Quantifying leachate discharge and assessing environmental risks of gully-type coal-based solid waste dumps in small watersheds: A refined hydrological modeling approach for mitigation strategies

Rainfall-induced leaching from extensive coal-based solid waste storage results in a long-term risk to watershed's water quality and safety. The leachate carries heavy metals and other contaminants, which migrate and accumulate through the watershed, leading to a persistent deterioration of downstream water environment. However, the lack of systematic research on the release, accumulation, and spatial-scale migration dynamics of leachate limits effective management of diffused leachate pollutions. This study presents a novel cross-scale coupling framework which integrates multi-source remote sensing data with Soil and Water Assessment Tool (SWAT) model, employing a strategy that transfers parameters from large basins to accurately quantify the hydrological processes in coal waste sub-basins. Additionally, a comprehensive analysis is performed on the hydrological characteristics, leachate generation, and watershed migration dynamics in gangue dump sub-watersheds, providing a new methodological framework for managing mining-related leachate pollution. The large basin model demonstrated strong performance (R² = 0.79, NSE = 0.66 for calibration; R² = 0.74, NSE = 0.59 for verification), while the sub-basin model exhibited excellent accuracy (R² = 0.94, NSE = 0.92 for calibration; R² = 0.81, NSE = 0.77 for verification). High-resolution drone data estimated the annual leachate production to be 3366.87 m³. Simulations revealed that leachate migration peaks in the summer months (July to September), significantly increasing downstream pollution risks. Risk assessments indicate that vegetation in land restoration areas reduces leachate production and migration via evapotranspiration and other processes. This study provides an adaptable methodological framework for managing mining-related leachate pollution and highlights the critical importance of optimal reclamation strategies for mitigating pollution and restoring degraded landscapes.