Impact of residential density on heavy metal mobilization in urban soils: Human activity patterns and eco-health risks in the Beijing-Tianjin-Hebei region

Urban residential density intensification poses increasing threats to soil quality, with heavy metal pollution becoming a critical challenge for ecosystem stability and public health. This study explored the speciation characteristics and bioavailability of soil metals across residential density gradients in the Beijing-Tianjin-Hebei megacity cluster, integrating Point of Interest (POI) data with geochemical analyses. Results showed that higher residential density significantly increased total soil metal loads (p < 0.001) and enhanced the mobility of Pb, Cu, and Zn (p < 0.05), with exchangeable fractions dominating their speciation profiles. Carcinogenic risk assessment identified children as the most vulnerable group, with high-risk zones concentrated in areas exceeding the regional average residential density in 2022. Principal component analysis (PCA) attributed soil metal accumulation primarily to industrial activities (28.2 % variance) and traffic emissions (25.8 % variance). Co-occurrence network and Partial Least Squares Structural Equation Modeling (PLS-SEM) suggested that human activities may indirectly influence metal dispersion by altering soil organic carbon (TOC) and nitrogen dynamics, rather than directly increasing bulk metal content. Specifically, Pb mobilization was driven by TOC-metal complexation, while Cu redistribution was linked to carbon-nitrogen interactions. By quantifying the ecotoxicological impacts associated with residential density, this study provides actionable insights for policymakers to optimize land-use zoning and mitigate heavy metal exposure risks in rapidly urbanizing regions.