Slope-Dependent Migration of Rare Earth Elements in the Topsoil Impacted by Ionic Rare Earth Mining

Introduction: As a unique strategic mineral resource of China, ionic rare earth ores are primarily distributed in southern subtropical hilly regions. The prevalent in-situ leaching technology has drastically altered the soil structure and geochemical environment of mining areas by disrupting the adsorption balance between rare earth elements (REE) and clay minerals, thereby activating and enriching REE in the topsoil. Current studies on REE migration in mining areas mostly focus on mining methods or soil properties, lacking quantitative analysis of how slope position affects REE migration processes. Method: Through simulated rainfall leaching experiments on abandoned mine soils across slope positions (uphill, middle slope, downhill), this study reveals slope-dependent REE migration. Results: 1) Leaching primarily altered light rare earth elements (LREE) in uphill and middle slope soils, but significantly impacted both LREE and heavy rare earth elements (HREE) in downhill soils; 2) Maximum REE accumulation occurred at 15 cm depth across all slope positions, with total rare earth elements (TREE) in mining areas (avg. 425.50 mg·kg⁻¹) exceeding Fujian background levels (236.77 mg·kg⁻¹); 3) Positive δCe anomalies in mining soils correlated with reduced REE mobility, evidenced by inverse relationships between δCe and fractionation indices (e.g., (La/Yb)N, R²>0.95). Conclusion: This study confirms that ionic rare earth mining enriches REE in topsoil but does not alter their chondrite-normalized distribution pattern, indicating mining primarily affects REE content rather than geochemical fractionation. Notably, slope position governs REE migration: uphill and middle slopes experience rapid surface runoff, causing loss of mobile LREE; downhill areas accumulate leachate, driving co-migration of LREE and HREE and REE enrichment at 15 cm depth. Additionally, δCe anomalies effectively reflect REE mobility, with their strong negative correlation to fractionation indices serving as a reliable tracer for environmental risk assessment. These findings advance understanding of REE geochemical behavior in mining areas and provide a scientific basis for targeted remediation (e.g., uphill soil and water conservation, downhill leachate control). conclusion: This rainfall simulation study demonstrates the following key findings: (1) Mining fundamentally alters REE distribution in topsoil. These differences become pronounced during leaching, where REEs migrate and enrich at approximately 15 cm depth. While mining changes the specific element concentrations and their migration behaviour during leaching, the overall chondrite-normalized distribution pattern remains consistent. (2) Ce constitutes a major fraction of REEs, and its anomaly (δCe) effectively characterizes mining-induced differences in soil REE composition. Increased positive δCe anomalies correlate with reduced REE enrichment in topsoil. (3) Rainfall simulation induces changes and migration of REEs in post-mining topsoil. In the mining area, LREE variations dominate the uphill and middle slope positions, while both LREE and HREE show significant changes downhill. Topsoil in the downhill position poses the highest potential risk for REE migration under leaching conditions.