The Conservative Behaviour of Trace Metals (Cd, Cu, Ni and Pb) and As in the Surface Plume of Stratified Estuaries: Example of the Rh ône River (France)

Abstract

Concentrations of dissolved trace elements (Cd, Cu, Ni, Pb and As) were determined over a 4-year period (1987 –91) during contrasting hydrological conditions in a typically stratified system: the Rh ône river estuary. Dissolved concentrations in the river water vary by nearly one order of magnitude. Estimated average values (nM) are: As, 26; Cd, 0 ·27; Cu, 35; Ni, 27; and Pb, 0 ·42. They indicate some degree of contamination for Cd, Ni and Pb, as compared to levels in pristine rivers and world averages. Dissolved concentration variations were interpreted in terms of a dilution of two sources of trace metals, i.e. anthropogenic discharges and leaching of surface soils. Concentrations in the river suspended matter (Cd, Cu, Ni and Pb) were measured during one survey (July 1987); comparison with published data also seems to indicate variable inputs to the estuarine zone. The distributions of As, Cu, Ni and Pb are conservative in the Rh ône mixing zone, whereas a small excess is sometimes observed for dissolved Cd. On the whole, trace metal concentrations in the suspended matter tend to decrease from the mixing zone to the open sea. They are only weakly controlled by the amount of terrigenous alumino-silicates. A sorption experiment shows that Cd fixation on the suspended matter is enhanced by increased amounts of particulate organic matter. The simultaneous use of in situand in vitrodistribution coefficients allows an estimation of the partitioning of particulate Cd between the reactive and non-reactive fractions of the suspended matter. This approach would confirm the mobilization of labile Cd in the saline surface plume.