Numerical Investigation of Evaporation-Driven Salt Uptake in Store-Release Soil Covers Over Hypersaline Tailings

Abstract

Placing moisture store-release (SR) soil covers over hypersaline tailings has been a promising way to rehabilitate closed tailings storage facilities. However, the performance of SR covers in limiting evaporation-driven salt migration remains less investigated. A one-dimensional numerical model that couples the transport of liquid water, water vapor, heat and solute was developed. The model was calibrated using monitored data obtained from an instrumented column comprising a monolithic silt cover overlying compacted bauxite residue. The validated model was then applied to investigate evaporation behavior and salt distribution in monolithic covers using three representative cover materials over both compacted and coarse tailings. Simulations of the two uncovered tailings indicated that tailings after compaction can significantly impede salt accumulation with depth, preventing surface salt crust formation. The simulated six cover scenarios suggested that a developing mixing zone of freshwater and saltwater in the cover during desiccation and its upper boundary can be termed the salinization plane, which represents saltwater intrusion. Under given atmospheric conditions, the maximum salinized depth in the cover was determined by the location where the salinization plane overlapped with the vaporization plane. In 1-year drying simulations, the fine sand cover was most vulnerable to salinization as over two-thirds of the depths were affected by salt uptake. Although the clay cover could effectively limit salt uptake and keep tailings saturated in the 1-year drying process, the silt cover was an optimal material for SR covers due to its robust ability to limit salt uptake and moderate water-release capacity.