Spatial distribution and environmental factors of catchment-scale soil heavy metal contamination in the dry-hot valley of Upper Red River in southwestern China

Abstract

The severe soil erosion in the dry-hot valley (DHV) in southwestern China may result in heavy metal pollution in the Red River. However, little is known concerning the level of contamination, spatial distribution and environmental controls of the heavy metals in the soils of the DHV. Heavy metal concentrations in agricultural soils in a typical DHV catchment were investigated. Soil samples from both the topsoil (0–20cm) and subsoil (20–40cm) were collected at sixty-two sites, and the soil physical and chemical properties of sand, silt, clay, and organic matter contents; soil pH; bulk density; and the concentrations of Cr, Ni, Cu, Zn, and Pb were determined. Topographic factors of slope gradient, slope aspect, elevation, and topographic wetness index at each site were calculated using DEM. Kriging and canonical correspondence analyses were used to assess the spatial distribution of and environmental controls on heavy metals. The results demonstrated that the concentrations of Cr, Ni, Cu, Zn, and Pb in the DHV were higher than most of the reported values for agricultural soils worldwide and that the DHV may constitute a potential source of heavy metal pollution in the Red River. Considering each heavy metal individually, the vertical distribution in the topsoil and subsoil was similar at all of the sampling sites. High concentrations of Cr, Ni, Zn and Pb were observed at low elevations and low slope areas, and the spatial distribution pattern of Cu was contrary to that of Cr, Ni, Zn and Pb. The concentrations of heavy metals in the topsoil were controlled by both soil and topographical factors, while in the subsoil, they were primarily controlled by soil-related factors. It is concluded that high background concentrations, uncontrolled use of chemical fertilizers, and mining are the sources responsible for heavy metal contamination in the DHV region.