73 3. Hydrogeology of the Poitou Threshold drawdown during a pumping test, at a distance r and time t, is denoted s(r,t) and reflects the total pressure change within the aquifer at the observation point, that is, the pressure variations in the fracture network. Two examples of pumping tests are presented in Figure 12. On the left, graph (a) shows the results of a pumping test conducted at the HES. The drawdown was measured in observation well M2 during pumping at well M21 at a discharge rate of 62 m³/h. Well M2 is located 103.7 meters from the pumping well. The drawdown derivative (ds) is shown in red and the drawdown curve in blue (HES) or green (Civaux). Initially, the derivative follows the trend of the drawdown, then decreases until approximately 10⁴ seconds. Thereafter, the derivative increases steadily until the end of the test. No wellbore storage disturbs the start of the drawdown curves. On the right, graph (b) presents the results of a pumping test conducted at Civaux 2. The drawdown was measured at piezometer P1, located 37.2 meters from the pumping well P2. The discharge rate for this test was 51 m³/h. The drawdown does not exhibit a linear relationship with the logarithm of time. The derivative appears to stabilize around 10⁴ seconds, then begins to increase again around 8.10⁴ seconds. To compare the two pumping tests, graph (c) shows the drawdown derivatives of both tests using the dimensionless parameters defined by Streltsova (1976), namely tD = Tt/(r²S) and sD = (2πs(r, t))/Q, where t is the pumping time, r is the distance to the pumping well, T is the transmissivity, S is the storage coefficient, s(r,t) is the measured drawdown, and Q is the pumping rate. The transmissivity and storage coefficient values used were those obtained from fitting the early-time data to the Cooper-Jacob straight line. The two presented pumping tests also exhibit a similar dimensionless radius around rD = 0.01, allowing them to be compared on the same plot ( rD r Km K b = / / ). Both pumping tests are plotted in graph (c) of Figure 12. The derivatives of both tests show very similar behaviour, structured in three distinct sequences 3: • early-time behavior: The derivative slope ranges between 0.4 and 0.7. This initial stage is interpreted as representing the flow contribution from the horizontal fracture network; • intermediate-time behavior: The drawdown rate decreases, and the derivative stabilizes. This sequence is interpreted as the contribution from the rock matrix; • late-time behavior: This phase reflects the combined influence of both fractures and matrix, where the pumping rate exceeds the crossflow from the matrix. As a result, drawdown does not stabilize but instead continues to increase over time. This method was used to estimate aquifer parameters by interpreting eight pumping tests conducted at the HES and three tests at the Civaux site 4. At the HES, for each test interpretation, two piezometers aligned with the pumping well were selected to analyse the drawdown curves. For both sites, the thickness of the matrix 2. © EDF 2024: these data are the property of EDF; any use is subject to EDF’s prior agreement. 3. © EDF 2024: these data are the property of EDF; any use is subject to EDF’s prior agreement 4. © EDF 2024: these data are the property of EDF; any use is subject to EDF’s prior agreement
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