Performance assessment of a monolithic soil cover over hypersaline bauxite residue under natural weather conditions

In arid and semi-arid regions, the function of a monolithic store-release soil cover for mine tailings with a thin thickness is underpinned by its capability of limiting net percolation and preventing salt capillary rise from hypersaline residue into the overlying cover, which is critical for mine closure and rehabilitation. An instrumented column filled with 600 mm of well-graded silty sand overlying 600 mm of compacted bauxite residue was constructed to investigate the hydrological performance of the store-release soil cover under a semi-arid climate. The water balance was analyzed by comparing water storage in the column derived from the moisture profile and that from cumulative evaporation and rainfall. Both methods agreed well during a two-year monitoring time, during which two dry and two wet seasons were identified. Results indicated that the upper 200 mm of cover soil acted as a moisture-releasing layer with a low depth-averaged degree of saturation (~0.5), while the lower 400 mm of cover soil behaved as a moisture-storing layer with a high depth-averaged degree of saturation (~0.7). The underlying bauxite residue remained saturated most of the time, except during the extremely dry season when an 850 mm unsaturated zone and a 50 mm dry soil layer developed. Evaporation-driven salt capillary rise was not evident throughout the monitoring time, due to the restraint from stored freshwater in the lower cover soil and the low hydraulic conductivity of the compacted residue. • The upper 200 mm cover soil acted as a moisture-releasing layer. • Controlling moisture fluctuation in the lower 400 mm cover is a key aspect of limiting salt capillary rise. • Evaporation-driven salt capillary rise was not evident throughout the two-year monitoring time. • Analysis of depth-averaged saturation in the cover and bauxite residue can assess the cover performance.