On the bioavailability of trace metals in surface sediments: a combined geochemical and biological approach.

The bioavailability of metals was estimated in three river sediments (Sensée, Scarpe, and Deûle Rivers) impacted by different levels of Cu, Cd, Pb, and Zn (Northern France). For that, a combination of geochemistry and biological responses (bacteria and chironomids) was used. The results obtained illustrate the complexity of the notion of "bioavailability." Indeed, geochemical indexes suggested a low toxicity, even in surface sediments with high concentrations of total metals and a predicted severe effect levels for the organisms. This was also suggested by the abundance of total bacteria as determined by DAPI counts, with high bacterial cell numbers even in contaminated areas. However, a fraction of metals may be bioavailable as it was shown for chironomid larvae which were able to accumulate an important quantity of metals in surface sediments within just a few days.We concluded that (1) the best approach to estimate bioavailability in the selected sediments is a combination of geochemical and biological approaches and that (2) the sediments in the Deûle and Scarpe Rivers are highly contaminated and may impact bacterial populations but also benthic invertebrates.

Dynamic behaviour of trace metals in the Deûle River impacted by recurrent polluted sediment resuspensions: from diel to seasonal evolutions.

An automatic trace metal monitoring station (ATMS) have been implemented in the river bank of the Deûle canal in the city of Auby (northern France), in the vicinity of metallurgical industries that have heavily polluted sediments, in Cd, Pb and Zn. Additionally, the fluvial traffic in this river is intense and surface sediments are resuspended several times per day. This paper is the following part of that published recently (Superville et al., 2014) in this journal. The main purpose of this paper is to present new information on diel and seasonal variations of trace metals in the Deûle River and highlight the potential role of sediments in the contamination of the water column. To this end, Cd, Zn and Pb were monitored at a high frequency (~1h(-1)) by voltammetry, distinguishing an electrolabile and an acid-leachable fraction. Our measurements revealed that (i) acid-leachable Cd is mainly bound to light particles such as clays and carbonates and its daily behaviour does not really depend on the intensity of sediment remobilisation process; (ii) acid-leachable Pb and Zn exhibit some seasonal variations which are explained here, suggesting they should at least partly depend on the ageing of hydrous ferric oxides in the sediments; (iii) in spring, the electrolability of Zn increases in agreement with the biological activity that may break some strong association between Zn and ligands and/or particles (e.g., organic matter, hydrous ferric oxides, etc.). This trend is observed for electrolabile Pb later in summer, but on a smaller scale since this metal is generally more strongly scavenged and less labile than Zn; and (iv) electrolabile Mn concentrations increase during the day due to desorption process of resuspended sediment particles in the water.

Bacterial metal resistance genes and metal bioavailability in contaminated sediments.

In bacteria a metal may be defined as bioavailable if it crosses the cytoplasmic membrane to reach the cytoplasm. Once inside the cell, specific metal resistance systems may be triggered. In this research, specific metal resistance genes were used to estimate metal bioavailability in sediment microbial communities. Gene levels were measured by quantitative PCR and correlated to metals in sediments using five different protocols to estimate dissolved, particle-adsorbed and occluded metals. The best correlations were obtained with czcA (a Cd/Zn/Co efflux pump) and Cd/Zn adsorbed or occluded in particles. Only adsorbed Co was correlated to czcA levels. We concluded that the measurement of czcA gene levels by quantitative PCR is a promising tool which may complement the classical approaches used to estimate Cd/Zn/Co bioavailability in sediment compartments.

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.

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.