Geochemical mapping and background concentrations of iron and potentially toxic elements in active stream sediments from Carajás, Brazil – implication for risk assessment

Abstract

The Companhia de Pesquisa de Recursos Minerais (CPRM - Geological Survey of Brazil) has conducted geochemical surveys in the south-central region of the Itacaiúnas River watershed (IRW) in the Carajás region. The surveyed area covers 9,650 km2 and the data obtained by CPRM is the basis for the present study. A total of 833 active stream sediment samples, including 144 duplicates were collected. The <0.177 mm fraction of all samples was digested by aqua regia and 53 elements were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS). From the results made available by CPRM, 14 elements, including Fe and some potentially toxic elements (PTEs, namely Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sn, V, and Zn) are used in our study to define the geochemical signature of stream sediments in the area. A multivariate statistics was employed to evaluate major associations among elements and GIS techniques to derive interpolated geochemical distribution maps. Geochemical background values were calculated by multiple statistical methods, and based on that a preliminary environmental risk assessment was undertaken. Strong evidence of natural control in the spatial distribution of studied elements were observed. Large differences in the spatial distribution of many elements are observed between the central and southern region of the IRW and are related to contrasts in local geological settings. Background values were established in three different sets of geochemical sample locations, one for the whole study area, a second and third for the Central and Southern regions of the IRW, respectively. These background values were compared to existing stream sediment guideline values for (eco)toxicological effects of the same elements. Each applied statistical method to derive background values yielded different results. This difference is mainly influenced by the central criterion of the statistical method (median, mean, 75th, 95th, 98th percentiles). The Median ±2*(Median Absolute Deviation) furnished the most consistent and realistic results. Six environmental indicators were calculated to evaluate potential areas of risk. The results indicated that active stream sediments of the study area are generally uncontaminated. Occasionally local natural contamination sources were identified in the area of the recently created Ferruginous Fields National Park (FFNP). Moreover, there is no a clear impact of the changes in land use and land cover, as well as mining activities (e.g. Sossego copper mine), on the geochemistry of stream sediments. At the sampling scale, the variation in the background concentrations of all investigated elements in the stream sediment samples is apparently much larger than any anthropogenic impact.