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Biogeochemical processes mediated by microorganisms in river sediments (hyporheic sediments) play a key role in river metabolism. Because biogeochemical reactions in the hyporheic zone are often limited to the top few decimetres of sediments below the water–sediment interface, slow filtration columns were used in the present study to quantify biogeochemical processes (uptakes of O 2 , DOC, and nitrate) and the associated microbial compartment (biomass, respiratory activity, and hydrolytic activity) at a centimetre scale in heterogeneous (gravel and sand) sediments. The results indicated that slow filtration columns recreated properly the aerobic-anaerobic gradient classically observed in the hyporheic zone. O 2 and NO 3 − consumptions (256±13 μg of O 2 per hour and 14.6±6.1 μg of N–NO 3 − per hour) measured in columns were in the range of values measured in different river sediments. Slow filtration columns also reproduced the high heterogeneity of the hyporheic zone with the presence of anaerobic pockets in sediments where denitrification and fermentation processes occurred. The respiratory and hydrolytic activities of bacteria were strongly linked with the O 2 consumption in the experimental system, highlighting the dominance of aerobic processes in our river sediments. In comparison with these activities, the bacterial biomass (protein content) integrated both aerobic and anaerobic processes and could be used as a global microbial indicator in our system. Finally, slow filtration columns are an appropriate tool to quantify in situ rates of biogeochemical processes and to determine the relationship between the microbial compartment and the physico-chemical environment in coarse river sediments.

International audience; DEBtox model is a biologically based model used to analyse aquatic toxicity data (Kooijman and Bedaux, 1996). To date, it has not been used to analyse sediment survival tests, mainly because this would require daily counting of survivors, which is difficult for benthic organisms. In the present study, we adapted survival toxicity tests with the midge Chironomus riparius to permit survival data analysis with DEBtox. To validate the adaptation proposed, we exposed organisms to two chemicals, copper and methiocarb. We exposed larvae of second, third and fourth instar to different concentrations of the tested compounds and two different diets to assess the influence of diet and instar on DEBtox parameter estimates. Daily counting of organisms did not affect survival and did not lead to much more effort when compared to usual tests. Moreover, the analysis profited much from use of the DEBtox model. It was possible, with only survival data to estimate and validate the kinetics of the compounds, to predict survival during the recovery of exposed organisms and to assess the influence of food availability on toxicity. Food availability did not influence methiocarb toxicity, but copper was more toxic to food-limited organisms when a threshold concentration was exceeded. Comparison of parameters estimate also helped in understanding the differences in toxicity responses between instars. For the two compounds we studied, the difference was entirely explained by differences in threshold values.; Le modèle DEBtox est un modèle à fondement biologique utilisé pour l'analyse des données de toxicité aquatique (Kooijman et Bedaux, 1996). Jusqu'à présent, il n'a jamais été utilisé pour analyser les données des tests de survie sur sédiment, ce qui est principalement du à la difficulté de suivre la survie des organismes benthiques jour après jour. Ici, nous avons adapté les tests de survie avec Chironomus riparius pour permettre l'analyse des données avec DEBTox. Pour valider notre approche, nous avons exposé les organismes à deux contaminants, le cuivre et le mercaptodiméthur (un pesticide). Trois stades larvaires différents (deuxième, troisième et quatrième) et deux régimes alimentaires (ad libitum et limitant) ont été considérés. Le décompte quotidien des survivants n'a pas altéré la bonne marche du test et n'a pas donné lieu à beaucoup plus d'effort qu'un test classique. De plus, nous avons pu tirer des bénéfices de l'utilisation de DEBtox. Il a ainsi été possible, à partir uniquement des données de survie, d'estimer la cinétique des composés, de prédire la survie au cours de la phase de dépuration, et de caractériser l'influence de la nourriture. Le manque de nourriture n'influe pas sur la toxicité du pesticide mais augmente celle du cuivre. Quant à la différence entre stades, elle s'est caractérisée uniquement par une différence entre les concentrations seuils d'effet.

International audience; Vertical flow constructed wetlands (VFCWs) have been very successful in France over the last 5 years. The sizing of VFCWs is still roughly based on organic load acceptance with slight clear water intrusion into the sewerage system which is often wrong in the context of small communities. To specify the hydraulic limits would be of great help to Water authorities in deciding at what point is it preferable to build separate sewers rather than adapt the wastewater treatment plant. The study of the hydraulic limits of reed beds, based on the knowledge of hydrodynamics in unsaturated porous media, shows the ability of the system to accept flow overloads. Measuring different parameters (flow, pollutant removal, infiltration rate (IR), pressure head profiles) in pilot and full-scale studies, we explain the hydraulic behaviour of the filter, and the role of reeds and batch frequency on the IRs. Consequently, new hydraulic limits with accompanying sizing rules and operational recommendations according to the level of deposit on the filter surface are suggested. The study shows the robustness of reed beds systems as designed in France to accept hydraulic overloads. Overloads up to ten times the dry weather flow are possible whilst still complying with the European standards.