The impact of different reclamation covers on the release of nitrate from various sources in coal gangue

Placing reclamation covers is an effective method to reduce NO₃^- release from coal gangue. This study considered the impacts of different reclamation covers on the contributions and release of NO₃^- from explosive and exchangeable NH₄⁺ (NH₄⁺-ex) sources in coal gangue using column experiments. The δ¹⁸O-NO₃^- values were measured to identify the sources of NO₃^- in coal gangue, and the calibrated and validated hydraulic and solute parameters were used to simulate the release of NO₃^- for all treatments (without cover, CK; 30 cm sandy loess, T1; 30 cm Pisha sandstone, T2; 30 cm mixed soil, T3; 15 cm Pisha sandstone overlying 15 cm sandy loess, T4; and 15 cm sandy loess overlying 15 cm Pisha sandstone, T5). The results indicated SEEP/W and CTRAN/W with optimized parameters could accurately simulate water movement and NO₃^- transport. The contribution of the explosive source was 80, 81, 86, 82, 84, and 84% and of the NH₄⁺-ex source was 20, 19, 14, 18, 16, and 16% for CK, T1, T2, T3, T4, and T5, respectively. The five covered treatments respectively decreased the total cumulative water volume by 5.80, 11.11, 7.73, 9.18, and 10.63% and the total NO₃^- cumulative mass released from the coal gangue by 5.59, 20.95, 7.41, 14.61, and 14.36% compared to CK. Our results suggest the reclamation covers significantly reduced the release of NO₃^- from coal gangue primarily by decreasing the release of NO₃^- derived from the NH₄⁺-ex source, with the most significant effect noted for the 30 cm Pisha sandstone cover (T2).