Transfert de biocides de la ville vers le milieu aquatique: exemple de l'agglomération parisienne

This work addresses biocide contamination in urban waters and in a river subject to strong urban pressure, with the aim of assessing the contribution of urban discharges, and proposing effective, comprehensive and concerted mitigation solutions. To this end, raw and treated wastewater, combined sewer overflows and surface waters were sampled up- and downstream the Paris conurbation. A series of 18 biocides were all quantified in the samples. Analysing the long-term dynamics of biocide concentrations in surface waters showed that even if the ban of several molecules as pesticides did have a noticeable impact on surface water contamination, it did not entirely eliminate the sources of these compounds. Indeed, several biocides were quantified in the river at concentrations that could present high risk to the aquatic ecosystem based on their comparison with predicted no effect concentrations. One of the causes of their presence is urban sources of biocides, whose mass loads (discharged and transiting in the river) were assessed using stochastic approach as Monte-Carlo method. An increase in biocide loads between up- and downstream the study site was ascertained for four molecules, so was the contribution of urban discharges to biocide transfer towards the receiving waters. Our results offer prospects for specific mitigation solutions at three scales: biocide emissions, their transport in the sewer system and their treatment before discharge.