Heavy Metal Contaminations in a Soil-rice System: Identification of Spatial Dependence in Relation to Soil Properties of Paddy Fields

In order to identify spatial relationship of heavy metals in soil-rice system at a regional scale, 96 pairs of rice and soil samples were collected from Wenling in Zhejiang province, China, which is one of the well-known electronic and electric waste recycling centers. The results indicated some studied areas had potential contaminations by heavy metals, especially by Cd. The spatial distribution of Cd, Cu, Pb and Zn illustrated that the highest concentrations were located in the northwest areas and the accumulation of these metals may be due to the industrialization, agricultural chemicals and other human activities. In contrast, the concentration of Ni decreased from east to west and the mean concentration was below the background value, indicating the distribution of Ni may be naturally controlled. Enrichment index (EI) was used to describe the availability of soil heavy metals to rice. The spatial distribution of EIs for Cd, Ni and Zn exhibited a west-east structure, which was similar with the spatial structures of pH, OM, sand and clay. Cross-correlograms further quantitatively illustrated the EIs were significantly correlated with most soil properties, among which; soil pH and OM had the strongest correlations with EIs. However, EI of Cu showed relative weak correlations with soil properties, especially soil pH and OM had no correlations with EI of Cu, indicating the availability of Cu may be influenced by other factors.