Daily variations of Zn and Pb concentrations in the Deûle River in relation to the resuspension of heavily polluted sediments.

Sediments in the Deûle River (northern France) are severely polluted by metals in the vicinity of several metallurgical plants. The aim of this paper is to identify the role played by the recurrent resuspension of these polluted sediments by boat traffic on the dissolved Pb and Zn concentrations in the river. For that purpose, several high-frequency on-line monitoring campaigns were performed. Our results clearly suggest that the resuspension of sedimentary particles into the overlying water significantly increased the dissolved electrolabile Pb and Zn contents. This increase does not persist for a long time because at night and during weekends and holidays, when the boat traffic strongly slows down, Pb and Zn concentrations drop again quickly. Our data also indicate that the biological processes (such as photosynthetic and respiration activities), diffusive and benthic fluxes, as well as dilution of pore water into the overlying water during sediment remobilization do not contribute significantly to the sharp increase of dissolved Pb and Zn concentrations in the river during the day. The good correlation between turbidity and electrolabile metal concentration allows us to discard slow AVS (Acid Volatile Sulfides) oxidation as well. Desorption of metals from sediment particles was clearly the most relevant process, responsible for the increase of dissolved Pb and Zn concentrations in the water column.

On the lability of dissolved Cu, Pb and Zn in freshwater: optimization and application to the Deûle (France).

A procedure to determine lead and zinc by anodic stripping voltammetry and copper by cathodic stripping voltammetry in natural river samples was developed and validated. Cu determination involves the adsorption of Cu complexes with 8-hydroxyquinoline (oxine) onto a hanging mercury drop electrode. All samples were studied at natural pH and following optimal conditions were found: an accumulation step at -1.3 V (all potential values in the paper are given versus the Ag/AgCl, [KCl]=3M reference electrode) during 30s for determination of Zn contents and at -0.7 V during 60s for determination of Pb contents. Concerning Cu analysis, the optimal oxine concentration was found to be 10(-5)M with a deposition potential of -1.1 V during 30s followed by an adsorption step at -0.25 V during 15s. This procedure was afterwards validated by using certified reference freshwater and performing an intercalibration exercise. Finally the method was successfully applied in the Deûle River, highly contaminated by dissolved Zn and to a lesser extent by Pb and Cu, due to past and present activities of metallurgical plants. Under these optimal conditions, metal concentrations measured by our voltammetric procedures in the Deûle River were found in the range 12.4-23.2 nM for Zn, 1.7-3.2 nM for Pb and 4.9-7.6 nM for Cu.

Evidence of highly dynamic geochemical behaviour of zinc in the Deûle river (northern France).

The monitoring of dissolved zinc in the Deûle river was undertaken at three different periods of the years 2008 and 2009. Electrolabile concentrations of Zn were estimated every 2 hours for several weeks by using an ATMS (Automatic Trace metal Monitoring System) based on voltammetric measurements using a solid Ag-Hg rotating disc working electrode. Complementary measurements were carried out with DGT (Diffusive Gradient in Thin films) pistons deployed directly in the river for 24 hours. Water samples filtrated at 0.45 µm were also analysed by HR-ICP-MS to estimate the total dissolved concentrations of zinc and other trace metals. High frequency monitoring of zinc over several weeks in the Deûle river indicated that the concentration could change significantly over short time periods. Resuspension of polluted sediment and biological activities are two main factors that control the behaviour of zinc in the Deûle river. Furthermore, in May 2009, daily cycles of the electrolabile zinc fraction have been observed at relatively constant total dissolved concentration. It is assumed that this particular behaviour of zinc is based on an exchange between colloids and/or nonelectrolabile forms and free cation and inorganic complexes at a daily time scale.