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Management of metal-contaminated sediments requires an accurate knowledge of metal distribution and metal exchanges between the solid and solution phase. The DGT technique (diffusive gradients in thin films) provides information about the characteristics of metal release from the particles to the porewaters. However, the species of metals that participate in the exchange are not well documented. The aim of this paper was to explore the DGT response of Co, Cd and Pb, to a range of reservoir sediments exhibiting gradients of physicochemical parameters. We also characterized the chemical partitioning of the 3 metals by sequential extraction to determine the nature of the sediment-bound metals species that could participate to the metal resupply to the porewaters. Results from DGT experiments and sequential extraction show that specific geochemical phases are key factors for the mobilization of individual metals towards DGT: dissolved organic matter and manganese oxides for Co, acid-volatile sulfides for Cd and particulate organic matter for Pb. Moreover,

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The tanning industry uses large quantities of Cr whose contribution to the contaminant burden of aquatic organisms is not yet fully understood. The present study investigated Cr bioaccumulation by indigenous chironomids in a freshwater ecosystem impacted by tannery effluents. Total Cr content in sediments and in chironomids was determined on several occasions. Chromium distribution among sediments and pore waters, and Cr speciation in overlying and pore waters were studied in detail to understand possible factors controlling Cr bioavailability to chironomids. Total chromium concentration ranged from 69 to over 3000 µg g-1 dry weight in sediments and from negligible to over 300 µg-1 dry weight in chironomids (values corrected for sediment gut content). Filterable (<0.45 µm) Cr concentration in overlying waters and pore waters from the surface sediment layers (upper 2 cm) ranged from 3 to 120 µg L-1 , with Cr(VI) representing 0.5-28% of the total filterable Cr. Chromium profiles in pore waters as determined by diffusive equilibration in thin films (DET) and diffusive gradient in thin films (DGT) were comparable. DGT-labile Cr accounted for less than 2% of the total Cr measured by DET. Although Cr concentrations in sedimentary and aqueous matrices were not directly proportional to Cr levels measured in chironomids, the available findings suggested that Cr inputs from tanneries were bioavailable to resident chironomids. These observations are of particular importance considering that Cr(III), putatively of limited bioavailability and ecotoxicological concern, is the predominant redox form of Cr in bed sediments impacted by tannery discharges. The companion paper provides further insight into Cr bioavailability and effects in tannery impacted ecosystems using a combination of in situ and laboratory approaches.