173 © EDP Sciences, 2026 DOI: https://doi.org/10.1051/978-2-7598-3934-6.c009 QUAL I TÉ GÉOPHYSIQUEAPPLIQUÉE The Deffend hydrogeological model T. Gaillard Hydrogeological knowledge before Hydrogeological Experimental Site (HES) of Poitiers Historical groundwater flow model of the Poitou Threshold The hydrogeology of the Poitou Threshold has been studied since the 19th century, beginning with the work of Longuemar (1856). This author proposed a conceptual model of groundwater flow based on vertical fracture networks that drain groundwater toward valley-bottom springs located at the contact with the Toarcian marls. This hydrogeological model was later adopted and used by academics (Welsch, 1912), as well as by public service practitioners and engineering consultants for the location of boreholes and the interpretation of dye tracer tests. Observing the presence of water at the base of numerous quarries, Welsch expanded Longuemar’s model by introducing the concept of “quarry water”, corresponding to a continuous groundwater table within the limestones of the Supra-Toarcian Aquifer. Figure 1 presents the historical hydrogeological models of Longuemar and Welsch. It is worth noting, however, that Longuemar had described perched springs in cliffs, 9
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