Benthic exchange of sedimentary metals (Cd, Cu, Fe, Mn, Ni and Zn) in the Deu ˆ le River (Northern France) Beatriz Lourino-Cabana, A Ludovic Lesven, A Gabriel Billon, A Lionel Denis, B,C Baghdad Ouddane A,E and Abdel Boughriet A,D A Universit e Lille Nord de France – CNRS UMR 8217 G eosyste `mes, Equipe Chimie et S ediment, Ba ˆtiment C8, 59655 Villeneuve d’Ascq Cedex, France. B Universit e Lille Nord de France – CNRS UMR 8187 LOG, Station Marine de Wimereux, 62930 Wimereux, France. C IRD – UMR 6535 LOPB, Universidad Auto ´ noma Metropolitana, Departamento de Hidrobiologı ´a/IRD, 09340 Iztapalapa, M exico DF, M exico. D Universit e Lille Nord de France, Rue de l’universit e, PO Box 819, 62408 B ethune, France. E Corresponding author. Email: Baghdad.ouddane@univ-lille1.fr Environmental context. Exchange processes at the water–sediment interface can release metals to riverine waters, having negative effects on organisms in the water column. We investigate the geochemical processes and metal exchange between the surface sediment and the overlying water under metal contamination conditions. Results suggest that the sediment can be a significant source of metal pollution in aquatic systems, particularly during anoxic events. Abstract. Experiments were performed on the Deu ˆle River (Northern France), which is strongly polluted by smelting plants, in the aim to investigate the influence of diagenetic processes and benthic macro-faunal activity on trace metal (Cd, Cu, Ni and Zn) and major metal (Fe, Mn) exchanges occurring at the water–sediment interface. Diffusive metal fluxes were determined from pore water metal concentration gradients measured in sediment cores. Benthic metal fluxes were evaluated using incubation chambers under dark conditions, and by further examining key variables (O 2 , CO 2 , redox potential and pH) affecting metal release and sequestration processes. As a whole, it was demonstrated that benthic fluxes were strongly dependent upon medium oxygenation and generation of colloidal iron oxides and hydroxides at the overlying water–sediment interface, raising the possibility of trace-metal adsorption and (co)precipitation. Additional keywords: colloids, diffusive flux, overlying water, pore water, sediment, trace metals. Received 6 July 2011, accepted 6 October 2012, published online 19 November 2012 Introduction In our recent work, [1] it was shown that microbial activity at the surface sediment of the Deu ˆle River (in Northern France) was accompanied by oxygen depletion and contributed to the pro- motion of early diagenesis [2] ; it was also revealed that the oxygen barely penetrated 3 mm into the sediment. A potential source of dissolved metals then appeared in the pore waters of these surface sediments, and could be liberated into the over- lying water through complex and dynamic processes governing exchanges of metals at the water–sediment interface. It is well known that a complete description of these exchanges necessitates examining both the effects of diagenetic reactions and the implication of benthic biotic activity (i.e. bioirrigation and bioturbation) directly on the dynamic biological compartments involved at the oxic–anoxic sedimen- tary interface. Briefly, it consists of assessing the fluxes of pore water metals diffusing across the water–sediment interface and those subject to advection within the sediment: (a) benthic chambers are used for determining benthic fluxes corresponding to both diffusive and advective fluxes [3–7] and (b) the detailed examination of concentration profiles of pore water metals observed from upper layers towards the overlying water at the top of sediment cores allows the assessment of diffusive metal fluxes. [4,8–11] In this latter case, centimetre-scale mea- surements are generally made on sediments in order to reveal (bio)geochemical features in concentration profiles v. depth. [12,13] In the present work, investigations were undertaken in the Deu ˆ le River. Our previous work [1,14] reported metal contamina- tion data on Deu ˆle sediments. Our investigations revealed extreme pollution in this river rarely observed in other aquatic media in the world. [15] Thus, some metal concentrations were found to be much higher than levels recommended by interna- tional guidelines (United States Environmental Protection Agency (USEPA), World Health Organization, French and European guidelines). The severity of such sediment contami- nation in this river is still a major concern in this part of Northern France. Indeed, soluble trace metals from the sedimentary medium could migrate upward to the sediment–water interface and, as bioavailable elements, create an important source of pollution by having negative effects on proximate living CSIRO PUBLISHING Environ. Chem. 2012, 9, 485–494 http://dx.doi.org/10.1071/EN12046 Journal compilation Ó CSIRO 2012 www.publish.csiro.au/journals/env 485 Research Paper