Sanitation systems discharge diffusion into sea and operational management of their impacts on bathing water quality The new European Directive concerning the management of bathing water quality drives the littoral communities towards operational management tools for an actual management of the beaches according to the impacts of the urban discharges. Beyond the progressive modernisation of the sanitation system itself, taking into account globally all the chronic factors impacting the beaches is only made possible by using decision-making systems based on a better knowledge of the sources of contamination and on the numerical modelling of the diffusion into the sea. Such a system, easy to operate, was developed by Veolia and Ifremer for the agglomeration of Dieppe (Normandy, France): it queries a database of pre-computed scenarios of conditions representative of rainfall, tide and wind force and direction to recommend when and on which beach control samples should be taken. The validation of this software and methodological tool over the two summer periods of 2005 and 2006 enables to proceed towards a more complex operational system, which takes into account the continuous evolution of all the impacting factors.