Contrasting Seasonal Responses and Source Signatures of Dissolved Rare Earth Elements in the Mun River, Thailand

Although dissolved rare earth elements (REEs) are increasingly utilized to explore riverine geochemical processes, their seasonal responses and source controls remain poorly constrained. This study investigated the sources of dissolved REEs in the Mun River and evaluated their responses to monsoon-driven hydrological processes. ΣREE ranged from 16.41 to 467.01 ng/L. Most river waters displayed negative Ce anomalies and positive Eu anomalies, whereas anthropogenic Gd enrichment was localized. ΣREE concentrations showed no significant seasonal difference in upper reaches but were significantly higher during wet season in the middle-lower reaches. Light REEs (LREE) and Middle REEs (MREE) generally exhibited stronger wet-season responses than Heavy REEs (HREE) in the middle and lower reaches. Positive matrix factorization (PMF) analysis identified three major sources: soil erosion and surface runoff (34.9% of ΣREE), bedrock weathering (16.6%), and mixed inputs influenced by agricultural runoff and wastewater discharge (48.5%). Cluster analysis further revealed a longitudinal transition from the upstream weathering-dominated conditions to enhanced material accumulation, organic complexation, and anthropogenic influences within the downstream region. These findings demonstrate how monsoonal precipitation regulates REE composition and sources by modifying catchment connectivity, surface runoff, and material transport, providing insights into seasonal mobility of trace elements in tropical agricultural basins.