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[Display omitted] • REEs existed in dissolved state in the stream under the effect of leaching solution. • REE fractionation in the stream was constrained by pH and ligand composition. • The secondary enrichment of REEs in the sediment was observed. • The outer-sphere complex of REEs determines whether REEs can be recycled. The exploitation of ion adsorption-type rare earth element (REE) deposits using in-situ leaching technology has become increasingly popular in recent years. However, the migration of REEs into the surrounding environment has raised significant concerns. This study aimed to systematically investigate the migration and species of REEs in two streams (northern and southern streams) surrounding a typical IAD in China. The results showed that in the northern stream, which was directly injected with the leaching solution, REEs were primarily present in the dissolved state as REE(SO 4) 2 − and REE(SO 4) +. Conversely, in the southern stream that did not receive indirect injection of leaching solution, REEs were present in both particulate and dissolved states (i.e., REE(CO 3) + and REE(CO 3) 2 −, REE(SO 4) 2 − and REE(SO 4) +). The southern stream had a higher enrichment of light REEs (LREEs) compared to the northern stream, with a L/H ratio of 1.8 and 1.6, respectively. This was attributed to the higher stability of dissolved complexes formed between heavy REEs (HREEs) and CO 3 2−. As pH increased in the northern stream, REEs were removed from stream water and deposited in sediment, leading to secondary enrichment of REEs in the sediment. X-ray absorption fine structure (XAFS) analysis revealed that REEs mainly formed outer-sphere (OS) and inter-sphere (IS) complexes in the sediment of northern and southern streams, respectively. Furthermore, a high proportion of ion-exchangeable REEs in the sediment of the northern stream indicated potential for recycling of REE resources, similar to those found in IADs. These findings contribute to the understanding of the transportation and secondary enrichment of REEs in the environment and provide a scientific basis for the comprehensive utilization of REE resources. [ABSTRACT FROM AUTHOR]