Rétention des produits phytosanitaires dans les fossés de connexion parcelle-cours d'eau

Pour lutter contre les pollutions diffuses en milieu rural, de nombreux programmes d'action se mettent en place. Le développement de recherches sur les connexions parcelle - cours d'eau devrait permettre de mieux comprendre le transfert et la dissipation des polluants dans ce milieu. En particulier, les fossés, structures relativement fréquentes dans les territoires cultivés, peuvent, a priori, avoir une fonction de court-circuit et donc faciliter le transfert des produits phytosanitaires, ou au contraire constituer des éléments de pondération de la pollution. Afin d'éclaircir ce point, une première série d'expérimentations a été menée par le Cemagref (Institut français de recherche pour l'ingénierie de l'agriculture et de l'environnement) dans des fossés de drainage agricole. Une solution aqueuse contenant trois herbicides aux caractéristiques physico-chimiques différentes (isoproturon, diuron et diflufénicanil), et un traceur (chlorures) a été injectée pendant quelques minutes dans quatre fossés. Des échantillons d'eau ont été prélevés à pas de temps fins à deux emplacements en aval du point d'injection. Après dosage par chromatographie au laboratoire, les résultats indiquent une diminution du flux et de la concentration maximale du pic de polluants comparativement à un traceur. En outre, la variation observée est corrélée aux propriétés physico-chimiques des produits, en particulier au coefficient de partage Koc. L'étude présentée montre que la surface de contact (liée à la nature du substrat) et le temps de contact (dépendant essentiellement des conditions d'écoulement) entre les polluants et le substrat sont les paramètres qui influent majoritairement sur la dissipation des produits phytosanitaires.The use of pesticide may lead to the contamination of surface and groundwaters. Agricultural nonpoint source pollution originates from land areas which intermittently contribute to the compound transfer to water. Several studies report on the occurrence of pesticides in surface water resources, with concentrations over the limit set by the 80/778 EEC directive for drinking water (0.1 µg/L for each substance and 0.5 µg/L for all pesticides). Numerous herbicides of different chemical families are detected in surface waters, especially triazines and ureas. Their concentrations vary with time and space partly in relation with application patterns and pluviometry. Maximum concentrations are linked to runoff, originating from agricultural fields and primarily occur right after the application periods.Many methods and levels of actions can be used to reduce water pollution. First, better agricultural practices can be set up, such as choosing the best dose and application period, controlling toxic substance impacts, combining with non-chemical practices. However, pesticide losses from fields can't be totally cancelled because of the complexity of the involved parameters (agricultural practices, climatic conditions, soil physical, chemical and biological properties …). In fact drift during application, runoff or drainage systems may still occur and have an effect on water quality. It may be then pertinent to evaluate to which extent the non treated areas between the fields and the surface water bodies can dissipate pesticide concentrations before they reach them. Pesticides leaving a plot in surface runoff may pass through various landscape components before reaching rivers ; including another field, a ditch, a small brook, a vegetative buffer zone. Besides, pesticide leaving the plot through drainage straightly moves agricultural ditches or streams. However, the contribution of each of these elements in pesticide dissipation is not well known, except for buffer zones (grassed or forest strips) (PATTY (1997). Cemagref (a French research institute), CEH Wallingford and ITCF (Institut Technique des Cereales et des Fourrages) attempt to extend their study to the other elements as agricultural ditches.This paper deals with the role of farm ditches and small streams in the transport and retention of pesticides from fields to the main river network. Their presence seems to play a significant role in the transfer of nonpoint source pollution (especially in the West of France). Indeed they can either accelerate pesticide transport or reduce it, according to their characteristics (length, flow, bottom sediment or soil characteristics, plants and organic matter contents, etc.). Since 1998, Cemagref has been investigating the retention of pesticides by several natural ditches with varied flows and substratum. A water solution containing three herbicides with different physico chemical properties (diflufenican, diuron and isoproturon) and potassium chloride, a tracer, is introduced with a pump in each ditch for about five minutes with a constant concentration. Water samples are collected in the ditches every two or five minutes at two distances from the injection point. The samples are stored in amber polyethylene terephtalate bottles and frozen. Laboratory analysis is performed by liquid-liquid extraction with dichloromethane and then liquid or gas chromatography depending on the compounds.The analysis of the water samples highlights a reduction of the maximum concentration and of the accumulative mass of each pesticide with distance compared to the tracer. Indeed, even if all the chloride ions used as tracers are not recovered at each sampling point (due to infiltration or lateral losses), we notice more significant losses for all the studied herbicides. The reduction can reach 70 % of the applied mass for diflufenican compared to the tracer. The retention of pesticides is also linked to their own physical and chemical properties. Thus, diflufenican, which has the highest sorption coefficient value, Koc, is also the most retained pesticide, whereas the total injected mass of isoproturon is recovered in most cases. Diuron has an intermediate behaviour.In brief, this field experiment proves that the surface and time of contact between pollutants and substratum are likely to play a major role in pesticide retention. An estimated adsorption capacity of each ditch has been assessed, which is based on laboratory sorption experiments on different natural substratum. Despite the few data, a relationship between diflufenican retention in ditches and the estimated adsorption capacity of each ditch has been underscored.This study also highlighted major limits of field experiments. For example, accurate flow measurements are really difficult to carry out with simple methods for low values. The conventional techniques can't be used with small water height or in ditch where the bottom is filled with plants or grass. Chloride ion was chosen in this study because it is easy to analyze, but the results showed an initial presence of chloride ion in the natural ditch water which incites to replace it by another tracer such as bromide with is not found in the environment in future field experiments.For all these reasons, some pilot experiments with a physical model (an artificial ditch of 8 m long and 0.4 m wide) are now designed. This equipment allows to adjust and control hydrodynamic parameters such as water flow, water height, and the nature and structure of the substratum. Then, it is possible to quantify both the role played by the substratum, mainly the organic matter content, and the role of the contact time. These parameters could be then taken into account in order to optimize further experiments on adsorption. The primary tests without substratum already give references for hydrodynamic measurements, as the stability of the water flow and the homogeneity of the initial solution concentration

Emploi des profileurs acoustiques à effet Doppler (aDcp) pour étudier la structure des écoulements en rivière

Depuis une quinzaine d'années, les profileurs de vitesse acoustiques à effet Doppler (aDcp) sont de plus en plus employés par les équipes hydrométriques pour le jaugeage des cours d'eau. Ces appareils offrent la possibilité d'investiguer la bathymétrie et les vitesses d'écoulement tridimensionnelles rapidement, de manière peu intrusive, sur de vastes domaines et pour une large gamme de conditions de terrain. Le principe de fonctionnement, les stratégies de déploiement et de traitement des données mises en oeuvre sont exposés en s'appuyant sur un cas d'étude expérimental: la station hydrométrique "Saint-Georges" sur la Saône à Lyon

Quantifier l'effet de choix du site dans l'incertitude des jaugeages ADCP par transects

HMEM, Durham, New Hampshire, USA, 09-/07/2017 - 12/07/2017International audienceStage-discharge rating curves are used to relate streamflow discharge to continuously measured river stage readings to create a continuous record of streamflow discharge. The stage-discharge relationship is estimated and refined using discrete streamflow measurements over time, during which both the discharge and stage are measured. There is uncertainty in the resulting rating curve due to multiple factors including the curve-fitting process, assumptions on the form of the model used, fluvial geomorphology of natural channels, and the approaches used to extrapolate the rating equation beyond available observations. This rating curve uncertainty leads to uncertainty in the streamflow timeseries, and therefore to uncertainty in predictive models that use the streamflow data. Many different methods have been proposed in the literature for estimating rating curve uncertainty, differing in mathematical rigor, in the assumptions made about the component errors, and in the information required to implement the method at any given site. This study describes the results of an international experiment to test and compare streamflow uncertainty estimation methods from 7 research groups across 9 institutions. The methods range from simple LOWESS fits to more complicated Bayesian methods that consider hydraulic principles directly. We evaluate these different methods when applied to three diverse gauging stations using standardized information (channel characteristics, hydrographs, and streamflow measurements). Our results quantify the resultant spread of the stage-discharge Quantifying the uncertainty of discharge measurements (or "gaugings") is a challenge in the hydrometric community. A useful tool to empirically estimate the uncertainty of a gauging method is the field inter-laboratory experiment (Le Coz et al., 2016). Previous inter-laboratory experiments conducted in France (in 2009, 2010, 2011 and 2012) showed that the expanded uncertainty (with a probability level of 95%) of an ADCP gauging made of six successive transects is typically around 5% under optimum site conditions (straight reach, uniform and smooth streambed cross-section, homogeneous flow, etc.) and may be twice higher under poorer site conditions. In practice, the selected cross-section does not always match all quality requirements which may result in larger uncertainty. However, the uncertainty due to site selection is very difficult to estimate with predictive equations. From 9 to 10 November 2016, 50 teams from 8 different countries, using 50 ADCPs simultaneously, conducted more than 600 discharge measurements in steady flow conditions (~14 m3/s released by a dam). 26 cross-sections with various shapes and flow conditions were distributed over 500 meters along the Taurion River at Saint-Priest-de-Taurion, France. A specific experiment protocol, which consisted of circulating every team over half of the cross-sections, was implemented in order to quantify the impact of site selection on the discharge measurement uncertainty. Beyond the description of the experiments, uncertainty estimates are presented. The overall expanded uncertainty of a 6-transect ADCP gaugings (duration around 720 seconds) is estimated to be around 6%.The uncertainty of the discharge measurements varies among the cross-sections. These variations are well correlated to the expert judgment on the cross-section quality made by each team. First results seem to highlight a relation between uncertainty computed for each cross-section and criteria such as flow shallowness and measured discharge ratio. Further investigations are necessary to identify the criteria related to error sources that are possibly meaningful for categorizing measurement conditions and site selection. Moreover, experimental uncertainty and the uncertainty predicted by analytical methods such as QRev, QUant, OURSIN, RiverFlowUA or QMSys software will be compared

The AVuPUR project (Assessing the Vulnerabiliy of Peri-Urbans Rivers): experimental set up, modelling strategy and first results

International audienceLe projet AVuPUR a pour objectif de progresser sur la compréhension et la modélisation des flux d'eau dans les bassins versants péri-urbains. Il s'agit plus particulièrement de fournir des outils permettant de quantifier l'impact d'objets anthropiques tels que zones urbaines, routes, fossés sur les régimes hydrologiques des cours d'eau dans ces bassins. Cet article présente la stratégie expérimentale et de collecte de données mise en ½uvre dans le projet et les pistes proposées pour l'amélioration des outils de modélisation existants et le développement d'outils novateurs. Enfin, nous présentons comment ces outils seront utilisés pour simuler et quantifier l'impact des modifications d'occupation des sols et/ou du climat sur les régimes hydrologiques des bassins étudiés. / The aim of the AVuPUR project is to enhance our understanding and modelling capacity of water fluxes within suburban watersheds. In particular, the objective is to deliver tools allowing to quantify the impact of anthropogenic elements such as urban areas, roads, ditches on the hydrological regime of suburban rivers. This paper presents the observation and data collection strategy set up by the project, and the directions for improving existing modelling tools or proposing innovative ones. Finally, we present how these tools will be used to simulate and quantify the impact of land use and climate changes on the hydrological regimes of the studied catchments

Comparaison de méthodes pour la mesure des matières en suspension dans les cours d'eau, en présence de sable

National audienceLa mesure des concentrations de matières en suspension (MES) dans les cours d'eau est un sujet d'étude complexe. Des questions se posent sur la répartition spatiale des particules dans le cours d'eau, notamment celle du sable en suspension. La taille des particules et leurs propriétés, mais aussi la capacité de la rivière à mobiliser ces matières sont des sujets qui intéressent la recherche et les gestionnaires de cours d'eau. Dans cette étude, nous nous focaliserons sur la mesure de la concentration et de la distribution granulométrique des MES non-organiques dans un échantillon prélevé manuellement ou de manière automatique dans un cours d'eau. Plusieurs méthodes de dosage en laboratoire existent dont la norme NF EN 872 préconisée pour la mesure des concentrations en MES dans la plupart des types d'eau. Cependant cette méthode présente des limites importantes en cas de présence de particules de diamètre supérieur à 63µm (sables). Nous illustrons dans cette étude le problème du sous-échantillonnage en présence de sable, en testant différentes méthodes utilisées en France et ailleurs sur une centaine d'échantillons de concentrations variées prélevés dans une rivière alpine. Notre étude montre que la méthode ASTM D3977 est plus précise que les autres méthodes testées pour mesurer les concentrations en MES en présence de sable dans l'échantillon

Estimation of flow discharge by an airborne velocimetry system

A low-cost airborne velocimetry system has been developed and tested, consisting of an action cam, a quadrocopter, ground reference points, and seeding material. The obtained video frames were ortho-rectified and geo- referenced by computer vision techniques, with the by-product that each image got scaled as well. Flow velocity vector fields were determined using a particle image velocimetry algorithm covering a total reach length of 310 m. The data generally confirm depth-averaged velocity profiles obtained by a 3D acoustic Doppler current profiler. Flow discharges are estimated from that. For areas with homogeneous flow conditions we conclude that a correction factor of 0.85-0.9 should be applied to reduce surface velocities to depth-averaged velocities

Quantification des flux de matières en suspension par adcp discussion méthodologique sur l'utilisation du logiciel sediview (drl software)

In the modern management of the water courses the quantification of the flow of suspended matters is particularly important. SPM constitute about 90% of the sedimentary provisions to the river mouth and are the vectors of the principal pollutants. The intensity of the backscatter by the particles in suspension of the acoustic Doppler courant profiler (aDcp) provide a transect location of relative concentrations. From sediment sampling, some software, such as Sediview, allow to calibrate aDcp transect and thus to derive the global solid flux. We propose in this work to evaluate the amount of samples necessary to obtain a satisfying calibration curve. It appears that, in « less than module conditions », a dozen samples provide a good software calibration, even if the samples are not statistically representative of the transect.Dans la gestion moderne des cours d'eau la quantification des flux de matières en suspension revêt un intérêt particulier. Les MES constituent prêt de 90% des apports sédimentaires à l'embouchure et sont le vecteurs des principaux polluant. L'intensité de la rétrodiffusion par les particules en suspension des profileurs de courant à effet Doppler (aDcp) fournit une spatialisation des concentrations relative sur un transect. A partir de prélèvements d'échantillons sédimentaires, certains logiciels proposent une calibration de l'aDcp sur le transect instrumenté pour obtenir une spatialisation quantifiée des concentrations de MES et, par intégration, le débit solide total. Le logiciel Sediview a été développé dans cet objectif. Ce travail propose d'évaluer le nombre de prélèvements nécessaires pour obtenir une courbe de calibration satisfaisante. Il en ressort que, dans des conditions inférieures au module, une douzaine d'échantillons suffit à une bonne calibration par le logiciel, même si les échantillons prélevés ne sont pas statistiquement représentatifs du transect instrumenté

Quantification des incertitudes sur les jaugeages par exploration du champ des vitesses

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