Trace elements determined in sediment cores from Nhecolãndia Pantanal by INAA

Abstract

Pantanal wetland is the world’s largest wetland, with an estimated area of about 200 000km2. This region is an alluvial plain and the Paraguay River and his tributaries run through the wetland due to the low declivity of the plain in the north-south and east-west direction; the water takes almost four months to cross the wetland, forming different ecosystems, such as floodplains, lakes of fresh or saline water, rivers, and others. In the last decades, this area has been affected by human activities, without the suitable planning. The sediment of an aqueous system represents a great source of study, as it is a deposit of solid material, formed by the wind, ice, or water on the surface of the earth and the deposition of organic material from animals that live there. From the sediment chemical analysis in combination with geochemical tools is possible to estimate a probable source of the elements chemistry, whether natural or anthropogenic. Trace elements from different sources tend to accumulate in the sediment fine fraction (silt + clay), and are commonly mobile, what could generate a transference of some metals to the environment. Therefore, the present work analyzed the fine (silt + clay) and the coarse (medium sand + fine sand) fraction of the sediment cores collected in four Salinas from Nhecolãndia Pantanal performing the elementary chemical characterization of these sediment fractions and verify a possible historical impact by anthropogenic sources and also to verify if some element could accumulate in the sand fraction since the sediments from these ponds have a high percentage of the coarse (sand) fraction; instrumental neutron activation analysis technique (INAA) was used in the determination. Four sediment cores were collected in ponds located at Embrapa farm, 19 33023:31"S and 56 4057:56"W in Nhecol andia Pantanal, Mato Grosso do Sul, using a PVC manual sediment sampler. The cores were slice every 2cm and dried at 50 C; after drying, the samples were ground and homogenized. The elements As, Ba, Ca, Ce, Co, Cr, Cs, Eu, Fe (%), Hf, K (%), La, Lu, Na, Nd, Rb, Sb, Sc, Se, Sm, Ta, Tb, Th, U, Yb, and Zn were determined (mg kg􀀀1) by Instrumental Neutron Activation Analysis (INAA) and the analysis were performed in the coarse and in the fine fraction of the sediment. The samples and reference material were irradiated for 8h at the IPEN Research Reactor IEA-R1. The element As, Ba and Fe concentrations showed higher values than those of the Upper Continental Crust, UCC, in the fine fraction of the sediment for the studied cores. For the coarse fraction only the elements As, Ba and Hf showed concentrations higher than the UCC values. The majority of the obtained results in the INAA determination for the coarse and fine fraction suggested that there is no expressive effect of the human activities in the region and this particular area until the moment.