Mapeamento geoquímico por sedimentos de corrente da bacia hidrográfica do Rio Capim-Pa: definição de valores de baseline geoquímico e aplicação de índices ambientais

Abstract

Geochemical mapping plays a key role in both mineral prospecting and environmental studies. Initially used to identify geochemical anomalies associated with mineral deposits, these studies have gained relevance in the field of environmental geochemistry, enabling the monitoring of soil, water, and sediment quality. The Geological Survey of Brazil (SGB) conducts geochemical survey projects across the national territory. One such project, carried out between 2009 and 2012, called "Multi-use Geochemistry," covered the Capim River Watershed (CRW) area and generated a geochemical sediment dataset for the region. The objective of this research was to use this available data to create multielemental geochemical maps and define baseline geochemical values, particularly for potentially toxic elements. The survey included the collection of 276 stream sediment samples with a sampling density of approximately one sample every 150 km². The samples were processed in the laboratory for analysis by ICP-OES and ICP-MS with hydride generation for Hg. In total, 52 elements were analyzed (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pd, Pb, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, U, V, W, Y, Zn, and Zr). For statistical treatment, statistical summaries were calculated along with box plots, histograms, and normal probability curves using log-transformed data for univariate statistics; Spearman correlation for bivariate statistics; and factor analysis (FA) to define multielemental associations in multivariate statistics. Three methods were applied to calculate the geochemical baseline (BG), including the Median ± 2*Median Absolute Deviation (mMAD), Tukey's Inner Fence (TIF), and Percentiles (98, 95, 90, 75). The results revealed important geochemical distribution trends, with a particular focus on potentially toxic elements. In total, 28 elements (Ag, Al, Ba, Bi, Ce, Co, Cr, Cu, Fe, Ga, Hf, Hg, La, Mo, Nb, Ni, Pb, Rb, Sc, Sn, Sr, Th, Ti, U, V, Y, Zn, and Zr) underwent more complex statistical tests, while 24 elements were not tested due to ≥30% of the data being below the detection limit. Correlation analyses showed that there were 7 correlations with significance values greater than 0.65 and 6 acceptable correlations with values between 0.575 and 0.65. It was observed that Cr levels exceeded the values stipulated by the resolution in all three methods analyzed: TIF (73.11 mg.kg⁻¹), mMAD (46.76 mg.kg⁻¹), and 98th Percentile (43.00 mg.kg⁻¹). In the lithological unit analysis, Cr anomalies were most pronounced for the siliciclastic rocks of the Barreiras Formation, with TIF (182.94 mg.kg⁻¹), mMAD (90.88 mg.kg⁻¹), and Percentiles 98th (66.28 mg.kg⁻¹), 95th (62.20 mg.kg⁻¹), 90th (52.60 mg.kg⁻¹), and 75th (40.52 mg.kg⁻¹), as well as in Lateritic Covers with TIF (81.76 mg.kg⁻¹) and mMAD (46.49 mg.kg⁻¹). The mMAD method was considered the most conservative and robust, without overestimating the upper limit values. Factor analysis allowed the identification of 7 factors, and along with the spatial mapping of the scores, it was possible to identify the geochemical associations, which primarily reflect the geological influence in these sediments, with lateritic terrains, iron oxyhydroxides, and aluminossilicatic clastic rocks being the main factors responsible for the distribution of the elements. Geochemical and environmental indices were calculated, such as the Enrichment Factor, Contamination Factor, Modified Contamination Degree, and Potential Ecological Risk Index. These indices indicated that, in several microbasins, there are signs of geological sources, but they mainly point to the northern, western, and southwestern areas of the CRW as the most contaminated. These approaches enabled the identification of contaminated areas, especially related to Cr, Cu, Hg, Ni, and Pb, whose concentrations exceeded the limits established by CONAMA Resolution 454/2012 at several points in the basin. However, it is worth noting that the normative uses reference values for total sediment, and the results of the present work are overestimated compared to this standard. Nevertheless, the use of environmental indices must be done with caution, as there are limitations that, when analyzed in isolation, may provide information not entirely aligned with local reality, either overestimating or underestimating data. It is important to highlight that, despite presenting higher concentrations of these elements, the values found are still lower than those found in geochemical provinces with mineralizations. This indicates a strong influence of superficial geochemical processes in the adsorption of Cr, Cu, Ni, and other potentially toxic metals in iron oxyhydroxides. Furthermore, the study reaffirms the use of the mMAD method as the most robust and suitable for defining baselines and the use of geochemical distribution maps as an essential tool for territorial management.