Spatial distribution characteristics of heavy metals in coal-grain composite zone and assessment of ecological risks

Coal-grain composite zone, as areas where coal resources and high-quality farmland overlap spatially, bear the dual responsibility of ensuring energy and food security. However, under the dual pressures of industrialization and agricultural modernization, the complexity, sensitivity, and vulnerability of their ecosystems are gradually increasing, posing significant challenges to their overall sustainable development. To clarify the spatial distribution characteristics of heavy metals in coal-grain composite zones and the ecological risks they face, this study systematically collected samples of coal gangue, soil, and crops from typical coal-grain composite zones. Correlation analysis and spatial distribution characteristic studies of heavy metals in multiple media (coal gangue, soil, and wheat) were conducted, and the ecological risks of soil in coal-grain composite zones were evaluated. The results show: Heavy metal concentrations show high variability and differences under biological and environmental influences; Cr, Cu, Pb, Zn and As show significant positive correlation in the coal gangue-soil-wheat system, heavy metals in different media in a similar enrichment mechanisms, mainly in the mining area along the coal railway and gangue mountain surrounding; the spatial distribution of heavy metals in wheat decreases from north to south affected by the expansion of the coal mining subsidence area to the south and the northward shift of the planting area; The heavy metals Hg and Cd are the primary ecological risk factors in coal-grain composite zones. Independent sources or migration mechanisms classify these areas as mildly to moderately polluted, with a probability of adverse biological toxicity ranging from 33% to 58%. It is recommended that ‘zonal control and precise monitoring’ be implemented in coal-grain composite areas. In the northern region, priority should be given to managing ‘mining + irrigation’ composite pollution, while in the southern region, efforts should focus on preventing the spread of ‘transportation + deposition’. Continuous monitoring of Hg and Cd migration should be conducted, and multi-scale evaluation methods should be integrated to provide scientific guidance and planning for the sustainable development of Coal-grain composite zone.