Influence of sampling strategy, lithology, vegetation and rainfall on metal background concentrations in sediment of the tropical Tuy River basin, Venezuela

Abstract

The Tuy River basin, north-central Venezuela, was selected to study the influence of the sampling strategy and analytical methods, lithology, rainfall and vegetation on the background concentrations of heavy metals in stream sediments. Stream bed sediment samples (105) were collected during four years of the period 1979–1986 at 15 sites. The sediments were air-dried at room temperature and sieved through a − 120 mesh (125 μm) stainless-steel sieve. The concentrations of Fe, Mn, Co, Cu, Cr, Ni, Ph, Zn and Cd were determined by atomic absorption spectrometry after digestion with a hot (80°C) concentrated mixture of HNO 3, HClO 4 and HF. The coefficient of variation (CV) due to analytical methods was <10%, while the CV among tributaries ranged from 13% to 45%. This allows the establishment of background values with a low number of samples (< 10). Tributaries with CV > 60% indicate contamination or mineralized areas. The large difference in average concentration among the tributaries (CV = 29 − 110%) allows distinction between areas with different lithology and vegetation. The northern and southern areas of the Tuy River basin exhibit felsic and mafic lithologies, respectively. The southern area can be further divided in two subunits: the southwestern with savanna vegetation and the southeastern characterized by tropical rainforest. The highest concentrations of metals were found in the southern area (3 times the northern). Especially high values were measured in the southeastern subunit and are likely due to the metavolcanic and mafic bedrock lithologies, and the high weathering intensities associated with this tropical forest area.