144 A new concept of karst development based on hydrogeology and geophysics Tracer tests Tracer test protocol Tracer tests were conducted to verify mass transfers between the different karst horizons developed in the Supra-Toarcian aquifer. Artificial hydrogeological tracing is an “experimental procedure aimed at making apparent and observable the actual movement of groundwater in an aquifer along one or more defined trajectories between a point of origin and one or more detection points, using an artificial tracer marking the water” (Castany and Margat, 1977, p. 180). These tests complement those carried out by researchers from the University of Poitiers on other boreholes (see Bodin et al., 2022). The protocol applied in these tests can be summarized as follows (Tab. 2): 1. a pumping operation is launched at a constant flow rate on MP6 until a pseudosteady flow regime is obtained; 2. the wellbore and tracer injection depth are selected from the results of GFTC logs. The targeted injection depth is generally a few tens of cm upstream of a flow horizon (from the well to the aquifer). The injection boreholes are (Fig. 16): – M19 at a depth of 111 m: injection No. 1 on 29/06/2021 at 9:25 a.m. UTC, – M19 at a depth of 85 m: injection No. 2 on 01/07/2021 at 1:50 p.m. UTC, – M05 at a depth of 111 m: injection No. 3 on 05/07/2021 at 3:40 p.m. UTC; 3. the tracer solution (5 g of uranine diluted in 2 L of water) is injected, followed by rinsing with a volume of 40 L of water. Pipes 2.5 m in length and 1.5 cm in internal diameter are connected down to the targeted injection depth. The total injection time, including rinsing, is less than 3 minutes; 4. tracer migration was monitored at the MP6 pumping borehole outlet pumped at 60 m3/h, using a fluorometer. The fluorometers were positioned at different depths in the observation boreholes M02, M12 and M21 in order to verify the nature of the flows (ascending or descending) between the different karst levels (Tab. 1); 5. the fluorometers used were Valeport Hyperion probes (3) and Albillia GGUN probes (4). The 7 fluorometers were previously calibrated with standard solutions whose concentrations were between 0 µg/L and 999 µg/L. Results The Breakthrough Curves (BTC) obtained following the 2 injections carried out on M19 are reported in Figures 17 and 18. The arrival times of the first concentration and the concentration peaks made it possible to better identify an upward flow in the M02 observation borehole and a downward flow in the M21 borehole. The tracer transfer times are longer in the case of the second injection at 85 m depth, which highlights a longer travel distance.
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