The discharge at sea of domestic effluents can present, in terms of its particulate phase, very different behaviors depending on the quantity discharged, on the type of purification treatment, as well as on the dumping method and on the dumping site. Consequently, it is extremely difficult to identify the relation between the deposits originating from the dumped waste and the modifications undergone by the substrate and by benthic populations. We have therefore focused our efforts on the particular case of East Toulon which is representative of most Mediterranean urban effluents discharges, in order to study the fate and impact of the particulate matter released by the sewage pipe (1,800 m long, 42 m deep) in the area close to its outfall.

The first phase consisted in searching for preferential deposit areas with the use of a radioactive tracer (hafnium) injected for 35 consecutive days on a continuous basis at the outlet of the physico-chemical purification plant. This tracer which fixes itself irreversibly on the particulate matter, was systematically tracked on the sea bottom with a sled carrying a nuclear detection probe. This enabled a cartography of deposits to be established both on a temporal (four times) and a spatial basis.

A multi-disciplinary sediment survey was then conducted, based on these results. Samples were collected at various distances from the outfall. In addition to standard parameters (particle size, matter content), the main toxicants originating from the discharge were examined. Finally, we used the same samples to carry out toxicological assays on sea urchins, based on sediment-induced alterations in embryogenesis, cell division and fertilization. Our findings revealed a non-monotonous trend in the embryo-toxicity of sediments which was satisfactorily consistent with the preferential deposit area of the (toxic) suspended matter originating from the discharge.

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