71 3. Hydrogeology of the Poitou Threshold The spatial extent of Type 2 voids forms a sub-horizontal, interconnected macroporous system, due to the low dip of the strata. Locally, these voids form aligned conduits only a few centimeters in diameter, which may evolve into cave networks, such as those observed in Lussac-les-Châteaux, in the Vienne Valley (Fig. 10). Pumping tests On the Poitou Threshold, the Aalenian and Bajocian limestones are located within the saturated zone. These two geological formations are exploited for drinking water supply (19 million m³/year according to SAGE Clain, 2011) and for agricultural irrigation (23 million m³/year). Transmissivity values, estimated using the Cooper-Jacob method (1946), range from 10–² to 10–³ m²/s. The storage coefficient ranges between 10–² and 10–⁴ depending on the borehole. These orders of magnitude were confirmed at the HES (Bernard, 2005). Two series of pumping tests were conducted in 2004 and 2005 after borehole drilling. Transmissivity values interpreted using the Cooper-Jacob method range from 2.2.10–³ to 4.4.10–³ m²/s. Storage coefficients exhibit substantial variability over three orders of magnitude, ranging from 3.6.10–⁴ to 2.8.10–¹. However, drawdown data cannot be consistently interpreted using a single straight line on a Cooper-Jacob semi-log plot, rendering the method ultimately inconclusive (Bernard, 2005, p. 125). Three families of drawdown curves have been identified (Bernard, 2005): 1) The first exhibits a drawdown that doesn’t follow a linear trend in the semi-log time domain; 2) The second displays two distinct linear segments in semi-log time, and 3) the third is characterized by a plateau or transition zone between two linear segments. To improve curve fitting, a solution for interpreting interference pumping tests in a fractally fractured medium of Euclidean dimension two was developed using the Figure 10 Macroporosity on D8 discontinuity (Lussac-les-Châteaux cliff).
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