Pollution characteristics and risk assessment of soil heavy metals in the Datong Basin, Loess Plateau

Abstract

To investigate the accumulation characteristics, ecological risks, and potential sources of soil heavy metals in an agro-pastoral ecotone affected by coal-related activities in the southern Datong Basin of the Loess Plateau, farmland soils surrounding representative functional zones were analyzed for As, Cd, Cr, Cu, Ni, Pb, and Zn. Pollution indices and the potential ecological risk index were used for contamination assessment, inverse distance weighting (IDW) was used for spatial analysis, and positive matrix factorization (PMF) and random forest coupled with SHapley Additive exPlanations (RF–SHAP) were used for source identification and environmental association analysis. Mean concentrations of all seven heavy metals were below the soil risk screening values for Shanxi Province, whereas Cd and Pb exceeded their respective soil background values. Cd and Pb showed slight enrichment, with mean single-factor pollution indices of 1.57 and 1.31, respectively. The Nemerow comprehensive pollution index ranged from 1.05 to 1.97, indicating slight overall contamination, while ecological risk remained low. Cd exhibited the greatest accumulation. Spatially, elevated As and Cd concentrations were scattered, elevated Pb and Cr concentrations were concentrated near roads and towns, and Cu and Ni hotspots occurred mainly in farmland and industrial areas, respectively. PMF identified four source factors characterized by Cu–Zn, As–Pb–Cd, Cd–Zn, and Ni–Cr, potentially associated with traffic-related activities, coal-related activities, agricultural and livestock inputs, and industrial activities, respectively. RF–SHAP revealed distinct associations of natural and anthropogenic factors with individual heavy metals. Overall, Cd should be prioritized for control, while the source apportionment results require further validation through isotopic tracing and in situ sampling of potential source materials.