Mapeamento geoquímico multielementar e valores de referência ambiental em um estuário tropical de macromaré: complexo estuarino São Marcos, Brasil

Abstract

Temporal-scale geochemical analysis, applied to estuarine environments located in tropical zones, becomes particularly effective when associated with advanced data-processing techniques and robust statistical models. Its application in macrotidal systems has proven to be a relevant tool for environmental studies in estuaries with port infrastructure, chiefly for differentiating between natural and anthropogenic concentrations of potentially toxic elements (PTEs). The definition of background values, based on dated sediment cores, is widely used to compose a robust geochemical history and to provide a source of locally relevant environmental quality reference values for the updating of standards that establish threshold values without considering regional heterogeneity. In Brazil, geochemical studies in tropical estuarine environments are sporadic and recent, especially along the North–Northeast coastal region. The CESM has worldwide relevance for constituting one of the largest estuarine systems on the South American Atlantic coast, characterized by a macrotidal regime with amplitudes greater than 7 meters and for hosting the largest port complex for cargo shipment, especially mineral exports, in Brazil. The present research geochemically characterized the São Marcos Estuarine Complex (CESM) in two distinct climatic seasons. The main objective was to investigate the geochemical signature of the CESM under distinct hydrological conditions, to establish natural background values through integrated temporal analysis from sediment cores collected with a Russian corer and dated by Carbon‑14, with ages prior to the colonization of Brazil. The methods for acquiring sediment samples involved the collection of six sediment cores, along which 15 samples were collected at 10 cm intervals, encompassing a temporal record of approximately 1,700 calibrated years for the establishment of historical natural geochemical background. In addition to the cores, 549 samples were collected with a “Gibbs”-type dredge in two consecutive campaigns, in 2020 (period of lower precipitation and low fluvial discharge) and in 2021 (greater precipitation and increased discharge), obtained from a previously established sampling grid, spatially distributed along the estuarine channel and the adjacent inner continental shelf. The sediment samples underwent drying, disaggregation, quartering, and sieving. Aliquots of approximately 20 g of the < 0.177 mm fraction were digested in a solution composed of hydrochloric and nitric acids (aqua regia) and analyzed to determine the concentration of 53 chemical elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Lu, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, Re, Sb, Sc, Se, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn, and Zr) by ICP‑OES and ICP‑MS. The determination of total organic carbon (TOC) and total sulfur (TS) employed infrared spectroscopy. Descriptive statistical analyses, tests to determine distribution normality, and subsequently multivariate statistical techniques were employed to identify multielement associations, using statistical packages in the RStudio software. For the calculation and determination of threshold (baseline) values and natural geochemical background, referenced methods for bottom sediments and dated sediment cores were used. Geochemical indices (Igeo, CF, ER, CD, PLI, RI) were also calculated for the assessment of sediment quality. Geospatial information was processed in a GIS environment, using algorithms in the QGIS software, to generate geochemical distribution maps. The natural background results obtained through sediment cores were determined according to the depositional environment and, in general, are relatively higher than the concentrations observed in current bottom sediments, as obtained by statistical methods, confirming the system’s current impoverishment. Stratigraphic analysis of the cores indicates a succession of sedimentary facies characteristic of tidal flats associated with mangroves, muddy tidal flats, mixed‑tide environments, and sandy bars, evidencing variations in depositional environments throughout the lithological record. The temporal evolution reveals two distinct phases: pre‑anthropogenic (> 100 years BP) and modern (< 100 years BP), with progressive geochemical impoverishment, possibly related to changes in sedimentary dynamics and diagenetic processes. The results of the bottom sediment samples showed the existence of five distinct geochemical groupings. The primary Co–Ni–Cr–V signature (correlations > 0.90) reflects inheritance from resistate minerals and/or Fe oxyhydroxides originating from the continent and proved temporally stable (< 5% variation). The association of the chalcophile elements Cu–Zn–Co was possibly more sensitive to conditions with lower pH and greater organic complexation arising from the increased input of continental water, increasing the mobilization of Zn (+51.9%) and Cu (+18.5%). The elements Fe–Mn show stable behavior, with strong correlation (0.92) and similar concentration variability, indicating association in oxides, such as (Fe,Mn)OOH, which precipitate under similar environmental conditions. The association and spatial distribution of the Pb–Sn–Ba group suggest an origin in the mixture of continental and marine sediment sources. The elements Cd–As–Mo, with lower correlation (< 0.7), showed distinct variations in the rainy period, wherein the increased supply of organic matter possibly favors the removal of As3+ by reduction and sulfide formation, while Cd is mobilized by an increase in pH. The spatial distribution of chemical concentrations followed estuarine hydrodynamics, with accumulation in lower‑energy zones. Despite moderate increases in environmental indices in 2021 (CD, PLI, RI), the values remained within quality limits. The analysis of the background and the geochemical signature of the CESM sediments demonstrates that the group of PTEs—Co, Cr, Cu, Mn, Mo, Ni, Pb, Sn, V, Zn—and Fe exhibit enrichment factors close to 1.0, whereas As, Ba, and Cd showed a slight elevation only in 2020. The result suggests impoverishment or stability relative to natural levels. This condition diverges from the pattern expected for anthropogenic contamination, which typically raises PTE concentrations sharply, especially those associated with human activities present in the drainage basin and in the surroundings of the estuary. This indicates that changes in the local hydrological regime, rather than pollution, are the main factors influencing the geochemical equilibrium of the CESM at the scale of the research conducted. The results thus demonstrated that the area maintains environmental quality close to natural conditions, with geochemical variations controlled mainly by sediment transport and depositional processes. The methodology developed, integrating multielement temporal analysis with the establishment of background through dated cores, constituted a robust foundation for the geochemical characterization of this large tropical estuarine complex, as well as for distinguishing between natural and anthropogenic variations.