Topography, weather and human activity effects on the behavior of metallic elements in a tropical catchment

Abstract

The geochemical behavior of metals in 11 rivers of the Lake Valencia drainage system, north-central Venezuela, was investigated by repeated sampling. Pb, Zn, Ni, Cu and Cr discharges from domestic and industrial activities have resulted in concentration increases of 2–16× above background levels, depending on the metal, as well as an increase of 13–27% of the labile fraction (% LF). No apparent effect on the metals' grain-size distribution was observed. Evidence suggests that chemical reactions, coupled with transport of sediments from the mountainous areas to lowland areas, produce: (1) an increase in metal concentration and % LF, which are concomitant with the pollution effect; and (2) a metals' grain-size redistribution toward fine fractions. Analysis of major elements, i.e. Na, K, Mg, Ca, Al and Fe, provides a suitable approach to discriminate the two effects mentioned above. The wet-dry season variability in metal concentration appeared to be controlled mainly by the input of flushed material from soil. This process can explain the large areal variability of concentrations compared with sampling and analytical variability. A physical dilution of Zn with less contaminated, flushed soil material was also observed.