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 poses serious environmental risks, while offering opportunities for resource recovery through mine land reclamation. This study investigates how soil cover thickness regulates rainfall infiltration in coal gangue backfill systems through laboratory testing of hydraulic parameters, field infiltration monitoring, and HYDRUS-based numerical simulations. Results show that a 50 cm soil interlayer effectively delays infiltration and reduces moisture at the soil–gangue interface by approximately 40%, achieving a balance between hydraulic barrier performance and cost efficiency. Increasing the soil cover thickness to 70–100 cm further improves water retention capacity but results in reduced economic benefits. For long-term rainfall conditions, a layered configuration consisting of alternating coal gangue and soil layers, capped with a 1 m surface layer, moderated moisture transfer and limited deep percolation, maintaining stable hydrological behavior throughout the simulation period. Multi-objective optimization using the NSGA-II algorithm identified a total cover thickness of 50–60 cm as the optimal configuration, reducing leachate generation by about 90%, controlling heavy-metal migration risk below 3%, and maintaining unit costs within 120–150 CNY m−2. These findings provide a quantitative and practical basis for designing sustainable soil cover systems that integrate solid waste utilization with environmental protection across different climatic regions.