Geochemistry and spatial distribution of heavy metals in Oxisols in a mineralized region of the Brazilian Central Plateau

The majority of the soils of the Central Plateau of Brazil are product of long time development over stable surfaces, usually associated with erosion and re-deposition cycles. In this context, to successfully study the geochemistry and spatial distribution of heavy metals, it is essential to understand the geology as well as the geomorphology of the landscape and the properties of the metals related to their mobility. The main objective of this work was to evaluate contents of naturally occurring heavy metals (Co, Cu, Mn, Ni, Pb and Zn) in some highly weathered tropical soils as related to their geochemistry, geology and geomorphology. Additionally, it was aimed to interpret the spatial distribution patterns of these metals and of major elements (Al, Fe, Ti and Mg), evaluating the association and taking the influence of the geology and geomorphology into consideration. In geo-referenced sites, ranging from 480 to 1040 m in altitude, samples were collected at two depths (0–20 and 60–80 cm), ground, sieved, and prepared for analysis. Heavy metals and major element contents were determined by extraction with aqua regia. Descriptive statistics, Pearson correlation coefficients, Principal Components Analysis (PCA) and spatial variability analyses were conducted and soil metal concentrations for the whole area were predicted by kriging (interpolation). A close association between Pb, Zn, and Mn was observed using PCA and Pearsonʹs correlation. Highest concentrations of Pb, Zn, and Mn were determined in soils between 650 and 550 m altitude surrounding the dolomite massif hills and Ambrósia and Fagundes mineral deposits. Copper and Fe were found to be geochemically associated with highest concentrations observed in soils formed from carbonaceous phyllite with quartzite layers. Cobalt and Ni concentrations were associated with areas characterized geomorphologically as alluvial–colluvial deposits at the lowest altitudes within the region. Although there is a marked dispersive action of tropical weathering on metal distribution in the region, Cu, Pb, and Zn were found spatially associated with their geological source. This probably happens because of greater affinity of those metals to Fe and Mn oxides formed predominantly under tropical conditions and present in higher concentrations near to the sources of these heavy metals. On the other hand, both the low affinity of Ni and Co for these oxide minerals and the regionʹs geomorphology allowed for their migration towards the drainage network to the regions of sediment accumulation at lower altitudes during the landscapeʹs evolution.