Geochemical mapping in stream sediments of the Carajás Mineral Province: Background values for the Itacaiúnas River watershed, Brazil

Abstract

Multi-elemental analysis of high-density (regional-scale) geochemical surveys is an important strategy for multi-purpose applications, particularly in addressing geochemical background concentrations in different sampling media. This approach was applied to the Itacaiúnas River Watershed (IRW), which is situated in the most prominent mining area of Brazil, the Carajás Mineral Province. Microcatchment-based mapping (~50 km2 each) covering the whole extent of IRW was delimited using remote sensing techniques and targeted for sampling. A total of 788 samples, including 27 duplicates, were collected in 2017. The <0.177 mm fraction of all samples was digested by aqua regia and 51 elements were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). Geochemical maps for Fe and potentially toxic elements (PTE; As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sn, V, and Zn) and microcatchment-based distribution maps based on threshold values were constructed for the whole IRW. The stream sediment geochemistry is mainly controlled by the local geologic setting and underlying lithology. Geochemical background (GB) values for 43 elements in stream sediments of the IRW were determined by a variety of methods (Tukey's inner fences -TIF, median ± 2*median absolute deviation - MAD, and percentile-based techniques). The results provided from the different methods show a wide range of values, with the MAD method being considered the most appropriate for deriving GB concentrations. A comparison of reference levels for PTE contemplated in the Brazilian regulation in stream sediments, the threshold concentrations obtained for the IRW area, and different study cases around the world is presented in detail. Our findings provide not only valuable information for selecting potential areas for mineral exploration surveys, but also for evaluating geochemical contaminant effects with time-varying treatments. Studies conducted to determine background values at regional scale are needed for environmental decision making, as well as to attest actions in cases of potential contamination. In the absence of these studies, misleading interpretations of the magnitude of contamination levels in a certain area may cause under- or overestimation of ecological and/or human-health risks of PTE.