INFLUENCE OF SAMPLING STRATEGY, LITHOLOGY, VEGETATION AND RAINFALL ONMETAL BACKGROUND CONCENTRATIONS IN SEDIMENT OF THE TROPICAL TUY RIVERBASIN, VENEZUELA

The Tuy River basin, north-central Venezuela, was selected to study th e influence of the sampling strategy and analytical methods, lithology , rainfall and vegetation on the background concentrations of heavy me tals in stream sediments. Stream bed sediment samples (105) were colle cted during four years of the period 1979-1986 at 15 sites. The sedime nts were air-dried at room temperature and sieved through a-120 mesh ( 125 mu m) stainless-steel sieve. The concentrations of Fe, Mn, Co, Cu, Cr, Ni, Pb, Zn and Cd were determined by atomic absorption spectromet ry after digestion with a hot (80 degrees C) concentrated mixture of H NO3, HClO4 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 num ber of samples ( < 10). Tributaries with CV > 60% indicate contaminati on 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 ar eas of the Tuy River basin exhibit felsic and mafic lithologies, respe ctively. The southern area can be further divided in two subunits: the southwestern with savanna vegetation and the southeastern characteriz ed by tropical rainforest. The highest concentrations of metals were f ound in the southern area (3 times the northern). Especially high valu es were measured in the southeastern subunit and are likely due to the metavolcanic and mafic bedrock lithologies, and the high weathering i ntensities associated with this tropical forest area.