IWRA Proceedings

AbstractIntermittent Mediterranean rivers show long draught periods interrupted by floods of high intensity and short duration. Their channel network structure is also highly variable in space and time. Water quality dynamics in these rivers is highly impacted by this specific hydrological behaviour. During the dry period, the river channel may dry up completely except in some reaches where anthropogenic point-source inputs (mainly waste water treatment plant effluents) contribute to maintain some pools. Accumulation of pollutants is observed in these reaches. During the first floods, flow is rarely continuous throughout the channel network, because of the existence of transmission losses. At the start of a storm, the advance of flood waves is limited by channel infiltration but also by localized storage in small pools. Total flood volumes therefore do not increase very much and even decreases downstream. Some floods observed in the upstream part of the river are not transferred to the outlet. This, in turn, influences the spatial and temporal dynamics of pollutant fluxes. Along the river course, pollutograph shapes show significant differences. Intense autumn floods flush away suddenly the pollutants from the soils and the river bed, but significant quantity of pollutants may remain trapped in the riverbed, if transmission losses are too high. Although the hydrological regimes of intermittent Mediterranean rivers are known, more knowledge is needed about the processes governing pollutant transport as well as the ranking of the origins of the pollutants. Thus, it is necessary to set up a special observation network in order to determine the temporal and spatial variability of the water and pollutant contributions to the river. All these phenomena will be illustrated on the case of the Vène River (67 km², South of France) over the observation period : from September 2003 to August 2006, on suspended solids, nutrients and faecal bacterias. The combination of continuous and event based monitoring coupled with the multi- scale sampling strategy allows a global understanding of the factors and processes influencing water quality at the catchment scale: concentration data is used to assess the contamination levels of the river and the bio-disponibility of pollutants, while by analysing the instantaneous fluxes the various contributions to the river might be established. Finally the total load is calculated in order to evaluate the exports at the catchment outlet. The multi-disciplinary approach followed herein improved our knowledge of the integrated catchment response and its impact for downstream environments.