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 sediment core, low-altitude lateritic plateaus; LA2-LA14), 36 surface soils, and 19 catchment crusts/rocks were collected in 2022-2023 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 PTEs 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, catchment soils, and crusts. Since the enrichment of PTEs is clarified by geogenic origin, these lake sediments are best suitable for establishing geochemical background (GB). This was calculated using a variety of methods, while the median ± 2 Median Absolute Deviation (mMAD) method was considered as most appropiate for this study. The new GB threshold values (estimated by mMAD) for As, Cu, Cr, Pb and Ni (most notably Cr and Ni) are significantly higher than those specified by the Brazilian CONAMA-L1 guidelines, as well as the regional GB values of Itacaiúnas River Watershed (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 anthropogenic risk assessment in the region. This is demonstrated by the results of the contamination factor (CF) of Cr and Ni in lake sediments, in which the CF values with respect to CONAMA limits and IRW GB values overestimated the contamination status. This finding made it clear that the site-specific GB values must be incorporated in the sediment quality guideline for improving contamination assessment and making any environmental decision of a given region.