Major, minor and trace elements in sediment samples from middle and lower Orinoco river, Venezuela

The concentrations of major (Fe and Al), minor (Na, K, Ca, Mg, Mn and Zn) and trace (Cr, Cu, Pb and Cd) elements were determined in samples of bottom sediments of the Caura, Apure and Orinoco rivers in order to verify the existence of geogenic or anthropogenic abundance of several elements in these sediments. The samples were subjected to acid digestion with aqua regia and then analyzed by atomic absorption spectrometry. The sediment samples of the Caura River showed the lowest values of minor and trace elements probably due to the presence of clays and hydroxides that have a low cation exchange capacity (kaolinite and gibbsite) and the low pH values showed by the Caura River. The sediment samples of the Apure and Orinoco rivers showed high concentrations of major, minor and trace elements probably due to the presence of minerals such as Fe-oxyhydroxides and clays produced by an intermediate weathering stage, which have the capacity to occlude and adsorb minor and trace elements. The enhancement of the element content in the sediment samples downstream the Orinoco River can indicate an increase in the sedimentation rate of fine-grained material supplied by the Apure River as the distance from the Apure-Orinoco confluence increases. The Orinoco River sediments showed an atypical abundance of Fe, Zn, Cr and Pb to the vicinity of the Matanzas industrial zone. This fact can be due to the wastewater coming from steel industries and the sedimentation of particles derived from iron ore mining industries.