Geochemical thresholds and bioavailability of potentially toxic elements in Amazonian soils: a risk-based framework for environmental management in mineralized watersheds

This study evaluates the distribution, mobility, and lithological control of potentially toxic elements (PTE: As, Ba, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sn, V, Zn), Al and Fe in surface soils of the Verde River Watershed (VRW), a highly mineralized and anthropogenically influenced basin within the Carajás Mineral Province (CMP), Brazilian Amazon. The CMP hosts world-class Fe ore reserves, large-tonnage IOCG (iron oxide–copper–gold), and Ni laterite deposits, while also being affected by small-scale Cu and Au mining, which contributes to regional environmental pressures. A total of 113 topsoil samples were analyzed for pseudo-total PTE (ICP–MS) and bioavailable (DTPA) fractions. Geochemical baselines were established using the modified median absolute deviation method, and multivariate analyses (clr-PCA, HCA), contamination indices (Igeo, EF, PLI, RAC), and chondrite-normalized REE patterns were applied to distinguish between geogenic enrichment and potential anthropogenic contributions. Results indicated that soil texture, mineralogy, and organic matter content are key drivers of PTE behavior. Soils developed over mafic–ultramafic lithologies in the CB showed higher clay and Fe/Al oxide content, promoting PTE retention. In contrast, felsic-derived soils in the CCD exhibited greater metal mobility, particularly for Cu, Zn, and Mn. RAC assessments identified Mn and Zn as the most mobile and potentially hazardous elements, while Fe remained largely immobile. The spatial variability of risk classifications aligned with lithological and pedogenic differences, underscoring the influence of geodiversity on contaminant dynamics. These findings contribute to a better understanding of metal behavior in tropical soil systems and offer a scientific basis for environmental monitoring and sustainable land management in mining-affected landscapes.