Mineral leaching chemicals transport with runoff and sediment from severely eroded rare earth tailings in southern China

Rare earth mining has led to severe soil erosion in southern China. Furthermore, the presence of the mineral leaching chemical ammonium sulfate in runoff and sediment poses a serious environmental threat to downstream water bodies. To study the transport characteristics of mineral leaching chemicals, soil on rare earth tailings was sampled in different positions, and runoff and sediment samples were collected along flow routes during three typical rainfall events. The results demonstrated that the NH4+ contents in the surface sediment deposits increased from the top of the heap (6.56 mg/kg) to the gully (8.23 mg/kg) and outside the tailing heap (13.03 mg/kg). The contents of SO42− in the different locations of the tailing heaps ranged from 27.71 to 40.33 mg/kg. During typical rainfall events, the absorbed NH4+ concentrations (2.05, 1.26 mg/L) in runoff were higher than the dissolved concentrations (0.93, 1.04 mg/L), while the absorbed SO42− concentrations (2.87, 1.92 mg/L) were lower than the dissolved concentrations (6.55, 7.51 mg/L). The dissolved NH4+ and SO42− concentrations in runoff displayed an exponentially decreasing tendency with increasing transport distance (Y=1.02*exp(-0.00312X), Y=3.34*exp(-0.0185X)). No clear trend with increasing distance was observed for the absorbed NH4+ and SO42− contents in transported sediment. The NH4+ and SO42− contents had positive correlations with the silt and clay ratio in transported sediment but negative correlations with the sand ratio. These results provide a better understanding of the transport processes and can be used to develop equations to predict the transport of mineral leaching chemicals in rare earth tailings.