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Fonctionnement hydrogéologique et hydrochimique du bassin crétacé de Tsagaan Els (Dornogobi, Mongolie)

Abstract : The Tsagaan Els basin is located in the Gobi desert, in Mongolia. This thesis aims to better understand the hydrogeology and hydro-chemistry of the basin in order to optimize the future mining projects. To this end, a 3D groundwater flow model of the basin has first been developed under MODFLOW. The latter was calibrated using piezometric network data and levels in a few nomadic wells. Several solutions were found with hydraulic conductivity values in agreement with pumping tests and recharge rates between 0.6-3.1 mm/yr in agreement with the current climate. The model helped us to extend our piezometric knowledge beyond AREVA’s mining licenses and to establish a regional water balance model. Groundwater age was then estimated with this model and with 14C dating. A comparison of the results of these two methods showed that calibrated solutions with higher recharge are more likely than those with lower recharge under the hypothesis that groundwater flow has not changed significantly during the last 30 kyrs. Meanwhile, a hydrochemical study using major ions, bromines and the stable isotopes of water (18O and 2H) showed that groundwater salinity mainly comes from dissolution of surficial evaporites: likely gypsum and halite and possibly thenardite. Even though evaporation is the main driving force of the groundwater flow, this process, when taking place in subsurface, seems to have no real influence on δ18O and δ2H values of the groundwater. Last a 3D local groundwater flow model centered on the terminal playa and based on the spatiotemporal evolution of the salinity, led us to a better understanding of the past and current hydrogeological functioning of the basin.
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Pierre Grizard. Fonctionnement hydrogéologique et hydrochimique du bassin crétacé de Tsagaan Els (Dornogobi, Mongolie). Sciences de la Terre. Université Paris sciences et lettres, 2017. Français. ⟨NNT : 2017PSLEM071⟩. ⟨tel-02173781⟩

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