77 3. Hydrogeology of the Poitou Threshold curve. This phenomenon is even more evident in the drawdown derivatives shown in graph (b), where a wave-like shape forms in the derivative curve before dropping off. Pumping test results from both the HES and the Civaux site 6 demonstrate that the heterogeneity of the supra-Toarcian aquifer can be effectively described using a dual-porosity conceptual model. This modelling approach is consistent with field data and highlights the role of sedimentary discontinuities in controlling groundwater flow. Conclusion The Aalenian, Bajocian, and Bathonian limestones constitute microporous aquifers affected by various forms of karstic dissolution. Tectonic structures do not account for the location or geometry of speleological networks, the most extensive of which—the Cuchon Cave—reaches 4,000 m in mapped extent. To explain the position of karstic outflow systems, a stratigraphic approach is required. Centimeter to decimeter-scale dissolution features have been correlated with the outcrop’s stratigraphy. In non-dolomitized limestones, ovoid dissolution features are found within Lower Aalenian strata (Opalinum Zone) and Lower Bajocian strata (Humphriesianum Zone). Additional voids are associated with stratigraphic discontinuities separating sedimentary sequences. The regional extent of these discontinuities accounts for the recurrence of similar dissolution features in all valleys intersecting the Poitou Threshold. Pumping tests conducted at two sites 7 on the threshold indicate that the SupraToarcian Aquifer behaves as a dual-porosity medium, with horizontal flow paths corresponding to bedding planes and discontinuities. The role of stratigraphy is often underestimated in karst studies. Nevertheless, several works have demonstrated its importance in the development of horizontal karst systems (Esteban and Klappa, 1983) and in explaining intrinsic permeability variations in carbonate oil reservoirs (Reynolds, 1993). More recently, stratigraphic controls have been invoked to explain the distribution of macroporous levels in Upper Cretaceous Chalk formations in both Normandy (Gaillard et al., 2022) and England (Farrant et al., 2022). The stratigraphic correlation of karst features has revealed basin-scale trends in the Anglo-Norman Basin, enabling the prediction of karstic horizons. This approach is also applicable to the limestones of the supraToarcian aquifer of the Poitou Threshold. 6. © EDF 2024: these data are the property of EDF; any use is subject to EDF’s prior agreement. 7. © EDF 2024: these data are the property of EDF; any use is subject to EDF’s prior agreement
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