Large-Scale Monitoring and Risk Assessment of Metal Contamination from Urban Areas in the Amazon River Basin

Abstract In this study we provide a comprehensive assessment of metal contamination in the Amazon River and its main tributaries, with a focus on urban contamination hotspots. A monitoring campaign was conducted across 40 sampling sites, analysing water concentrations of As, Cd, Cu, Cr, Fe, Mn, Pb, Ni, and Zn. Metal concentrations were evaluated against national and international water quality standards and compared between urbanized and less impacted regions to delineate pollution patterns. Additionally, an ecological risk assessment was conducted for individual metals and for metal mixtures using Species Sensitivity Distributions (SSDs). The results of this study show that Fe levels in the Amazon River were exceptionally high, with potential implications for aquatic species distribution across the basin. Exceedances of water quality standards were identified for four metals (Fe, Pb, Mn, Cu), with metal enrichment observed particularly in Manaus, Macapá, and Belém. Acute multi-substance potentially affected fraction (msPAF) of species in the Amazon River and its tributaries indicated insignificant risks, while chronic values ranged from 3% to 33%. In urban areas, acute msPAF values were generally low, with the exception of two samples taken in Manaus that exhibited values of 11% and 16%. Chronic riskvalues, however, were consistently high in these urban locations, reaching up to 72% of species potentially affected and reflecting sustained ecotoxicological stress. Cu, Zn and Mn were identified as the primary contributors to chronic toxicity, with Cu and Zn dominating in Manaus, and Mn in Macapá. These findings underscore the need for stricter industrial discharge regulations and improved wastewater treatment systems to preserve Amazonian freshwater ecosystems.