Stratigraphy-driven natural background for potentially toxic elements in a heavily mined Amazonian watershed

Abstract The Verde River watershed (VRW) in southeastern Amazonia is undergoing extensive channel modification and sediment disturbance from illegal alluvial gold mining, yet site-specific geochemical baselines capable of separating natural from mining-derived metal enrichment remain lacking. We integrated morphostratigraphic reconstruction with high-density geochemical analyses of potentially toxic elements (PTE: As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sn, V, Zn) in stratigraphically constrained alluvial deposits to establish pre-anthropogenic background (natural) values and quantify enrichment in recently reworked sediments. Over the past ∼7600 years, sediment geochemistry has been shaped by shifts in provenance between mineralized mafic and felsic lithologies and by climatic variability, producing naturally elevated Cu (93–455 mg kg −1 ), Cr (124–352 mg kg −1 ), and Ni (40–107 mg kg −1 ) that in some cases exceed Brazilian and international sediment quality guidelines (threshold, probable, and severe effect levels) without anthropogenic input. While these natural concentrations generally indicate low ecological risk, comparisons between lithology-specific backgrounds and residues from illegal mining sites show that Cu, Co, and Ni are the most consistently and significantly enriched PTE, whereas Hg, historically the focus in Amazonian mining studies, remains below moderate contamination factors at most sites. These results highlight that stratigraphy-anchored backgrounds are critical for distinguishing geogenic from anthropogenic signatures in tropical alluvial systems, thereby improving ecological risk assessment and guiding targeted monitoring and remediation in mining-impacted Amazonian watersheds.