TRENDS IN THE HEAVY-METAL CONTENT (CD, PB, ZN) OF RIVER SEDIMENTS IN THE DRAINAGE-BASIN OF SMELTING ACTIVITIES

The geochemical and granulometric characteristics of sediments from three sites in the Lot River basin contaminated by heavy metals (cadmium, lead and zinc) were studied over a period of three years. Total heavy metal concentrations show little variation with time. However, a correlation between the heavy metal concentration and the fraction of sediment particles smaller than 20 mu m in the Lot River at Capdenac-Gare, a moderately polluted site, shows the occurrence of yearly variations: concentrations are highest during the winter and spring, periods of high water discharge. Correlations between various components of the sediments and heavy metals indicate that the heavy metal content is probably not controlled by a single geochemical component but by several. Depending on the degree of contamination of the site (no pollution, moderate pollution or high pollution) and the heavy metal investigated, high metal concentrations may be associated to iron, manganese, organic and inorganic carbon, calcium, sulfide and small particles. Heavy metal transport in the Lot River, downstream from the confluence with the polluted Riou Mort, takes place mainly in particulate form. Zinc and Cd input is definitely related to the Riou Mort.