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

Abstract. 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. In this paper, the characteristics of mineral-leaching chemicals in surface soil samples collected in the field were studied. In addition, NH₄⁺ and SO₄²⁻ transport via soil erosion was monitored using runoff and sediment samples collected during natural rainfall processes. The results demonstrated that the NH₄⁺ contents in the surface sediment deposits increased from the top of the heap (6.56 mg kg⁻¹) to the gully (8.23 mg kg⁻¹) and outside the tailing heap (13.03 mg kg⁻¹). The contents of SO₄²⁻ in the different locations of the tailing heaps ranged from 27.71 to 40.33 mg kg⁻¹. During typical rainfall events, the absorbed NH₄⁺ concentrations (2.05, 1.26 mg L⁻¹) in runoff were significantly higher than the dissolved concentrations (0.93, 1.04 mg L⁻¹), while the absorbed SO₄²⁻ concentrations (2.87, 1.92 mg L⁻¹) were significantly lower than the dissolved concentrations (6.55, 7.51 mg L⁻¹). 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, which can provide a scientific foundation for erosion control and soil management in rare-earth tailing regions in southern China.