Metallic contaminant profiles in sediment cores from Lake Valencia, Venezuela.

Total extractable concentrations of AI, Fe, Mn, Pb, Cu, Zn, Hg and organic carbon (Corg) were determined along four sediment cores from Lake Valencia. In one selected core, the percentage labile fractions were determined in order to establish anthropic enrichment factors, the depth reached by the contaminants and their mobilities. The distribution patterns of concentration versus depth reflect the original concentrations of the settling sediments. Only Mn seems to show diagenetic cycling. No enrichment of Fe and Mn occurs in the upper part of the cores, in spite of good water circulation and aeration of bottom lake waters during five months of the year. This immobility can be attributed to the high pH of water (7.6 - 8.9). Pb, Zn, Cu and Hg are enriched in the upper part of the cores, probably reflecting the input of contaminants. These elements show enrichment factors of up to 3.0. Background concentrations of the studied metals are reached below a depth of 30 cm.