Exploring multi-media geochemical relationships in the southeastern Amazonian basin: A way forward to define source and background levels of potentially toxic elements in lake sediments.

Abstract

Understanding geochemical source-sink relationships is an important aspect for developing background values of potentially toxic elements (PTEs) in a lake basin. This approach was studied in the Araguaia belt of Amazonia, Brazil. A total of 96 sediments (from 13 lake cores LA1-LA13), 36 surface soils, and 19 catchment rocks were collected in 2022 and chemical analysis of these samples was performed in the fine fraction (< 177 μm) using XRF and ICP-MS. Results revealed that the PTEs concentration was significantly different (p < 0.05) between sediment, soil, and catchment rock, with more accentuated enrichment in rock followed by soils. The upper continental crust (UCC) normalization pattern shows that high enrichment of Fe, Ti, Ni, and Cr, and moderate enrichment of As, P and Cu in a few sediments, similar with catchment soils and crusts. Thus, the enrichment of PTE in lake sediments is mainly controlled by dominant catchment lithology, whereby mafic and ultra-mafic bodies are the main source of Cr, Ni and Cu, whilst meta-mafic (Couto Magalhães Formation) rocks are a primary source of As. Principal component analysis (PCA) also supported a strong source-sink geochemical relationship. It identified the major bedrock geochemical signature Cr-Ni-Co as 'mafic rocks' and Al-Ti-Nb-Ga-Zr-Hf-U-Th as 'resistant minerals', and REE group, which were nearly similar between lake sediments and catchment soils and rocks. Since the enrichment of PTE is clarified by geogenic origin, these lake sediments are best suitable for establishing geochemical background (GB). The new GB threshold values (estimated by mMAD method) for Cu, Cr, Pb and Ni are substantially higher than those specified by the Brazilian CONAMA guidelines, as well as the regional GB values of IRW. Since these high values were already evidenced in catchment materials, the new GB threshold values should be considered as a more realistic reference value for environmental risk assessment in the region. This is demonstrated in the anthropogenic risk assessment of PTEs (especially Cr and Ni) in lake sediments, which, when calculated using CONAMA guidelines, showed high to very high contamination, but mostly indicated low contamination with respect to new GB. This finding made it clear that the site-specific GB values must be incorporated in the sediment quality guideline and monitoring programme for contamination assessment and making environmental decision of a given region.