Evaluation des capacités d’écoulement et de stockage d’un aquifère karstique dynamique par caractérisation géologique et modélisation pluie-débit. Sources de Dardennes, Toulon, France.

Abstract : The aim of this PhD is to contribute to the karst aquifers understanding by (1) identifying the factors influencing the groundwater storage and flow and by (2) locating and quantitatively estimating water resources within the carbonate sedimentary series. Measurements of porosity and isotopic geochemistry on carbonate rocks are compared in order to locate and explain created voids within the sedimentary series. Balanced cross-sections and structural analyses were performed to evaluate the geometry of the aquifer at depth. Continuous data acquisition (specific conductivity, temperature and water depth-pressure) and hydrochemical data (major ions, stables isotopes of water and carbon-13) allow to study the aquifer dynamism and functioning. A daily rainfall-discharge modeling approach was used to estimate the flow part available for drinking water and the part that generates karst flash-floods. This study was conducted on the Dardennes aquifer, near Toulon, in France. Dardennes groundwater is used to supply drinking water to the Toulon city and flows downstream to the Las river generating important floods. The recharge area of this aquifer is estimated at 70 km². The unsaturated zone thickness can reach more than 500 meters in average. The aquifer is composed of early Cretaceous of Urgonian limestone and upper Jurassic limestones and dolomites. In this region, several tectonic phases structured the aquifers. Porosity measurements show very low values in the Urgonian limestones and high values in the upper Jurassic dolomites. These previous rocks were affected by diagenetic transformations and karstification phases resulting in the voids creation available for groundwater flow. Limestones from early Cretaceous have a low porosity and are intensely karstified and fractured, with many shallow karst features (sinkholes, caves, lapiaz...). During the Pyrenean-Provence compression, major thrust structures were created implicating the impervious basement, rising-up near the surface. Moreover, a syncline structure is identified at depth, under the Dardennes springs. In 3D, this implies an important groundwater reserve located at depth. A rainfall-discharge model was applied with three years of daily discharge data. This work also aimed at improving the KarstMod modeling platform. The model proposed includes three reservoirs: Epikarst, Matrix and Conduit. A hydrograph separation of springs discharge was carried out at annual and floods scales. It allows to separate the baseflow (from the Matrix reservoir) available for the drinking water supply and the quickflow (from the Conduit reservoir) generating flash floods. Depending on the year, the baseflow proportion varies between 27 and 61% of the total discharge, respectively for a rainy or a dry year. During flood in high-water periods, the quickflow proportion increases considerably to more than 90% of the total springs discharge. The analysis of the rainfall and karst hydraulic head relationship showed that the system has the capacity to store quick infiltration water when it has not been recharged previously. At annual scale, model results have been compared with the current groundwater management, showing a balanced current exploitation. In new perspective, it would be interesting to establish active management, combining a greater groundwater exploitation, floods mitigation and stream ecology respect.
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Cécile Baudement. Evaluation des capacités d’écoulement et de stockage d’un aquifère karstique dynamique par caractérisation géologique et modélisation pluie-débit. Sources de Dardennes, Toulon, France.. Géologie appliquée. AMU - Aix Marseille Université, 2018. Français. ⟨NNT : 2018AIXM0109⟩. ⟨tel-02178760⟩

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