Spatiotemporal distributions and potential sources of sediment and waterborne heavy metals in lowland lakes and rivers of the Ecuadorian Amazon

The recent rapid expansion of mineral and petrochemical extraction throughout Amazonia has intensified concerns about the extent and origins of heavy-metal contamination in one of the world’s most biodiverse freshwater ecosystems. While most recent studies have emphasized anthropogenic sources (direct releases, produced waters, spills, roads, and other sources to the environment from human activities), the role of natural geochemical processes remains comparatively understudied. Here, we assess seasonal and spatial variation in heavy metal concentrations in water and sediments of the Napo and Pastaza drainage basins in eastern Ecuador, with an assessment of potential environmental and health risks. We analyzed total and bioavailable concentrations of ten heavy metals (Cd, Co, Cr, Cu, Fe, Hg, Mn, Pb, V, Zn) in water and in sediment samples using inductively coupled plasma–optical emission spectrometry (ICP-OES). Metal concentrations often exceeded background levels in the Amazon Basin, with some values comparable to those in mining-impacted areas. Geological factors such as black shale weathering and volcanism likely explain regional patterns. Despite occasional high values, enrichment factors were generally low. Key physicochemical variables influenced metal distribution, and most metals were found to be immobile and biologically unavailable. Seasonal and spatial variations in metal concentrations reflected trends seen in other tropical floodplain systems, indicating common, widespread drivers of aquatic metal levels across Amazonia. Important physicochemical factors, such as hardness, pH, organic matter, and dissolved oxygen, showed habitat-specific effects on metal concentrations. Although distribution coefficients (Kd) and mobility factors (MF) suggested that most sites had metals that were immobile and biologically unavailable, few locations showed high chances of metal migration, which could pose environmental and human health risks. Overall, our results imply limited immediate threat to ecosystems or human populations but highlight the need for continued monitoring of identified hotspots.