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

Stormwater 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.