Three Gorges Reservoir, soil samples were collected according to altitudinal difference under four different land use types including farmland, woodland, orchard and residential zone. The fraction composition of Cd in the soils was determined by chemical sequential extraction. The key factors affecting the spatial distribution of Cd and its fractions in the riparian soils were deciphered. The results showed that the average content of Cd in the soil was 0.65 mg/kg. The acid extractable fraction(41.21%) of Cd was the dominant fraction in the soils, followed by residual, reducible and the oxidizable fractions. In general, there is no significant difference in the concentrations of Cd and its fractions in the soils under different land use types, indicating slight local anthropogenic contribution to Cd in the riparian soils. Both the content of Cd and its fractions in the soils showed a significant negative correlation with altitude, and their content had a marked change at the altitude of 160-165 m. This indicates that sediment deposition caused by water-level fluctuation may be the main factor controlling the spatial distribution of Cd in the riparian soils. The physicochemical properties of soils, especially the fine particles, induce an important influence on the spatial distribution of Cd in the riparian soils. Further research should be focused on the effects of sediment properties on the migration and fraction transformation of Cd in the riparian soils.