Three-dimensional assessment of heavy metal contamination in soil affected by urbanization at the urban-rural interface of Chengdu

Urbanization, particularly the transformation of agricultural land into urban areas, significantly impacts soil quality, especially concerning the concentration and distribution of heavy metals. This study investigates the spatial distribution of six heavy metals (As, Cd, Cr, Pb, Ni, and Zn) in soils at the urban-rural interface of Chengdu, focusing on areas transitioning from agricultural land to urbanized land and back to agricultural land. The study analyzes the three-dimensional spatial distribution of heavy metals and assesses the impact of urbanization on soil contamination. It was found that the top 1-m soil layer exhibited higher concentrations of heavy metals compared to the bedrock and deeper soil layers. A combination of geochemical assessments, including the Ratio of Secondary Phase to Primary Phase (RSP) and the Risk Assessment Code (RAC), was used to evaluate the ecological risks posed by these metals. The findings indicate that Cd is the most hazardous contaminant, with contamination levels being particularly high in older urban areas (N2, Shuangliu) and at river confluences (N6, Huayang). Soils in the old urban area not only had a higher total amount of heavy metals but also had a higher proportion of its heavy metal exchangeable form. These areas face significant ecological risks, and the study suggests that targeted soil remediation strategies should be developed, with a focus on urban soil reclamation and risk mitigation. • The surface soil (<1.10m) exhibits higher concentrations of heavy metals compared to the bedrock and deeper soil layers. • The risk of heavy metal pollution is more pronounced in the older urban areas and at river confluences than in other regions. • Urbanization-driven changes in land use are strongly correlated with increased soil heavy metal pollution.