Rare earth elements as emerging contaminants: Multi-source release, geochemical tracing, and implications for environmental monitoring and management.

Rare earth elements (REEs) are increasingly vital to green and high-tech industries, but their expanding use has led to widespread environmental contamination. This review evaluates REE release pathways, tracing methodologies, and knowledge gaps that limit effective environmental management. We synthesize REE inputs across mining, agricultural, industrial, and medical sectors. In mixed-source environments, overlapping geochemical signatures complicate source identification, a challenge exacerbated by global REE recycling rates below 1% and dissipation rates exceeding 90%. Against this backdrop, we systematically compare a suite of source-tracing approaches, including REE anomaly calculation, isotopic tracing (e.g., strontium, neodymium, and lead isotopes), receptor modeling (e.g., positive matrix factorization and chemical mass balance), and speciation analysis (e.g., high-performance liquid chromatography coupled with inductively coupled plasma-mass spectrometry and diffusive gradients in thin films). We identify two major methodological bottlenecks. First, the proliferation of over seven normalization systems and more than 30 anomaly equations yields results that are not readily comparable across studies. This is further complicated by region-specific baselines, particularly for gadolinium. Second, the predominant reliance on total concentrations obscures REE speciation and bioavailable fractions. To address these limitations, we propose a tiered monitoring framework that couples screening-level anomaly detection with quantitative source apportionment. This framework prioritizes standardized calculations, local baseline mapping, and bioavailability-based risk assessment. Advancing REE governance requires a shift from passive monitoring to a proactive framework that links environmental occurrence, geochemical tracing, and bioavailability-informed risk assessment.