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Réponse d'un système aquifère multicouche aux variations paléoclimatiques et aux sollicitations anthropiques - Approche par modélisation couplée hydrodynamique, thermique et géochimique

Abstract : The study of the hydrodynamic parameters and the piezometric level of the aquifers is not enough to understand the hydrogeological behaviour of large sedimentary basins. It is necessary to take into account the whole of the layers and their hydrogeological behaviour on the long term. These aquifer systems are indeed too often analysed on the short term on the supposition that the hydrodynamic state was stabilized before the human exploitation. Thus it is under a new vision that the understanding of the aquifer of the Infra-Molassic Sands is approached in the aim of integrating it in a hydrodynamic model with the overall of the south-Aquitaine multi-layered aquifer system.
The interpretation of the isotopic and geochemical data showed that only vertical flows in addition or not to a horizontal flow can explain the results and it made it possible to present the idea of a major phase of recharge at the end of the Pleistocene. The reconstitution of the local paleoclimate was undertaken in order to propose a scenario of the potential periods of recharge of the south-Aquitain aquifer system over the last 40,000 years. These results revealed the instability of the conditions of recharge of this multi-layered aquifer system and thus its transitory state.
A major study of the geological, hydrogeological and geothermical characteristics was undertaken to suggest a hydrodynamical and thermal 3D model. This model makes it possible to study the recent evolution but also the long-term trends. This model is made up of eleven layers and the extension is approximately 32,000 km². The calibration of the temperature in the model was carried out. The calibration in transient state of exploitation with the pumping and the activity of two storages of gas was done. The simulation of the eustatic variations on the last 100,000 years showed the weak influence of those variations on the Infra-Molassic Sands groundwater. The simulation of the variations of recharge according to the paleoenvironmental fluctuations over 40,000 years, final purpose of this work, was carried out and enabled to establish three principal axis of flow in coherence with the geochemical constraints. The importance of the vertical and horizontal flow could be done using water budget. To finish with, the piezometric fall currently observed would be the principal consequence of the pumping of the last thirty years and of the natural drainage of the system started 150 years ago, following an important recharge during the Little Ice Age.
This study shows the existence of exchanges between the various layers (aquifers and aquitards), the important recharge by vertical drainage in different areas and the transient-state groundwater flow over several hundreds to several thousands of years. In short, the behaviour of this type of multi-layered aquifer of regional extension requires to be detached from the common vision of a functioning which would have begun only with the anthropic exploitation on a hydrodynamical stabilized system.
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Contributor : Olivier Douez <>
Submitted on : Tuesday, December 18, 2007 - 4:10:16 PM
Last modification on : Thursday, January 4, 2018 - 11:10:12 AM
Long-term archiving on: : Tuesday, September 21, 2010 - 3:03:24 PM

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Olivier Douez. Réponse d'un système aquifère multicouche aux variations paléoclimatiques et aux sollicitations anthropiques - Approche par modélisation couplée hydrodynamique, thermique et géochimique. Hydrologie. Université Michel de Montaigne - Bordeaux III, 2007. Français. ⟨tel-00198733v2⟩

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