Characteristics and Evaluation of Soil Rare Earth Element Pollution in the Bayan Obo Mining Region of Inner Mongolia

The Bayan Obo deposit is the largest light rare earth ore deposit in the world, which releases rare earth elements (REEs) to the surrounding environment through long-term mining processes. To inform restoration plans, it is necessary to investigate the concentration, spatial distribution, pollution level, and ecological risk of REEs. Sample analyses showed that the average total concentration of REEs in this area is 6064.95 mg·kg-1, which was higher than the background levels of control soils (207.44 mg·kg-1), Inner Mongolia (150.95 mg·kg-1), and China (184.72 mg·kg-1). Light REEs (LREEs) accounted for 83%-99% of the detected REE, and Ce was the dominant element. Areas with high REE concentrations were mainly located near the source bed, and the distribution was extremely inhomogeneous, being greatly affected by external interference. Chondrite normalized REE patterns of different functional areas were similar and normalized curves inclined to the right, indicating district fractionation between the LREE and heavy REEs (HREEs). Significant negative Eu anomalies and positive Ce anomalies were observed in the soils based on δCe and δEu values. La/Yb, La/Sm, and Gd/Yb ratios all indicated that the soils were LREE-enriched, whereas the LREEs were more fractionated than the HREE. Four methods were employed to evaluate the pollution and ecological risk of the detected soil REEs. The average values of Ce, Nd, Pr, and La reached heavily contaminated levels based the geo-accumulation index (Igeo). The modified degree of contamination method showed that the average mCd values of REEs in different functional areas ranged from 7.14 to 31.38. The tailings pond had a high level of pollution, residential and industrial areas had a very high pollution level, and the mining area and waste dump showed extremely high levels of pollution. Based on the pollution load index, the tailings pond is moderately polluted while all other functional areas are severely polluted. The potential ecological risk index values ranged from 120.99 to 6376.46, with REEs in soils posing high strong risk, very strong risk, strong risk, moderate risk, and low risk in 33%, 16%, 12%, 30%, and 9% of the sampling sites, respectively. Based on these findings, measures for controlling current pollution and potential ecological risks from REE in the soils of the Bayan Obo mining region are urgently needed.