Rare Earth Elements as Emerging Contaminants in Negro River Sediments During an Extreme Drought Event (Central Amazon)

Extreme climate events are intensifying in the Amazon Basin, with the 2023 drought marking the most severe on record in the Negro River, exposing extensive sediment banks and raising concerns about geochemical alterations in this acidic, organic-rich blackwater system. Rare earth elements (REEs), increasingly recognized as emerging contaminants and geochemical tracers, were analysed in surface and profile sediments from Anavilhanas Archipelago and from Manaus. Samples from Anavilhanas Archipelago exhibited low ΣREE concentrations and fractionation patterns consistent with natural mafic sources. In contrast, Manaus sediments, particularly those exposed to air, showed elevated ΣREE levels (up to 167.6 mg/kg), enhanced ΣLREE/ΣHREE ratios, and positive cerium anomalies, indicating oxidative processes and potential anthropogenic inputs. Positive europium anomalies across all sites reflected plagioclase retention under variable weathering conditions. Ecological risk assessment using the Potential Ecological Risk Index (PERI), indicated moderate risk, with Eu, Tb, Ho, Tm, and Lu contributing most, elements linked to electronic waste and industrial activity. Expansion of the Manaus Free Trade Zone has increased e-waste generation, much of it improperly discarded into rivers, contributing to REE accumulation. This study provides the first evaluation of REE fractionation and ecological risk in Negro River sediments during an extreme drought, establishing a geochemical baseline and underscoring the need for systematic monitoring and regulatory frameworks to mitigate REE-related risks in climate stressed Amazonian watersheds.