Résumé
Densely urbanized areas generate a wide range of contaminants exerting pressures on the quality of downstream rivers. As urban areas are in permanent extension, assessing the potential evolution of water quality and the effects of mitigation measures is a real challenge. Modelling tools could be helpful in decision making but require sound field data for calibration steps. The present study focuses on the urban river Lez that crosses the area of Montpellier city. Due to the Mediterranean climate, extreme precipitations intensify the transfer of contaminants by generating land runoff and significant sewer overflow. The spatiotemporal variability in microbiological and chemical contamination of the Lez river and its tributaries is characterized through field campaigns. Three low flow campaigns and a flood event are presented here, with a focus on the most urbanized sub-catchment: Verdanson. Hydrological conditions are shown to have a strong impact on the water quality in terms of concentrations of fecal indicator bacteria, organotins, trace metals and polycyclic aromatic hydrocarbons. As a first step towards the modelling of the water quality during a flood event, a sensitivity analysis was performed to test how land cover could be properly be taken into account in the modelling process. The analysis reveals that discretizing the catchment into land cover-homogeneous subcatchments enhances the suitability of water quality model outputs, keeping good outputs of the hydrological model. Consequently, a rigorous description of land cover is required to properly assess the impacts of territorial dynamics on water quality.