Dynamics of toxicity within different compartments of a peri-urban river subject to combined sewer overflow discharges

International audienceCombined Sewer Overflows (CSO) in small pen-urban streams and rivers are potentially toxic for their biocenosis. Improving the management of CSO discharges requires better knowledge of their dynamics and toxicity. In view to characterizing this toxicity, we sampled the different compartments (benthic and hyporheic zone) of a pen-urban stream located near the city of Lyon in France. The samples were taken at different distances from a CSO and at three period characteristic of different hydrological conditions. Their toxic effects were assessed by bioassays on the dissolved fraction (D. magna, V. fished and B. calyciflorus bioassays) and on the particle fraction (V. fished and H. incongruens bioassays). The results highlighted significant toxicity of the particulate fraction for the benthic and hyporheic samples, in particular downstream of the CSO, but with high spatio-temporal variability. This variability can first be attributed to the variability of CSO discharge sampling as a function of season and rainfall, and the dynamics of polluted particles (trapping of transported particles in infiltration zones, mobilization during floods). These parameters play a fundamental role in the distribution of pollutants according to the geomorphology of stream facies. Regarding dissolved pollutants, the chemical exchanges taking place at the "water-sediment" interface trigger the transfer of pollutants from one phase to another, after which the dispersion of these pollutants is governed by hydraulic flows. Finally, critical zones and periods are identified for the pen-urban river toxicity studied: benthic sediments under mean flow downstream: hyporheic sediments after a storm event downstream, close to the CSO. Recommendations are made on the basis of the knowledge obtained to optimize the management of these discharges

Évolution des caractéristiques physico-chimiques et écotoxicologiques des sédiments accumulés dans un bassin de retenue-décantation : premiers résultats du projet ANR CABRRES.

National audienceDetention basins are valuable facilities for urban stormwater management from the standpoint of both flood control and mitigating particulate pollutants by means of settling processes. Trapped sediments represent specific waste. The amount and the characterization of these accumulated sediments are also a very important issue for practitioners because these solids have to be frequently removed and managed as waste. A physical and chemical characteristics (metals, PAHs, pesticides) and ecotoxicological evaluation of these contaminated sediments have been carried out in the framework of CABRRES French programme. Sediments accumulated over about 6 months and 6 years in retention/detention basin located in the east of Lyon, France, were sampled at five different points. These preliminary results show: spatial variability for Cd, Pb, Zn and Σ16 PAHs, with the higher contamination at one point; and an ecotoxicological risk for accumulated sediments over about 6 years. These preliminary results may contribute to improve the management of detention basins in term of maintenance frequency and appropriate ways to recycle sediments.Les bassins de retenue-décantation sont très souvent utilisés pour la gestion des eaux pluviales dans les villes. Ils permettent de diminuer les risques d’inondations et de réduire les rejets de polluants parti culaires dans les milieux récepteurs (sol/nappe). Leur fonctionnement sur le long terme pose cependant le problème de la gestion des sédiments accumulés en leur sein. Afin de suivre leurs transformations (physique, chimique, écotoxicologique), une caractérisation physicochimique – éléments traces métalliques, hydrocarbures aromatiques polycycliques (HAP), pesticides – ainsi qu’une évaluation écotoxicologique ont été réalisées sur les sédiments accumulés dans un bassin de rétention de grande taille localisé dans l’est lyonnais (France). Cinq points ont été échantillonnés sur deux périodes d’accumulation des sédiments à la suite du curage du bassin (6 mois et 6 ans). Les résultats de cette étude préliminaire ont mis en évidence : une hétérogénéité spatiale en matière de contamination pour Cd, Pb, Zn et Σ16HAPs, avec des teneurs en polluants élevées en un point particulier; et un risque écotoxique pour les sédiments accumulés depuis 6 ans. Ces résultats préliminaires donnent des orientations en matière de fréquence de curage de ces bassins et de choix de filières de traitement et de valorisation des sédiments

Caractérisation des sédiments d’un bassin de retenue-décantation des eaux pluviales et éléments pour la gestion

National audienceStormwater runoffs are loaded with metals, PAHs, pesticides and pathogenic bacteria, conveyed for many in particulate form. One of the techniques used to manage this contaminated stormwater is to use retention basins in order to allow settling of the suspended solids. The deposits thus set up represent zones of contamination and a particular waste. The deposits thus constituted represent waste that must be managed. The objectives of this paper are: i) to characterize the sediments of a stormwater retention basin, ii) to identify the variability and heterogeneity of its microbiological, physical, chemical and ecotoxicological characteristics and to highlight the key factors that could explain this variability, iii) establish correlations between characteristics and identify trends in the change of sediment quality. The deriving results show that the median diameter of the sediments is between 50 and 150 μm. They are mostly contaminated with PAH, metals and 4-n-nonylphenol. Concerning the microbiological characterization, the bacteria “cores” observed in the sediments are composed of the bacterial genus which may contain pathogenic forms: Mycobacteria, Pseudomonas, Aeromonas, Nocardia. Nocardia counts are higher in sediments composed of fine and light particles (grain size between 1.1 and 3.3 μm). The ecotoxicity analysis of these sediments using the “ostracodes” test revealed that “fresh” sediments collected during a storm event were more ecotoxic than old ones. Sediment characteristics trended to become homogenous after 5 years of monitoring. Regarding investigations on the identification of the sources of microbiological contaminants observed in the retention basin, socio-urban observations revealed a direct relationship between the microbiological contamination of runoff and the presence of items and marks leave by socio-economic activities undertaken on the watershed. These results are useful for the managers in order to guide the choice or to stimulate the development of sectors or strategies of treatment and valorization of these sediments