Spatial interrelations and multi-scale sources of soil heavy metal variability in a typical urban–rural transition area in Yangtze River Delta region of China

Information and knowledge about the spatial variability and multi-scale sources of soil heavy metal variability are important for risk assessment, soil remediation, as well as effective management recommendations. Topsoil samples (0–20 cm) ( n = 330) were collected from Luhe County, China. Geostatistical multivariate factorial kriging with robust estimates of variograms, spatial overly analysis and indicator kriging were applied to explore the correlations among soil heavy metals (Cu, Zn, Pb, Cr, Ni, Cd, and Hg) across different spatial scales, identify the sources of spatial variability, and evaluate the potential risk of soil contamination. High variability of soil heavy metals was observed and the correlations among the selected soil heavy metals in the study area depend on spatial scale. Copper, Zn, Cr, and Ni at short-range scale (2 km) are mainly controlled by geology (bed rock), while land use greatly affected the long-range variations of Cd and Hg. The strong correlations between Cr and Ni at short- and long-range (11 km) scale indicate that the sources of Cr and Ni are predominately geochemical. However, anthropogenic activities had profound impacts on the concentrations of Cu, Zn, Pb, Cd, and Hg. The potential contamination risk, defined here as measured soil heavy metal concentrations which exceeded the background values of Chinese Environmental Quality Standard for Soils, was observed in Cu, Zn and Pb, and the areas covered 3%, 3.4%, and 1.4%, respectively. The possible sources of Cu and Zn contamination can be considered as the current and historic industrial emissions from Dachang industrial zone, and the potential areas of Pb contamination could be attributed to the industrial emissions from Dachang industrial zone, small factories and heavy traffic flows in/nearby downtown area of the county. While the Hg contamination risk was closely related to the widely use of Hg-related pesticide during 1940–1970s.