Occurrence, spatial distribution, and sources of rare earth elements in soils of large urban agglomerations in Northern China

The inadvertent release of rare earth elements (REEs), emerging as pollutants in agriculture and industry, has contributed to urban soil contamination. However, the driving factors and primary sources of their accumulation in urban soils remain inadequately understood. This study systematically investigated the concentration and spatial distribution of REEs in the Beijing-Tianjin-Hebei urban agglomeration (BTHUA) soils and identified pollution sources using gradient boosting decision tree (GBDT) and positive matrix factorization (PMF) models. Results revealed there was a significant risk of point-source pollution from REEs in BTHUA soils, primarily located in areas with heavy traffic and industrial clusters. Analysis using the GBDT model indicated that soil REEs enrichment was influenced by factors such as soil characteristics (humidity index, HI; soil total organic carbon, TOC), and industrial and urbanization activities (enterprise density, ED; distance from road, Distance). The PMF model results demonstrated that natural sources (48.04 %) were the main contributors to REEs in BTHUA soils, while industrial and transportation sources (31.87 %) and agricultural sources (20.09 %) may significantly affect the spatial heterogeneity of REEs in urban soils. These findings provide guidance for the risk management of REEs in BTHUA soils.