RésuméThis manuscript presents the research led in the context of a Doctorate in Geology and Hydrogeology. The study focuses on the specific case study of the Furfooz karst system, located in a Carboniferous mudmound of Tournaisian-Visean Age (Waulsort Formation), in Southern Belgium. This massif supported a heterogeneous dolomitization that takes the form of dolomite patches. An intensive karstification occurs through the massif. Within the saturated zone of the massif, an underground river infiltrates, from the surface Lesse River, through a swallow hole called “Le Chantoir des Nutons”. From there, the underground river flows on about 1.2 km to a diffuse resurgence (“Trou de la Loutre”), and crosses two meanders of the surface river on its path. The underground river flows through contrasted geometries: small conduits vs. underground voluminous lakes (the “Trou Qui Fume” lake and the “Puits des Vaux” lake). Several cave networks give access to the river. The Furfooz karst is investigated, in the present work, through two main approaches: the hydrogeological functioning of the karst system, and the karstogenesis of the massif.
The hydrogeological approach concentrates on the study of the underground river that drains the conduit system. A complete monitoring of the river (water-level data, physico-chemical data, gauging of flow-rates) was set up for several years to characterize the functioning of the river. The study is then focused on solute transport issues. Fifteen tracer tests were conducted on the site, in various discharge conditions, and from different injection points. They highlighted a complex functioning of the Furfooz system, mainly due to the presence of the lakes on the tracer path. Double-peaked breakthrough curves (BTC) appear downstream the lakes. These results are very sensitive to the discharge conditions. The double-peaked BTCs are interpreted, in a conceptual model, as the result of a double advection-dispersion phenomena through two exchanging flow zones that takes place within the lake. This conceptual model ought to be nuanced, as single-peaked BTCs are observed downstream the lakes at very low discharge conditions. The conceptual model, based on field information, is then confronted to physical and numerical modelling. The simulations show that the conceptual model is relevant to explain the double-peaked BTCs observed on the field. The dimensioning achieved through the modelling is constrained by some strong field hypotheses.
To a smaller extent, this manuscript develops theories about the karstogeneis of the Furfooz massif. Some specific geochemical processes of dedolomitization and calcite cementation are revealed. These processes are geometrically related to karst cavities in the dolomitized zones of the mudmounds. Analyses (cathodoluminescence, isotopic analyses, fluid inclusions measurements, and Laser Induced Breakdown Spectroscopy imaging) tend to show that these geochemical changes could also be related in time with the ongoing karstification process. This first set of observations open doors to further investigations.
|la date de réponse||6 déc. 2018|
|Sponsors||Université de Namur|
|Superviseur||Vincent HALLET (Promoteur), Pauline Collon (Copromoteur), Johan YANS (Président), Nathalie Dörfliger (Jury), Jean Marc Baele (Jury) & Malcolm Field (Jury)|