*3.1. Aquifer Piezometry*

Long-term piezometric monitoring was set up more than a year before injection [6]. During the 2019–2020 hydrological cycle, the average water table of the site reached its maximum in mid-April 2019 with 47.67 m ASL and its minimum at the beginning of October 2019 with 46.80 ASL (Figure 8a). Since that date, i.e., 49 days before the experiment, the piezometric level rose regularly as a result of the autumn recharge.

**3. Results** 

*3.1. Aquifer Piezometry* 

the presence of a packer.

for the PZ2TER and the PZ3.

**Figure 8.** Piezometric monitoring of the experimental site: (**a**) piezometry from the start of monitoring; and (**b**) piezometry before and after the injections. **Figure 8.** Piezometric monitoring of the experimental site: (**a**) piezometry from the start of monitoring; and (**b**) piezometry before and after the injections.

*3.2. Tracing of the Injected Water*  PZ2 remained sealed for 24 h after the injections to limit the risk of hydrogen degassing. When it was reopened, despite the leaching induced by the injection of 5 m3 of hydrogenated water that followed the injection of 1 m3 of tracer-holding water, the tracer concentrations still reached 208 µg·L−1 for lithium, 117 µg·L−1 for uranine, and 3.9 µg·L−<sup>1</sup> During the injection, the average piezometric level of the site remained stable at 46.90 m ASL, with the exception of PZ2BIS (+0.07 m) and PZ3 (+0.03 m), which were affected by the injections (Figure 8a). This was probably also the case for PZ2TER, located between the two previous piezometers, but it was not possible to measure its water table due to the presence of a packer.

Long-term piezometric monitoring was set up more than a year before injection [6]. During the 2019–2020 hydrological cycle, the average water table of the site reached its maximum in mid-April 2019 with 47.67 m ASL and its minimum at the beginning of October 2019 with 46.80 ASL (Figure 8a). Since that date, i.e., 49 days before the experiment,

During the injection, the average piezometric level of the site remained stable at 46.90 m ASL, with the exception of PZ2BIS (+0.07 m) and PZ3 (+0.03 m), which were affected by the injections (Figure 8a). This was probably also the case for PZ2TER, located between the two previous piezometers, but it was not possible to measure its water table due to

At PZ2BIS, a first piezometric peak of +0.07 m occurred 0.25 h after the start of the first injection (see circled 1 in Figure 8b), and then the water table returned to its original level just before a new peak of +0.11 m, which occurred 0.33 h after the start of the second injection (see circled 2 in Figure 8b). The water table returned to its initial state 2 h after the end of the second injection. This piezometer was therefore influenced by the average overpressures of 1.4 bar induced by the successive injections and, to a lesser extent, same

the piezometric level rose regularly as a result of the autumn recharge.

for helium (Figure 9). Depending on the tracer, this represents 0.3–2% of the concentration injected the day before. Since the injection induced an overpressure, this phenomenon is probably due to the retention of part of the tracers within the chalk's porous matrix (which is of micrometric dimension), and then to their slow release when the natural flow of the aquifer resumed. This phenomenon has previously been demonstrated at other sites where the chalk shows dual porosity [29,30]. Thus, in the injection well, the tracer concentration remained higher than the background noise for 12.9 days for lithium and helium At PZ2BIS, a first piezometric peak of +0.07 m occurred 0.25 h after the start of the first injection (see circled 1 in Figure 8b), and then the water table returned to its original level just before a new peak of +0.11 m, which occurred 0.33 h after the start of the second injection (see circled 2 in Figure 8b). The water table returned to its initial state 2 h after the end of the second injection. This piezometer was therefore influenced by the average overpressures of 1.4 bar induced by the successive injections and, to a lesser extent, same for the PZ2TER and the PZ3.

### and until the end of the monitoring (t ≥ 49.9 days) for uranine, on which date the analyses *3.2. Tracing of the Injected Water*

still showed the presence of 0.29 µg·L−1 of this tracer. This retention phenomenon therefore generated a decreasing but clearly significant background noise for about two weeks after injection. At PZ2BIS, 5 m downstream, three successive tracer plumes were observed (Figure 10b): PZ2 remained sealed for 24 h after the injections to limit the risk of hydrogen degassing. When it was reopened, despite the leaching induced by the injection of 5 m<sup>3</sup> of hydrogenated water that followed the injection of 1 m<sup>3</sup> of tracer-holding water, the tracer concentrations still reached 208 µg·L −1 for lithium, 117 µg·L −1 for uranine, and 3.9 µg·L −1 for helium (Figure 9). Depending on the tracer, this represents 0.3–2% of the concentration injected the day before. Since the injection induced an overpressure, this phenomenon is probably due to the retention of part of the tracers within the chalk's porous matrix (which is of micrometric dimension), and then to their slow release when the natural flow of the aquifer resumed. This phenomenon has previously been demonstrated at other sites where the chalk shows dual porosity [29,30]. Thus, in the injection well, the tracer concentration remained higher than the background noise for 12.9 days for lithium and helium and until the end of the monitoring (t ≥ 49.9 days) for uranine, on which date the analyses still showed the presence of 0.29 µg·L <sup>−</sup><sup>1</sup> of this tracer. This retention phenomenon therefore generated a decreasing but clearly significant background noise for about two weeks after injection.

At PZ2BIS, 5 m downstream, three successive tracer plumes were observed (Figure 10b):


i.e., approximately 10 min after the start of injection, at a concentration of 3913 µg·L −1 for lithium, 2667 µg·L −1 for uranine (again inducing saturation of the fluorimeters), and 160 µg·L −1 for helium. The tracer dilution factors reached 2.56, 3.75, and 9.28, respectively. This plume is interpreted as the result of leaching of the tracers contained in the porous matrix of the aquifer rock induced by the arrival of water at a slight overpressure from the second tank.

3. In the third, weaker and more spread-out plume (see <sup>3</sup> in Figure 10b; note the logarithmic axes), uranine was detected by the fluorimeter. It peaked at 189 µg·L −1 on 20 November 2019 at 02:45 (t = 0.531 days) and decreased very slowly, finally lasting about a month; 0.24 µg·L <sup>−</sup><sup>1</sup> of uranine remained at t = 26.8 days, lithium and helium having disappeared by t = 12.9 days. When the concentration peak passed through, the dilution factor reached 52.9 for uranine, the only tracer measured using the recording fluorimeter (because the peak occurred in the middle of the night), which must have corresponded to a dilution factor of about 36 for lithium and about 130 for helium. *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 11 of 27 *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 11 of 27

**Figure 9.** Evolution of the tracers at PZ2 (injection well). **Figure 9.** Evolution of the tracers at PZ2 (injection well). **Figure 9.** Evolution of the tracers at PZ2 (injection well).

and (**b**) detail of the first day of monitoring. **Figure 10.** Breakthrough curves for the tracers at PZ2BIS (5 m downstream): (**a**) during the first three days of monitoring; and (**b**) detail of the first day of monitoring. **Figure 10.** Breakthrough curves for the tracers at PZ2BIS (5 m downstream): (**a**) during the first three days of monitoring; and (**b**) detail of the first day of monitoring.

As regards the other downstream piezometers, a peak of tracers is clearly noticeable at PZ3 (10 m downstream) and PZ4 (20 m downstream) at t = 2 days with respective concentrations of 9.75 and 9.08 µg·L −1 for uranine (Figure 11a). These peaks seem to be shifted to t = 6 days for lithium with respective concentrations of 15 and 10 µg·L −1

(**a**) (**b**)

(**a**) (**b**)

(see circled 1 in Figure 11b), and for helium with respective concentrations of 1 and 2 µg·L −1 (Figure 11c). At PZ4, a second lithium peak is also observed at t = 20 days with a maximum concentration of 24 µg·L −1 (see circled 2 in Figure 11b). This distinct behavior of piezometers PZ3 and PZ4 has been observed previously. It means that the tracers arrive as quickly and at the same concentrations at PZ3 as at PZ4, which is twice as far from the injection well, with respective speeds of 5 and 9.5 m·day−<sup>1</sup> . It is assumed that the groundwater takes a dual flow path, which is more marked at PZ4 than at PZ3, with a rapid circulation within a more permeable (probably fissured) zone, and a slow circulation within the aquifer's porous matrix. (**a**) (**b**) **Figure 10.** Breakthrough curves for the tracers at PZ2BIS (5 m downstream): (**a**) during the first three days of monitoring; and (**b**) detail of the first day of monitoring.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 11 of 27

**Figure 9.** Evolution of the tracers at PZ2 (injection well).

**Figure 11.** Breakthrough curves for the hydrogeological tracers at the other downstream piezometers: (**a**) uranine; (**b**) lthium; and (**c**) helium. **Figure 11.** Breakthrough curves for the hydrogeological tracers at the other downstream piezometers: (**a**) uranine; (**b**) lthium; and (**c**) helium.

*3.3. Dissolved Hydrogen*  In the water samples obtained by pumping, the dissolved hydrogen is extracted by the method of partial degassing by mechanical agitation, after which its content is measured with a portable Biogas analyzer with a detection threshold of 0.5 µg·L−1. The dissolved CH4 and H2S are also measured by the same method, with respective detection limits of approximately 1 and 0.6 µg·L−1. In parallel, the gases dissolved in the water of the PZ2TER are analyzed by Raman and Infrared spectrometry [25]. The results obtained are as follows (Figure 12): 1. No trace of dissolved CH4 or H2S is detected in any piezometer using the method of partial degassing by mechanical agitation. Regarding the piezometers located far downstream, namely PZ5 at 30 m and PZ6 at 60 m, traces of each of the two tracers are observed there approximately two weeks after injection (about 0.6 µg·L −1 for uranine, about 10 µg·L −1 for lithium), and helium is detected six days after injection with a concentration lower than 1 µg·L −1 (Figure 11a). Although the peak concentration of tracers has not yet been reached when the monitoring was stopped, it can be estimated that the corresponding transfer rates here are less than 1–2 m·day−<sup>1</sup> . The dilution ratio is from 0.01% to 0.1%, which means that the impact of the hydrogen injected into the aquifer cannot be measured at these distances, under the experimental conditions created. Note that this result is in line with that obtained by PHREEQC modeling during the previous helium injection test [6].

2. No trace of dissolved H2 is detected at PZ1 located 20 m upstream of the injection

3. At the injection well (PZ2), the dissolved H2 concentration, which is 1.76 mg·L−1 at the time of injection, is still 0.084 mg·L−1 when the well was reopened, i.e., 1.8 days after the start of the injection. This residual concentration represents 5% of the concentra-

4. At the main monitoring piezometer (PZ2BIS), located 5 m downstream, the concentration peak passed 2 h after the injection started (i.e., 0.08 days) with a value of 0.63 mg·L−1. This corresponds to a theoretical transfer velocity of 60 m·day−1 what is not a representative value of the mean transfer velocity of the water in the aquifer (which is approximately 3–10 m·day−1 according to Gombert et al. [7]) because it is strongly

5. At the monitoring piezometer PZ2TER located 7 m downstream, the dissolved H2 concentration peak detected by Raman spectrometry passed through 9.7 h after the start of injection (i.e., 0.40 days) with a value of 0.17 mg·L−1 [25]. This corresponds to a theoretical transfer velocity of 17 m·day−1; value still strongly influenced by the in-

6. At the PZ3 piezometer, located 10 m downstream, the first traces of dissolved H2 appeared at 1.05 days and the concentration reached its maximum of 1.7 µg·L−1 after

7. At the PZ4 piezometer, located 20 m downstream, the first traces of dissolved H2 appeared at 1.2 days and the concentration peaked a first time at 1.5 µg·L−1 after two days and a second time at 1.74 µg·L−1 after 2.8 days. This corresponds to transfer

2.02 days, which corresponds to a transfer velocity of 5 m·day−1.

speeds of around 10 and 7 m·day−1, respectively.

well.

jection.

tion of the injected water.

influenced by the injection.
