Effect of land use, geology, and seasonality on hydrogeochemical baseline variations in a watershed impacted by human activities in the eastern Amazon

Watershed monitoring is vital for ecosystem sustainability and contamination source identification. Such studies are scarce in the Amazon region. To address this, an integrated study was conducted to assess variations in baseline concentrations of Fe and potentially toxic elements in the Gelado Creek Watershed (GCW), eastern Amazon. This watershed, situated in the Carajás Mineral Province, hosts two world-class Fe mines with tailing dams. It has also faced extensive deforestation due to rapid human settlement and artisanal mining. Surface water samples were collected at 12 sites within the GCW during 25 monthly field campaigns from April 2016 to April 2018, and physicochemical analyses were conducted. Data processing included statistical treatment, geospatial characterization, multivariate analysis, and hydrogeochemical baseline determination. Time-series maps and multivariate hydrogeochemical patterns revealed four main catchment groups. One group, influenced by a preserved forested area, was considered a reference for determining the hydrogeochemical background. The other three were influenced by pasturelands, urban areas, and mining tailing dams. The highest concentrations of Fe, Ag, Ba, Cd, and Hg were observed at the site influenced by an urban area. High concentrations were also observed in areas with predominant pastureland due to soil exposure and runoff. Moderate to high dissolved Mn concentrations were registered, potentially from water-sediment-tailings interactions and high precipitation events. This hypothesis requires validation through robust chemical analysis and detailed assessment of extreme precipitation. This integrated assessment is a powerful tool for identifying contamination sources, highlighting the need for similar studies to understand geogenic and anthropogenic impacts in the Amazon.