International audience | Consumption of water polluted by faecal contaminants is responsible for 2 million deaths annually, most of which occur in developing countries without adequate sanitation. In tropical aquatic systems, streambeds can be reservoirs of persistent pathogenic bacteria and high rainfall can lead to contaminated soils entering streams and to the resuspension of sediment-bound microbes in the streambed. Here, we present a novel method using fallout radionuclides (Be-7 and Pb-210(xs)) to estimate the proportions of Escherichia coli, an indicator of faecal contamination, associated with recently eroded soil particles and with the resuspension of streambed sediments. We show that using these radionuclides and hydrograph separations we are able to characterize the proportion of particles originating from highly contaminated soils and that from the resuspension of particle-attached bacteria within the streambed. We also found that although overland flow represented just over one tenth of the total flood volume, it was responsible for more than two thirds of the downstream transfer of E. coli. We propose that data obtained using this method can be used to understand the dynamics of faecal indicator bacteria (FIB) in streams thereby providing information for adapted management plans that reduce the health risks to local populations.

International audience | Consumption of water polluted by faecal contaminants is responsible for 2 million deaths annually, most of which occur in developing countries without adequate sanitation. In tropical aquatic systems, streambeds can be reservoirs of persistent pathogenic bacteria and high rainfall can lead to contaminated soils entering streams and to the resuspension of sediment-bound microbes in the streambed. Here, we present a novel method using fallout radionuclides (Be-7 and Pb-210(xs)) to estimate the proportions of Escherichia coli, an indicator of faecal contamination, associated with recently eroded soil particles and with the resuspension of streambed sediments. We show that using these radionuclides and hydrograph separations we are able to characterize the proportion of particles originating from highly contaminated soils and that from the resuspension of particle-attached bacteria within the streambed. We also found that although overland flow represented just over one tenth of the total flood volume, it was responsible for more than two thirds of the downstream transfer of E. coli. We propose that data obtained using this method can be used to understand the dynamics of faecal indicator bacteria (FIB) in streams thereby providing information for adapted management plans that reduce the health risks to local populations.

Dans les bassins versants méditerranéens où les zones cultivées en vigne occupent une part importante de l'espace, les pratiques culturales sont un facteur important d'apparition de risques de pollution diffuse des eaux par les pesticides. Au cours de la thèse, une démarche de modélisation couplée pression-impact a été développée. La question de recherche a porté sur la prise en compte dans le modèle construit de la dynamique des variables d'état du milieu modifiées par les actions culturales. Deux variables ont été identifiées : l'infiltrabilité de la surface du sol et la concentration en matières actives au sol. A l'échelle du bassin versant, pour réaliser l'évaluation des impacts, on s'est appuyé sur l'utilisation du modèle hydrologique distribué MHYDAS qui permet de simuler les flux d'eau et de polluants selon différents scenarii de pratiques et climatiques. Pour représenter la répartition spatio-temporelle des opérations culturales sur chacune des parcelles du bassin versant, le modèle décisionnel DHIVINE a été construit. Il permet de simuler les itinéraires techniques des viticulteurs. Pour chaque exploitation du bassin versant, le modèle est basé sur une représentation de la conduite technique annuelle du vignoble sous forme de plans d'activités. Ces plans mettent en oeuvre des indicateurs d'états de l'agrosystème et prennent en compte les caractéristiques des ressources productives de l'exploitation ainsi que les arbitrages à réaliser entre opérations culturales et parcelles concurrentes. Des modèles biophysiques ont été développés pour simuler les indicateurs de la décision ainsi que l'impact des différentes pratiques sur les états de surface du sol qui conditionnent l'infiltrabilité du sol en surface. Une typologie fonctionnelle des états de surface basée sur leurs propriétés d'infiltrabilité du sol a été utilisée pour développer une démarche visant une modélisation continue dans le temps de l'infiltration. Elle intègre une modélisation parallèle de la dynamique des états de surface du sol nu et du développement de la couverture herbacée par les adventices. Un couplage des modèles a été réalisé sous la plate-forme OpenFLUID. La démarche adoptée ici permet de représenter les dynamiques différentes des variables d'état du milieu en fonction de différentes pratiques culturales et semble adaptée pour l'évaluation des impacts en terme de pollution de l'eau par les pesticides à l'échelle de bassins versants viticoles méditerranéens. | In Mediterranean catchments in which vineyards account for a major share of the area, cropping practices are an important factor with respect to increasing risks of diffuse pesticide pollution of water. This study was about developping an approach for pressure-impact modelling. The research question considered dynamics of agrosystem status variables modified by cropping actions. Two variables were identified : soil surface infiltrability and active material concentration on the soil. On a catchment scale, to assess impacts, we used the distributed hydrological model MHYDAS to simulate water and pollutant flows according to different cropping practice and climatic scenarios. To represent the spatiotemporal distribution of cropping operations on each plot, the decision model DHIVINE was built. It simulates vinegrowers's crop management sequences. For each farm in the catchment, the model is based on a representation of the vineyard annual technical management strategy in the form of activity plans. These plans involve agrosystem status indicators and take into account the characteristics of productive resources on the farm and arbitration between cultivation operations and competing plots. Biophysical models were developped to simulate decision indicators and impacts of cropping practices on soil surface characteristics which are drivers of soil surface infiltrability. A functional typology of soil surface states based on their soil infiltrabilty properties was used to develop an approach for temporally continuous modelling of infiltration. It was based on a parallel modelling of dynamics (i) of soil surface characteristics of bare soil and (ii) of weed cover development. Model coupling was realized with OpenFLUID platform. The approach developped herein allows simulating dynamics of agrosystem status variables as a function of cropping practices. It seems adapted for assessing hydrological impacts at the catchment scale in Mediterranean environments.