Optimizing Soil Cover Systems for Coal Gangue Reclamation: Balancing Leachate Control, Environmental Risk, and Cost

The large-scale accumulation of coal gangue not only occupies land resources but also generates serious environmental risks, while offering potential for resource recovery through mine land reclamation. This study examines how varying soil cover thickness regulates rainfall infiltration in coal gangue backfill systems, using a combination of laboratory column experiments, field-scale infiltration monitoring, and HYDRUS simulations calibrated with measured hydraulic properties. Under extreme short-term rainfall, a 50-centimeter intermediate soil layer delayed infiltration and reduced moisture at the soil–gangue interface by about 40 percent, substantially limiting leachate generation. Increasing the thickness to 70–100 centimeters further delayed infiltration but yielded diminishing benefits. For prolonged rainfall, a layered structure of five meters of coal gangue alternated with half-meter soil layers and topped with a one-meter surface layer moderated moisture transfer and reduced deep percolation. Even under extreme rainfall of 50 centimeters per day, the system maintained stable hydrological behavior. Multi-objective optimization with the NSGA-II algorithm indicated that a total soil cover thickness of 50 to 60 centimeters achieves an effective balance between leachate control, environmental risk reduction, and cost, and can be applied in both arid and humid regions. These results provide a practical basis for designing sustainable cover systems that integrate solid waste utilization with environmental protection.