Variação sazonal, transporte e partição de hg no gradiente fluviomarinho do Rio Jaguaribe - CE

Abstract

Coastal environments operate as connections between terrestrial and oceanic environments and in the control of transfers of heavy metals from fluvial waters to the ocean and may cause the deposition of metals associated with suspended material or the release of dissolved form, which completely changes metal vailability to the environment. Hg is an element naturally present in the biosphere, but can also occur via the contamination of the water, biota, soils and atmosphere and is associated with various dissolved or suspended compounds in the aquatic environment, presenting a potential risk to living beings. The partitioning of Hg between the dissolved (Hg-D) and particulate (Hg-P) phases is one of the most important factors to consider in the mobilization and transport of Hg in aquatic systems. In this context, the aim of this work was to analyze the chemistry of Hg partitioning along the estuary of the Jaguaribe river and the adjacent coastal region and estimate the flux of Hg to the adjacent continental shelf through different tidal cycles and seasons and try to identify their controlling factors. Sampling was conducted at two points in the Jaguaribe estuary (Aracati-CE, region with the greatest influence of fresh water, and Fortim-CE, region with the highest marine influence) in the estuarine plume (PE) and the continental platform (PC) and two distinct periods, the rainy season and dry season. For the rainy season, the Hg-D was detectable only in Aracati and in the PE with an average of 10.1 ng.L-1 and 5.6 ng.L-1 respectively. For PC and Fortim the values were lower than the limit of detection (L.D = 0.1 ng.L-1). The average concentration of Hg-P, for the same period was 0.5 ng.L-1 in Aracati, 1.5 ng.L-1 in Fortim, 3.0 ng.L-1 in PE and, 0.12 ng.L-1 in the PC. For the dry season the average concentration of Hg in Aracati was 3.4 ng.L-1; in Fortim was 2.6 ng.L-1, in PE was 0.7 ng.L-1 and in the CP was 2.9 ng.L-1. The average concentration of Hg-P for the same period was 0.25 ng.L-1 in Aracati, 0.38 ng.L-1 in Fortim; 0.18 ng.L-1 in PE and 0.16 ng.L-1 in the PC. The estimated average annual flow of Hg-T from the Jaguaribe river to the continental shelf was ~ 126 kg. The balance between the inflow and output of Hg-T in the estuary indicates that is occurring the retention of this material for the two studied periods. But for the period of lowest discharge the situation is considered more critical, because the retention of Hg is quantitatively larger as well as its residence time in the estuary, increasing the reactivity of the Hg and producing Hg-D, which includes the most bioavailable form of Hg, increasing the possibility of incorporation by the biota and biomagnification in the food chain.