A framework for unveiling multi-pathway mechanisms of soil heavy metal pollution: Evidence from five major urban agglomerations in China.

Urban agglomeration (UA) development exacerbates heavy metal contamination in farmland soils. Spatial patterns and region-specific drivers of this pollution, however, are rarely quantified. This study establishes an integrated framework to reveal ecological risks and region-specific drivers of Cd, Hg, As, and Pb across five major UAs in China. By integrating BorutaSHAP, XGBoost-SHAP algorithm and structural equation model, we identify dominant factors and their interactions. Results show the Pearl River Delta (PRD) has the highest risk, driven mainly by Cd and Hg, followed by Chengdu-Chongqing (CC) and Yangtze Middle Reaches (YMR), while Yangtze River Delta (YRD) and Beijing-Tianjin-Hebei (BTH) show lower overall risks with localized hotspots. Spatially, PRD and YRD exhibit core-concentrated patterns, whereas CC, YMR, and BTH display peripheral-diffused patterns. Industrial emissions are the predominant contributors across UAs, with soil properties exerting supplementary influence in CC and YMR. Notably, despite reductions in direct industrial inputs to soil, the indirect impacts mediated by hydrology are significantly pronounced, particularly in PRD and BTH. Systematic governance measures that suppress pollution cross-media transfer is therefore urgent for heavy metal pollution control in farmland under rapid UA expansion.