Geoquímica do material particulado em suspensão em um sistema estuarino tropical, sudeste do Brasil

Abstract

The aim of this study is to assess the variation of the geochemistry of suspended particulate matter (SPM) in the tropical estuarine system composed of the adjacent Piraquê-Açu and Piraquê-Mirim (ES) rivers using SPM sampled with sediment traps (45 µm mesh) installed in the drainage channels. We collected SPM during different tidal stages and seasonal periods and analysed the metals using inductively coupled plasma mass spectrometry according to EPA 3051A, the mineralogy using X-ray diffraction, the particle size using laser ray diffraction, and organic matter (OM) using calcination. The traps collected enough SPM to perform analyses for each sampling period. The SPM consists of silt size particles, and the percentage of OM is between 20 and 40%. The particles are dominated by kaolinite and quartz, and gibbsite, haematite, goethite, and pyrite are also present. These materials are found in the Barreiras Formation, through which the drainage system cuts. The metal concentrations were higher in the summer: Al (3208.32 g kg ). The Piraquê-Açu River had higher metal concentrations than the Piraquê-Mirim River, and the geochemical indices of both rivers indicate that they naturally contribute to the estuarine system of the Piraquê-Açu and Piraquê-Mirim rivers (SEPAPM). However, anthropic interferences influence these indices at the confluence of the two channels.Keywords: sediment trap; metals; mineralogy; Piraquê-Açu. INTRODUCTIONEstuaries are transitional environments that act as biogeochemical filters that retain dissolved and particulate material, including pollutants.1-4 Because they are largely influenced by hydrodynamic processes (waves, currents, and tides), estuaries distribute these elements and determine whether the system is a retainer or an exporter.5 Several studies have addressed the chemical composition and transport of suspended particulate matter (SPM; ≥ 0.45 µm) from estuarine regions to the adjacent coastal zones, mainly to quantify the continental contribution to the oceans. [6][7][8][9][10][11] However, few studies have been conducted in estuaries located in small and medium river basins in tropical regions, especially in those where human impacts are not yet evident. Understanding the geochemistry of SPM and the processes that influence it in estuarine systems has become a challenge when considering conditioning parameters such as rainfall, river discharge, the concentrations of major and trace elements and the physicochemical variables that may modify the geochemical behaviour of the SPM.To address these issues, the estuarine system of the Piraquê-Açu and Piraquê-Mirim rivers (SEPAPM) was studied to 1) evaluate if the geochemistry of the SPM of these two adjacent fluvial channels with different drainages varies 12,13 and 2) test if a sediment trap can be used to collect SPM considering the difficulty of sampling SPM with conventional methods, which require filtering many litres of water to acquire sufficient material for the geochemical analysis. Study areaThe SEPAPM is located in the municipa...