Distribution, sources and environmental risk assessment on potential toxic elements in soils along coast of Sanmen Bay

The accumulation of potential toxic elements (PTEs) in soil through the food chain poses a serious threat to human health. Therefore, it is important to investigate the concentration of PTEs in soil for ecological and environmental safety. To identify the content, distribution, and main sources of PTEs in soils along coast of Sanmen Bay, Zhejiang, soil samples were collected in 36 sites in four sub-basins along Sanmen Bay. The analyses of clay minerals, pH, 10 PTEs, Si, Al, Fe, and TOC in the soils were performed. Source analysis of PTEs was performed using correlation and principal component analysis. Assessment on environmental risk of PTEs based on the Soil Environmental Quality (China National Standard GB 15618—2018), Enrichment Factor (EF), Nemerow Pollution Index (PI), and Potential Ecological Risk Pollution Index (RI) was conducted. Results show that the clay mineral composition of soils in different geomorphic units varied significantly. In comparison, the proportions of kaolinite and chlorite in the intermontane plain soils were relatively high (>30% on average), and that of illite in the marine plain was relatively high (>60%). The intermontane plain soils are mainly from local weathered parent rocks, while the marine accumulation plain soils are mainly modified by sediment from the Changjiang (Yangtze) River during the historical period. The concentration of soil PTEs shows an increasing trend from upstream to downstream in all sub-basins. The order in average enrichment of PTEs from high to low was Hg, Cd, Sb, As, Pb, Ni, Cr, Co, Zn, and Cu. In addition, 27.8%, 2.8%, and 2.8% of the sites were in minor enrichment, moderate enrichment, and moderately-severe enrichment in Hg, respectively; 13.9%, 8.3%, 8.3%, and 16.7% of the sites were in minor enrichment in Cd, Sb, As, and Pb, respectively. PI values showed that 86.1%, 2.8%, and 2.8% of the sites were moderately polluted, heavily polluted, and severely polluted, respectively. RI values showed that 16.7% of the sites ranked in ecological grade of moderate risk, whose Hg and Cd were amounted to 44.8% and 25.9%, respectively. The correlation and principal component analysis suggested that Cu, Zn, Cr, Ni, Co, As, and Sb were mainly in natural origination from soil-forming parent rocks, soil mechanical composition, soil pH, etc. On the other hand, Pb, Cd, and Hg were mainly anthropogenic from activities of industry, agriculture, and transportation.