175 9. The Deffend hydrogeological model drillers and hydrogeologists to the presence of a “fault”. The facture model proposed by Longuemar and Welsch therefore seemed to be confirmed until the research on the Hydrogeological Experimental Site (HES) of the Poitiers University. The Poitiers HES groundwater model By the 1970s, the stratigraphy of the Middle Jurassic formations in the Poitou Threshold was already well established, and research conducted at the University of Poitiers helped bridge a century-long gap in paleontological investigations of the Poitou carbonate platform. However, it was not until the establishment of the Hydrogeological Experimental Site (HES) near Poitiers that new data on the karst systems of the Poitou Threshold became available. Initial results demonstrated that the geometry of karst levels within the saturated zone is predominantly horizontal rather than vertical (Mari and Porel, 2008; Mari et al., 2009). Moreover, these horizontal karst levels are not randomly distributed but are strongly constrained by stratigraphy (Gaillard, Moreau, and Mari, 2024). Testing and experiments conducted at the HES established a direct link between stratigraphy and hydrogeology, leading to a substantial revision of earlier theories regarding the spatial organization of karst horizons in the Poitou Threshold. The hydrogeological model developed from HES investigations is based on the superposition of several karst horizons within the Supra-Toarcian Aquifer. Borehole Optical Televiewer (OPTV) logs reveal that fractures predominantly affect the unsaturated zone and are less frequent in the saturated zone. Groundwater flow occurs mainly through karstified levels that show very little fracturing. The caves visible in cliff outcrops are relic features of these ancient karst systems (Gaillard et al., 2024; Gaillard, 2026). Current groundwater flow is partly routed toward overflow springs discharging into valleys, and partly beneath the valleys of the Clain and Vienne rivers. Figure 2 presents a block diagram of this conceptual model, referred to as the Deffend model Figure 2 The Deffend model.
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