Synthèse bibliographique sur le rejet en mer de l'effluent mixte urbain-industriel de La Salie. Validation de terrain des résultats obtenus par modélisation mathématique.

The compilation of data collected during the last decade, as part of the institutional survey of bathing waters, urban wastes, registered protected areas etc, demonstrates that the treated effluent discharged at La Salie Ocean (Bay of Arcachon) can be divided into two distinct types. The first is of urban origin and mainly of microbiological concern. Whilst, the second is due to the chemical treatment of wood for paper making and is rich in dissolved organic matter. The results of a specific survey conducted in the summer of 1997 at 8 stations close to the discharge outlet, confirm a significant impact of the discharges over the southern zone, as previously predicted by mathematical models. These results lead to the reclassification of the water quality on 4 of 8 points, as bathing water of intermediate quality (Category B). Nevertheless, estimations of dilutions show that despite the recent morphological modifications to the mouth of the Bay, dilution of the wastes remains both high and rapid. At present, it appears clear that although there is an impact of the discharges this effect is of little consequence outside of the prohibited zone which extends 400 m North and South of the discharge outlet. Nevertheless, in order to improve water quality in the future, the local authorities should as a priority implement tertiary treatment of urban effluents to reduce microbiological loads. Whereas, the paper mill should act to reduce the organic matter loads in the effluents.