Seasonal controls on dissolved rare earth element signatures in large tropical rivers: interplay of monsoonal discharge, lithology, and anthropogenic inputs

Understanding the geochemical behavior of dissolved rare earth elements (REEs) in large tropical rivers is critical for tracing global silicate weathering and anthropogenic fluxes to the oceans. This study investigates REE dynamics in two major river systems, the Godavari and the Mahanadi, which drain contrasting lithologies (Deccan basalt vs. heterogeneous metamorphic terrains) and discharge into the Bay of Bengal. In the Godavari River, water samples were collected along the upper 725 km-stretch draining basalts, while sampling in the Mahanadi was conducted in the lower reaches along a 350 km-stretch from downstream of Hirakud dam. Rare earth element (REE) concentrations were analysed in water samples, using a Quadrupole ICP-MS. Additionally, bedrocks, bed sediments and suspended sediments from the Godavari were analysed for REEs. Based on a comprehensive three-year seasonal dataset ( n = 181), the results show that REE distributions are primarily governed by the interplay of monsoonal hydrology and specific mineralogical weathering. PAAS-normalized REE patterns reveal significant heavy REE (HREE) enrichment in both rivers, consistent with preferential adsorption of light REE (LREE) onto siliciclastic sediments and Fe-Mn oxyhydroxides. Superimposed on this baseline pattern, the total dissolved REE load rises sharply during high-flow monsoon conditions, when intensified weathering and desorption of REE from colloidal particles supplement the dissolved pool. A pronounced positive europium anomaly (Eu/Eu * ) during the pre- and post-monsoon periods (collectively termed “non-monsoon” hereafter) is attributed to the selective weathering of plagioclase minerals and colloidal interactions, evidenced by a negative correlation between Zr/Na ratios and Eu/Eu * . Furthermore, the study identifies a fundamental shift in weathering regimes: the Mahanadi exhibits a transport-limited regime with REE-pH sensitivity, whereas the Godavari maintains a stable REE chemistry despite seasonal fluctuations reflecting a weathering-limited regime. Critically the emergence of lanthanum (La/La * ), samarium (Sm/Sm * ) and gadolinium (Gd/Gd * ) anomalies across both systems indicate that dispersed anthropogenic contaminants from industrial and wastewater effluents are now overprinting natural geological signatures. As REEs are increasingly classified as critical raw materials for global green technologies, our results provide a baseline for identifying the natural background versus anthropogenic enrichment of these strategic elements in densely populated tropical catchments.