An improved method for tracing soil erosion using rare earth elements

PURPOSE: The use of rare earth elements (REEs) as tracers provides a high-precision method for quantitative determinations of soil particle movement in soil erosion studies. In this study, a new calculation method was developed and tested to improve the precision of the REE tracing method and to expand the application of this method to areas with coarse-textured soils. MATERIALS AND METHODS: This study used purple soil to simulate a catchment with data based on a field survey of a small catchment located in the Three Gorges Area in China. Eight different powdered rare earth oxides, which included La₂O₃, CeO₂, Nd₂O₃, Sm₂O₃, Eu₂O₃, Tb₄O₇, Ho₂O₃ and Yb₂O₃, were applied as tracers to describe soil movement in this scaled catchment during three simulated rainfall events of 1.0, 1.5 and 2.0 mm min⁻¹ rainfall intensity. Leaching experiments were conducted to investigate the vertical migration of REEs in soil layers. Particle size distributions (PSDs) and REE concentrations for each soil particle size class (1–2, 0.5–1, 0.25–0.5, 0.1–0.25, 0.075–0.1, 0.05–0.075, 0.02–0.05, 0.005–0.02 and <0.005 mm) were analysed to evaluate the precision of the proposed calculation method. RESULTS AND DISCUSSION: Most REEs remained in the first layer during leaching. The scanning electron microscopy-energy dispersive X-ray (SEM-EDX) mapping images showed that more REEs were adsorbed by small particles (≤0.1 mm), with large specific surface areas, than by large particles (>0.1 mm). During the three rainfall events, the coarsest size classes (1–2 and 0.5–1 mm) of the sediment samples were less than that of the soil. In contrast, the other classes, including <0.075 mm, showed the strongest adsorption for REEs, and the weight percentage of grains in eroded sediment was more than that in the source soil. The accuracy of the new proposed calculation method increased by 24.37, 20.25 and 3.84 % for the first, second and third storm, respectively, compared with the uncorrected mass of soil loss from the scaled catchment. CONCLUSIONS: The REEs bonded well with purple soil particles and the leaching of REEs from the tagged layer to the lower layers was minimal. The <0.075-mm particle size class had the strongest adsorption capacity for REEs. The soil loss estimates were improved with the new calculation method.