Facteurs d'échelle dans la hiérarchisation des écoulements au sein d'un aquifère karstique : Analyse multi-échelles des propriétés hydrodynamiques et de transport de l'aquifère du Lez

摘要

Karst aquifers represent an important groundwater resource, upon which a large portion of the population depends, particularly around the Mediterranean regions. Although heavily exploited, these aquifers remain poorly understood, particularly because of the difficulty to characterize their functioning due to flows heterogeneities. In order to improve knowledges of these systems, a multi-scale characterization of hydrodynamic and transport properties has been performed in the Lez karstic aquifer located in Northern Montpellier region. The study focuses on i) the identification of relevant hydrogeological structures and the spatial and temporal variabilities of hydrodynamic properties of the system, and ii) on transport properties of heterogeneities at different scales. The estimates hydrodynamic properties at several scales of investigation (borehole, local and regional) enabled to discuss the importance of the tested hydrogeological structures according to the used methods for parameter estimation. The increase of transmissivity with investigated scale and the variability of observed storativity are directly related to the major hydrogeological structures and overall system connectivity. Comparison of observations at different scales highlights the importance of the Jurassic-Berriasian stratigraphic interface which plays a main role on the connectivity of the system, and is a major structural and hydrodynamic link between karstic network at local (Terrieu Experimental Site) and regional scale. The study of mass transfer from local to regional scales gives informations about transport properties of the main drainage network between the injection and the recovery points. Tracer tests have shown flows channeling which depend of structures geometry. Moreover, a power law seems to fit the increasing of dispersivity with scale. All results provides a better knowledge of the hydrodynamic and hydrodispersives properties of these complex media at different scales, essential to model this kind of aquifers.