Heavy metals in sediments from the Paraguay River: a preliminary study.

The first results ever obtained on heavy metal concentrations (Fe, Mn, Cr, Cu, Zn and Pb) in the Paraguay River surface sediments are presented. Samples collected at 11 locations, along a distance of 570 km, between the cities of Bahia Negra and Alberdi in Paraguay, for six different periods between November 1991 and 1993, were analyzed. The Paraguay sediments appeared to have features of an unpolluted river even though significant amounts of domestic and industrial effluents are discharged near the river channel. The relative heavy metal enrichment in sediments between Bella Vista and Asunción, caused by local domestic sewage and industrial outfalls, is less than for the shale standard values. The heavy metal content of the sediments exhibited seasonal variations. Enhanced organic matter content and biochemical oxygen demand of the river load in winter caused most likely a retainment of the heavy metals in a dissolved state. Consequently, the sediments deposited in the winter were relatively depleted in these elements.