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Abstract: We analysed Si isotopic signatures (δ30Si signature) of dissolved silicon (DSi) in the Scheldt (Belgium) tidal river (four stations) and in its main tributaries (Zenne, Dender, Grote Nete and Kleine Nete). For each station, we analysed four samples representative of the four seasons. These are the first δ30Si data at the basin and seasonal scales measured in a watershed under high anthropogenic pressure. In the Scheldt main stem, we show that the role of diatoms on the silicon biogeochemical cycle and δ30Si signatures is dominant in summer and sometimes in fall. Out of the diatoms' growing season, the Scheldt main stem exhibits unexpected large δ30Si signatures variations, especially in winter. Although these isotopic variations are not reflected in DSi concentrations, they are consistent with δ30Si signatures recorded in the main tributaries. This indicates that except for summer diatom blooms, δ30Si signatures in the tidal river result from a conservative mixing with tributaries. Overall we suggest that the variations are mainly set by land use and lithology at the sub-basin level. Forest cover (inducing clay dissolution favoured by the presence of humic acids) and wetland cover (yielding to dissolution of biogenic silica) are highly and significantly correlated with δ30Si (R2 >0.85). Both environments tend to render the tributaries δ30Si signatures lighter while agriculture cover has the reverse effect on δ30Si values (R2 =0.70, p-value<0.08). In contrast, urban zone cover does not have any noticeable effect on Si isotopic signatures of the Scheldt tributaries. Despite this effect of land use, we estimate a mean annual δ30Si signature output from the Scheldt tidal river to the saline estuary at 0.95‰, well comparable to other rivers. This results from the mixing of tributaries within the tidal river which levels off at the outlet the Si isotopic differences of the sub-basins. In the Scheldt watershed, it thus seems that Si isotopes could trace anthropogenic activities at the sub-basin scale. In contrast, at the outlet, the variety of land-use and lithology found over the whole watershed are merged into a mean isotopic signature similar to the world river average. As it is now recognised that change in land use alter the silicon biogeochemical continental cycle, our data show that use of Si isotopic signatures might constitute a useful proxy to better trace these modifications on contrasted (sub-)basins. [Copyright &y& Elsevier]

Abstract: The study of trace metal speciation has benefited from a great deal of interest leading to the development and the diversification of sequential extraction schemes (SES), which triggered the need for harmonization by the standard, measurements and testing programme (SM & T). However, some uncertainties still persist in the application of the harmonized 3-step protocol, because of the difficulty in conceptualizing a technique that can be applied to environmental samples irrespective of their nature. The alternative method proposed in this study is based on the solubility of the sample components progressively dissolved during the course of an acidimetric titration by 1molL−1 HCl. The major HCl-reactive mineralogical components are identified using mass balance calculation of H+ consumed by their dissolution together with the amount of major elements released into solution. The speciation of minor and trace elements is investigated by comparing their titrations to those of the major elements. This approach is much simpler than SES because it uses simple non-selective H+ at room temperature instead of a complex experimental design of so-called specific reagents. The different mineral components of the solid are no longer operationally defined and the problem of selectivity is irrelevant to the titration approach. The method was applied to several sediment samples from the Scheldt estuary and the particulate phase was further examined by Scanning Electron Microprobe and X-ray Diffraction techniques. The nearly complete consumption of H+ in the suspension is balanced by the total dissolution of carbonates and Fe-oxyhydroxides. In contrast to the speciation inferred from the Tessier SES, the acidimetric titration has demonstrated that the carbonate phase does not significantly contain trace metals with the exceptions of 40% of the Mn and 30% of the Co. In contrast, the Fe-oxyhydroxides seem to play a major role and account for 70% of Pb and 20% of Cr, in addition to 60% of P and additional amounts of 20% Co and 40% Mn. 70% of the Cu also occur in the oxyhydroxide phase, more likely coprecipitated with gibbsite. 90% of the Cd and 85% of Zn can be attributed to the Acid Volatile Sulfide (AVS) phase as evidenced from S titration. The acidimetric titration method not only provides information on the speciation of trace metals but also allows the quantification of their reactivity and mobility, if one considers that the titration roughly mimics pH changes that may occur as a result of chemical disequilibrium in the environment. The results demonstrate the potential of the acidimetric titration as an alternative to SES protocols in geochemical and environmental regulation studies. This method is applicable to a wide variety of environmental materials with little or minor adjustments. [Copyright &y& Elsevier]

Predicting metal concentrations in surface waters is an important step in the understanding and ultimately the assessment of the ecological risk associated with metal contamination. In terms of risk an essential piece of information is the accurate knowledge of the partitioning of the metals between the dissolved and particulate phases, as the former species are generally regarded as the most bioavailable and thus harmful form. As a first step towards the understanding and prediction of metal speciation in the Scheldt Estuary (Belgium, the Netherlands), we carried out a detailed analysis of a historical dataset covering the period 1982-2011. This study reports on the results for two selected metals: Cu and Cd. Data analysis revealed that both the total metal concentration and the metal partitioning coefficient (Kd) could be predicted using relatively simple empirical functions of environmental variables such as salinity and suspended particulate matter concentration (SPM). The validity of these functions has been assessed by their application to salinity and SPM fields simulated by the hydro-environmental model SLIM. The high-resolution total and dissolved metal concentrations reconstructed using this approach, compared surprisingly well with an independent set of validation measurements. These first results from the combined mechanistic-empirical model approach suggest that it may be an interesting tool for risk assessment studies, e.g. to help identify conditions associated with elevated (dissolved) metal concentrations., (Copyright © 2013 Elsevier B.V. All rights reserved.)