Environmental background in Amazonian rivers near the industrial pole, northern Brazil

The lack of a robust long-term environmental baseline has hindered risk management in the industrial complex region of Abaetetuba and Barcarena, Eastern Amazon. This study fills that gap by analyzing a 41-year time series (1980-2021; N = 19,292) and applying statistical tools such as the Median Absolute Deviation (MAD) to establish the geochemical background. Results for key physicochemical parameters indicate typical tropical conditions (temperature near 30 °C; slightly acidic pH: 6.8 ± 1.0) and low buffering capacity (alkalinity < 30 mg l⁻¹). Nutrient data revealed clear anthropogenic enrichment, with total phosphorus (TP) occurring in excess (0.2 ± 1.9 mg l⁻¹) and a low N/P ratio (2.5). Four metallic elements exceeded legal thresholds: aluminum (Al, 0.0009-1.31 mg l⁻¹), iron (Fe, 0.002-1.14 mg l⁻¹), cadmium (Cd, 0.0001-0.001 mg l⁻¹), and lead (Pb, 0.0001-0.019 mg l⁻¹). While Al and Fe concentrations are linked to regional geology and exhibited negative temporal trends, Cd and Pb require particular attention due to their association with rising anthropogenic pressures. Overall, the study successfully identified environmental thresholds, generating the first 41-year background reference for the region. This contribution is crucial for distinguishing natural variability from long-term contamination and significantly strengthens environmental monitoring and impact assessment in Amazonian aquatic ecosystems.