*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 CH<sup>4</sup> 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):


A detailed analysis of the evolution of the dissolved hydrogen concentration at the main monitoring piezometer PZ2BIS during the injection day shows that the first peak appears only 38 min after the start of the hydrogenated water injection at 0.36 mg·L −1 (see circled 1 in Figure 13), following which a second more significant peak reaching 0.63 mg·L <sup>−</sup><sup>1</sup> occurred after 2 h (see circled 2 in Figure 13). These two peaks occurred during the injection of hydrogenated water, which lasted 2.5 h. Thus, at this close distance to the injection well, PZ2BIS appears to be strongly disturbed by the experimental conditions and particularly the overpressure induced. Similar to the tracers, the theoretical transfer speed it provides is not representative of the natural flow of the aquifer water, but instead of a flow disturbed by the successive injections.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 13 of 27

**Figure 12.** Comparative evolution of dissolved H2 and tracer concentrations at the different piezometers: (**a**) PZ1 (20 m upstream) and PZ2BIS (5 m downstream); and (**b**) PZ1 and PZ3 (10 m downstream) and PZ4 (20 m downstream). A detailed analysis of the evolution of the dissolved hydrogen concentration at the **Figure 12.** Comparative evolution of dissolved H<sup>2</sup> and tracer concentrations at the different piezometers: (**a**) PZ1 (20 m upstream) and PZ2BIS (5 m downstream); and (**b**) PZ1 and PZ3 (10 m downstream) and PZ4 (20 m downstream). *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 14 of 27

main monitoring piezometer PZ2BIS during the injection day shows that the first peak

**Figure 13.** Detailed evolution of the dissolved H2 and tracer concentration at PZ2BIS (5 m downstream) during the day of injection. **Figure 13.** Detailed evolution of the dissolved H<sup>2</sup> and tracer concentration at PZ2BIS (5 m downstream) during the day of injection.

### *3.4. Oxidation-Reduction Potential 3.4. Oxidation-Reduction Potential*

until after 2.8 days.

At the PZ2 injection well, the baseline showed an average oxidation-reduction potential of +192 mV prior to the injections [6]. When the measurements are resumed with the reopening of the well the day after the injections (t = 1.01 days), the oxidation-reduction potential is still low, with a value of +94 mV and it did not regain its initial value until t = 2.8 days onwards (Figure 14a). At PZ2BIS, the oxidation-reduction potential decreased from +154 mV prior to the At the PZ2 injection well, the baseline showed an average oxidation-reduction potential of +192 mV prior to the injections [6]. When the measurements are resumed with the reopening of the well the day after the injections (t = 1.01 days), the oxidation-reduction potential is still low, with a value of +94 mV and it did not regain its initial value until t = 2.8 days onwards (Figure 14a).

the water remained reductive for at least 1.8 days and did not return to its normal value

1. The first is not very marked (−22%) and reached +120 mV at 14:15, i.e. 15 min after injection from the first tank containing helium and hydrological tracers. It corresponds to the passage of less oxidizing water, probably because of its deoxygenation

2. The second is very clear (−190%) and it reached −139 mV at 15:45, i.e. 55 min after injection from the second tank containing dissolved hydrogen. This drop is in fact

Due to the persistence of reducing conditions in the aquifer, in particular because of the release of the hydrogenated water stock present in the chalk's porous matrix in the immediate surroundings of the injection well, the oxidation-reduction potential remained at low values for the first day after the injection at PZ2BIS. It then increased in regular fashion at a mean speed of +59 mV·day−1 as a result of three mechanisms acting in unknown proportions: (i) dilution of the plume injected into the aquifer; (ii) partial degassing of the hydrogen; and (iii) potential chemical or biochemical reaction between the hydro-

Again, at PZ2BIS, a detailed examination of the first day of the experiment shows the

induced by the introduction of dissolved helium.

gen and certain elements present in the water or the aquifer rock.

synchronous with the second tracer peak.

existence of two successive minima (see circled 1 and 2 in Figure 15):

**Figure 14.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at PZ1 (20 m upstream), PZ2 (injection well), and PZ2BIS (5 m downstream): (**a**) PZ1 and PZ2; and (**b**) PZ1 and PZ2BIS. The gap in the redox potential values corresponds to the change of probe. **Figure 14.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at PZ1 (20 m upstream), PZ2 (injection well), and PZ2BIS (5 m downstream): (**a**) PZ1 and PZ2; and (**b**) PZ1 and PZ2BIS. The gap in the redox potential values corresponds to the change of probe.

At PZ2BIS, the oxidation-reduction potential decreased from +154 mV prior to the injections to a minimum of −139 mV during the injection (Figure 14b).Despite the shift in the values due to a difference in the calibration of the measuring probes, it is noted that the water remained reductive for at least 1.8 days and did not return to its normal value until after 2.8 days.

Again, at PZ2BIS, a detailed examination of the first day of the experiment shows the existence of two successive minima (see circled 1 and 2 in Figure 15):


For the piezometers located further downstream, we consistently noted the existence of fluctuations in the oxidation-reduction potential during the first two days (Figure 16). These are in fact artifacts arising from the increased frequency of measurements, which Due to the persistence of reducing conditions in the aquifer, in particular because of the release of the hydrogenated water stock present in the chalk's porous matrix in the immediate surroundings of the injection well, the oxidation-reduction potential remained at

low values for the first day after the injection at PZ2BIS. It then increased in regular fashion at a mean speed of +59 mV·day−<sup>1</sup> as a result of three mechanisms acting in unknown proportions: (i) dilution of the plume injected into the aquifer; (ii) partial degassing of the hydrogen; and (iii) potential chemical or biochemical reaction between the hydrogen and certain elements present in the water or the aquifer rock. (**b**) **Figure 14.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at PZ1 (20 m upstream), PZ2 (injection well), and PZ2BIS (5 m downstream): (**a**) PZ1 and PZ2; and (**b**) PZ1 and PZ2BIS. The gap in the redox potential values corresponds to the change of probe.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 15 of 27

(**a**)

**Figure 15.** Detailed evolution of the oxidation-reduction potential and the tracer concentration at PZ2BIS (5 m downstream) during the day dissolved hydrogen was injected. **Figure 15.** Detailed evolution of the oxidation-reduction potential and the tracer concentration at PZ2BIS (5 m downstream) during the day dissolved hydrogen was injected.

For the piezometers located further downstream, we consistently noted the existence of fluctuations in the oxidation-reduction potential during the first two days (Figure 16). These are in fact artifacts arising from the increased frequency of measurements, which For the piezometers located further downstream, we consistently noted the existence of fluctuations in the oxidation-reduction potential during the first two days (Figure 16). These are in fact artifacts arising from the increased frequency of measurements, which demonstrate intraday fluctuations that were not visible during the looser monitoring on the other days. Apart from this, there is no evidence of an impact of the injection of hydrogenated water on the oxidation-reduction potential once the distance downstream of the injection well reaches 10 m. *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 16 of 27 demonstrate intraday fluctuations that were not visible during the looser monitoring on the other days. Apart from this, there is no evidence of an impact of the injection of hydrogenated water on the oxidation-reduction potential once the distance downstream of the injection well reaches 10 m.

**Figure 16.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at the other downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m). **Figure 16.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at the other downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m).

(**a**) (**b**) **Figure 17.** Comparative evolution of the dissolved oxygen and the tracer concentrations at the main piezometers: (**a**) PZ1

the injection on this parameter is already no longer visible at this point.

6.27 mg·L−1 has the same order (Figure 17a) of magnitude. This shows that the impact of

In contrast, at PZ2BIS, the mean concentration dropped from 6.24 mg·L−1 prior to the injections to a minimum of 4.17 mg·L−1 at t = 0.82 days (Figure 17b). As with the oxidationreduction potential, there are also significant fluctuations in the dissolved oxygen concentration during the first two days following the injections, probably also linked to the increase in the frequency of measurements and the disturbances induced by operations at

*3.5. Dissolved Oxygen* 

the piezometers.

(20 m upstream) and PZ2 (injection well); and (**b**) PZ1 and PZ2BIS (5 m downstream).

### *3.5. Dissolved Oxygen* injections to a minimum of 4.17 mg·L−1 at t = 0.82 days (Figure 17b). As with the oxidation-

stream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m).

*3.5. Dissolved Oxygen* 

At the PZ2 injection well, the mean concentration of dissolved oxygen during baselining equaled 6.30 mg·L −1 [6]. The day after the injections, the first measurement taken at 6.27 mg·L <sup>−</sup><sup>1</sup> has the same order (Figure 17a) of magnitude. This shows that the impact of the injection on this parameter is already no longer visible at this point. reduction potential, there are also significant fluctuations in the dissolved oxygen concentration during the first two days following the injections, probably also linked to the increase in the frequency of measurements and the disturbances induced by operations at the piezometers.

the injection on this parameter is already no longer visible at this point.

At the PZ2 injection well, the mean concentration of dissolved oxygen during baselining equaled 6.30 mg·L−1 [6]. The day after the injections, the first measurement taken at 6.27 mg·L−1 has the same order (Figure 17a) of magnitude. This shows that the impact of

In contrast, at PZ2BIS, the mean concentration dropped from 6.24 mg·L−1 prior to the

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 16 of 27

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

(**c**) (**d**)

**Figure 16.** Comparative evolution of the oxidation-reduction potential and the tracer concentration at the other down-

the injection well reaches 10 m.

demonstrate intraday fluctuations that were not visible during the looser monitoring on the other days. Apart from this, there is no evidence of an impact of the injection of hydrogenated water on the oxidation-reduction potential once the distance downstream of

**Figure 17.** Comparative evolution of the dissolved oxygen and the tracer concentrations at the main piezometers: (**a**) PZ1 (20 m upstream) and PZ2 (injection well); and (**b**) PZ1 and PZ2BIS (5 m downstream). **Figure 17.** Comparative evolution of the dissolved oxygen and the tracer concentrations at the main piezometers: (**a**) PZ1 (20 m upstream) and PZ2 (injection well); and (**b**) PZ1 and PZ2BIS (5 m downstream).

In contrast, at PZ2BIS, the mean concentration dropped from 6.24 mg·L <sup>−</sup><sup>1</sup> prior to the injections to a minimum of 4.17 mg·L <sup>−</sup><sup>1</sup> at t = 0.82 days (Figure 17b). As with the oxidation-reduction potential, there are also significant fluctuations in the dissolved oxygen concentration during the first two days following the injections, probably also linked to the increase in the frequency of measurements and the disturbances induced by operations at the piezometers.

A detailed inspection of the first day of the experiment clearly shows the existence of three successive minima on 19 November 2019 at 14:45 with 5.13 mg·L −1 , on 19 November 2019 at 15:45 with 5.77 mg·L −1 , and on 20 November 2019 at 10:35 with 4.17 mg·L −1 (Figure 18):


Regarding the other piezometers located further downstream, fluctuations are systematically noted during the first two days of monitoring with dissolved oxygen concentrations below 6 mg·L −1 : this is also considered as an artifact associated with the higher frequency of measurements (Figure 19). Apart from these fluctuations, as the distance downstream of the injection well approaches 10 m, there is no evidence of an impact due to the injections on the dissolved oxygen concentration in the water.

18):

18):

tions.

tions.

to the injections on the dissolved oxygen concentration in the water.

due to the bubbling of helium.

bling of hydrogen (see circled 2 in Figure 18).

due to the bubbling of helium.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 17 of 27

bling of hydrogen (see circled 2 in Figure 18).

**Figure 18.** Detailed evolution of the dissolved oxygen and tracer concentrations at PZ2BIS (5 m downstream) during the first day of dissolved hydrogen injection. **Figure 18.** Detailed evolution of the dissolved oxygen and tracer concentrations at PZ2BIS (5 m downstream) during the first day of dissolved hydrogen injection. frequency of measurements (Figure 19). Apart from these fluctuations, as the distance downstream of the injection well approaches 10 m, there is no evidence of an impact due

A detailed inspection of the first day of the experiment clearly shows the existence of three successive minima on 19 November 2019 at 14:45 with 5.13 mg·L−1, on 19 November 2019 at 15:45 with 5.77 mg·L−1, and on 20 November 2019 at 10:35 with 4.17 mg·L−1 (Figure

A detailed inspection of the first day of the experiment clearly shows the existence of three successive minima on 19 November 2019 at 14:45 with 5.13 mg·L−1, on 19 November 2019 at 15:45 with 5.77 mg·L−1, and on 20 November 2019 at 10:35 with 4.17 mg·L−1 (Figure

1. The first one, synchronous with the first tracer peak, corresponds to the passage of water from the first tracer tank (see circled 1 in Figure 18). This water is deoxygenated

2. The second one, synchronous with the second tracer peak, corresponds to the rapid passage of water from the second tank. This water is deoxygenated due to the bub-

3. The third one, synchronous with the third tracer peak, corresponds to the slow passage of water from the second tank (see circled 3 in Figure 18). It signals the arrival of the main plume of hydrogenated water, which circulated more slowly within the aquifer and still contained under-oxygenated water for about a day after the injec-

1. The first one, synchronous with the first tracer peak, corresponds to the passage of water from the first tracer tank (see circled 1 in Figure 18). This water is deoxygenated

2. The second one, synchronous with the second tracer peak, corresponds to the rapid passage of water from the second tank. This water is deoxygenated due to the bub-

3. The third one, synchronous with the third tracer peak, corresponds to the slow pas-

sage of water from the second tank (see circled 3 in Figure 18). It signals the arrival of the main plume of hydrogenated water, which circulated more slowly within the aquifer and still contained under-oxygenated water for about a day after the injec-

**Figure 19.** Comparative evolution of the dissolved oxygen during the dissolved hydrogen injection experiment at PZ1 (20 m upstream piezometer) and at the downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m). **Figure 19.** Comparative evolution of the dissolved oxygen during the dissolved hydrogen injection experiment at PZ1 (20 m upstream piezometer) and at the downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (60 m).

### *3.6. Other Physicochemical Parameters 3.6. Other Physicochemical Parameters*

bubbling with other gases.

and PZ2BIS (5 m downstream): (**a**) over a period of one month; and (**b**) during the first 24 h.

Regarding the electrical conductivity, we note the existence of natural variations within the aquifer, recorded at PZ1 upstream of the injection well: at this piezometer, before injection and up to six days after, the average conductivity is 550 µS·cm−1; it then decreases to 492 µS·cm−1 after this date. This fluctuation, which is observed upstream of the area affected by the injections, therefore affects the entire aquifer with successive repercussions on all piezometers: at PZ2BIS, the conductivity thus dropped from 558 to 495 µS·cm−1 (Figure 20a). On the other hand, a detailed analysis of the first day following the injection again shows the same three successive decreases in concentration already encountered at this piezometer with the previous physicochemical parameters (Figure 20b). Regarding the electrical conductivity, we note the existence of natural variations within the aquifer, recorded at PZ1 upstream of the injection well: at this piezometer, before injection and up to six days after, the average conductivity is 550 <sup>µ</sup>S·cm−<sup>1</sup> ; it then decreases to 492 <sup>µ</sup>S·cm−<sup>1</sup> after this date. This fluctuation, which is observed upstream of the area affected by the injections, therefore affects the entire aquifer with successive repercussions on all piezometers: at PZ2BIS, the conductivity thus dropped from 558 to <sup>495</sup> <sup>µ</sup>S·cm−<sup>1</sup> (Figure 20a). On the other hand, a detailed analysis of the first day following the injection again shows the same three successive decreases in concentration already encountered at this piezometer with the previous physicochemical parameters (Figure 20b).

The minimal values are reached on the day of injection at 14:30 with 490 µS·cm−1, at 15:20

passage of the plumes of water from the two tanks, as well as the slow release into the aquifer of water that has remained trapped in the porous matrix around the injection well. We show below that this correspond mainly to a decrease in the cumulative concentration of the dominant ions (Ca2+, Mg2+, and HCO3) of this bicarbonate-calcic groundwater: this is interpreted as a consequence of the precipitation of CaCO3 and MgCO3 from the degassing of dissolved CO2 following the saturation of the water with He or H2. On the other hand, these decreases are immediately followed by a peak in conductivity reaching 744 µS·cm−1 for the first, 707 µS·cm−1 for the second, and, later (t = 2.80 days), 582 µS·cm−1 for the third. These are the highest values recorded since the implementation of baseline monitoring over more than a year ago. These cycles of decrease and increase should therefore be related to the injections conducted: it seems that the observed increases can result from the dissolution of the carbonaceous aquifer rock by the injected water, which became aggressive to calcite, following the degassing of CO2 induced by the

(**a**) (**b**) **Figure 20.** Comparative evolution of the electrical conductivity (EC) and the tracer concentration at PZ1 (20 m upstream) (60 m).

The minimal values are reached on the day of injection at 14:30 with 490 <sup>µ</sup>S·cm−<sup>1</sup> , at 15:20 with 564 <sup>µ</sup>S·cm−<sup>1</sup> , and at 16:00 with 502 <sup>µ</sup>S·cm−<sup>1</sup> . As before, this corresponds to the rapid passage of the plumes of water from the two tanks, as well as the slow release into the aquifer of water that has remained trapped in the porous matrix around the injection well. baseline monitoring over more than a year ago. These cycles of decrease and increase should therefore be related to the injections conducted: it seems that the observed increases can result from the dissolution of the carbonaceous aquifer rock by the injected water, which became aggressive to calcite, following the degassing of CO2 induced by the bubbling with other gases.

Regarding the electrical conductivity, we note the existence of natural variations within the aquifer, recorded at PZ1 upstream of the injection well: at this piezometer, before injection and up to six days after, the average conductivity is 550 µS·cm−1; it then decreases to 492 µS·cm−1 after this date. This fluctuation, which is observed upstream of the area affected by the injections, therefore affects the entire aquifer with successive repercussions on all piezometers: at PZ2BIS, the conductivity thus dropped from 558 to 495 µS·cm−1 (Figure 20a). On the other hand, a detailed analysis of the first day following the injection again shows the same three successive decreases in concentration already encountered at this piezometer with the previous physicochemical parameters (Figure 20b). The minimal values are reached on the day of injection at 14:30 with 490 µS·cm−1, at 15:20 with 564 µS·cm−1, and at 16:00 with 502 µS·cm−1. As before, this corresponds to the rapid passage of the plumes of water from the two tanks, as well as the slow release into the aquifer of water that has remained trapped in the porous matrix around the injection well. We show below that this correspond mainly to a decrease in the cumulative concentration of the dominant ions (Ca2+, Mg2+, and HCO3) of this bicarbonate-calcic groundwater: this is interpreted as a consequence of the precipitation of CaCO3 and MgCO3 from the degassing of dissolved CO2 following the saturation of the water with He or H2. On the other hand, these decreases are immediately followed by a peak in conductivity reaching 744 µS·cm−1 for the first, 707 µS·cm−1 for the second, and, later (t = 2.80 days), 582 µS·cm−1 for the third. These are the highest values recorded since the implementation of

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 18 of 27

(**c**) (**d**) **Figure 19.** Comparative evolution of the dissolved oxygen during the dissolved hydrogen injection experiment at PZ1 (20 m upstream piezometer) and at the downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6

*3.6. Other Physicochemical Parameters* 

**Figure 20.** Comparative evolution of the electrical conductivity (EC) and the tracer concentration at PZ1 (20 m upstream) and PZ2BIS (5 m downstream): (**a**) over a period of one month; and (**b**) during the first 24 h. **Figure 20.** Comparative evolution of the electrical conductivity (EC) and the tracer concentration at PZ1 (20 m upstream) and PZ2BIS (5 m downstream): (**a**) over a period of one month; and (**b**) during the first 24 h.

We show below that this correspond mainly to a decrease in the cumulative concentration of the dominant ions (Ca2+, Mg2+, and HCO3) of this bicarbonate-calcic groundwater: this is interpreted as a consequence of the precipitation of CaCO<sup>3</sup> and MgCO<sup>3</sup> from the degassing of dissolved CO<sup>2</sup> following the saturation of the water with He or H2. On the other hand, these decreases are immediately followed by a peak in conductivity reaching <sup>744</sup> <sup>µ</sup>S·cm−<sup>1</sup> for the first, 707 <sup>µ</sup>S·cm−<sup>1</sup> for the second, and, later (t = 2.80 days), 582 <sup>µ</sup>S·cm−<sup>1</sup> for the third. These are the highest values recorded since the implementation of baseline monitoring over more than a year ago. These cycles of decrease and increase should therefore be related to the injections conducted: it seems that the observed increases can result from the dissolution of the carbonaceous aquifer rock by the injected water, which became aggressive to calcite, following the degassing of CO<sup>2</sup> induced by the bubbling with other gases.

Regarding pH, we note that it is already varying cyclically from 6.8 to 7.4 at PZ1 (upstream) before the injections. As is the case for the conductivity, we assumed that these are natural groundwater fluctuations since they are measured upstream of the injection well, and they affect the entire experimental site up to the most downstream piezometers: at PZ2BIS, the pH thus varies from 7.0 to 7.7 (Figure 21a). On the other hand, the detailed analysis of the first day following the injection shows, at this piezometer, a weak but sudden increase in pH from 7.2 to 7.4 when the first two tracer peaks passed through (Figure 21b). This behavior is to be linked to the degassing of dissolved CO2, following the dissolution of He and H2. The pH then decreased to 6.9 after t = 2.88 days, i.e., for the duration of the passage of the third plume of water, while at the same time the water at PZ1 showed an increase of natural origin. One could possibly interpret this increase as an impact of the slow arrival of hydrogenated water with the main aquifer flow.

slow arrival of hydrogenated water with the main aquifer flow.

**Figure 21.** Comparative evolution of the pH and the tracer concentration at PZ2BIS (5 m downstream): (**a**) over a period of one month; and (**b**) during the first 24 h. **Figure 21.** Comparative evolution of the pH and the tracer concentration at PZ2BIS (5 m downstream): (**a**) over a period of one month; and (**b**) during the first 24 h.

### *3.7. Dominant Ions (Ca2+, Mg2+ and HCO3−) 3.7. Dominant Ions (Ca2+, Mg2+ and HCO<sup>3</sup>* −*)*

The saturation of the water in the first tank with helium and then that of the second tank with hydrogen caused the degassing of the natural gases initially present, mainly an admixture of CO2, N2, and O2. The degassing of CO2 notably upsets the calco-carbonic equilibrium in the water, which seems to have led to the precipitation of CaCO3 at the bottom of the tanks and MgCO3 to a lesser extent, totaling a probable loss of around 29 mg·L−1 of dissolved elements. The water injected into the aquifer is therefore undersaturated with calcite and dolomite, which explains the observed decrease in the concentrations of Ca2+, Mg2+, and HCO3− in the samples taken during the first day (Figure 22). The cumulative concentration of these three ions decreased from 401 mg·L−1 before injection to a minimum of 372 mg·L−1 during injection, which corresponds to a 9.1% drop in their molar concentration shortly after the passage of the injected plumes. A maximum of 427 mg·L−1 is then observed at t = 2.80 days. This is interpreted as the dissolution of the carbonaceous aquifer rock by the injected water, which became more aggressive with respect to calcite and dolomite. The concentrations then reached their normal baseline values between the third and sixth days, with a cumulative concentration of 405 mg·L−1 until the end of monitoring. The saturation of the water in the first tank with helium and then that of the second tank with hydrogen caused the degassing of the natural gases initially present, mainly an admixture of CO2, N2, and O2. The degassing of CO<sup>2</sup> notably upsets the calco-carbonic equilibrium in the water, which seems to have led to the precipitation of CaCO<sup>3</sup> at the bottom of the tanks and MgCO<sup>3</sup> to a lesser extent, totaling a probable loss of around 29 mg·L −1 of dissolved elements. The water injected into the aquifer is therefore undersaturated with calcite and dolomite, which explains the observed decrease in the concentrations of Ca2+ , Mg2+, and HCO<sup>3</sup> − in the samples taken during the first day (Figure 22). The cumulative concentration of these three ions decreased from 401 mg·L <sup>−</sup><sup>1</sup> before injection to a minimum of 372 mg·L <sup>−</sup><sup>1</sup> during injection, which corresponds to a 9.1% drop in their molar concentration shortly after the passage of the injected plumes. A maximum of 427 mg·L −1is then observed at t = 2.80 days. This is interpreted as the dissolution of the carbonaceous aquifer rock by the injected water, which became more aggressive with respect to calcite and dolomite. The concentrations then reached their normal baseline values between the third and sixth days, with a cumulative concentration of 405 mg·L <sup>−</sup><sup>1</sup> until the end of monitoring. *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 20 of 27

Regarding pH, we note that it is already varying cyclically from 6.8 to 7.4 at PZ1 (upstream) before the injections. As is the case for the conductivity, we assumed that these are natural groundwater fluctuations since they are measured upstream of the injection well, and they affect the entire experimental site up to the most downstream piezometers: at PZ2BIS, the pH thus varies from 7.0 to 7.7 (Figure 21a). On the other hand, the detailed analysis of the first day following the injection shows, at this piezometer, a weak but sudden increase in pH from 7.2 to 7.4 when the first two tracer peaks passed through (Figure 21b). This behavior is to be linked to the degassing of dissolved CO2, following the dissolution of He and H2. The pH then decreased to 6.9 after t = 2.88 days, i.e., for the duration of the passage of the third plume of water, while at the same time the water at PZ1 showed an increase of natural origin. One could possibly interpret this increase as an impact of the

**Figure 22.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week following the injection of dissolved hydrogen. **Figure 22.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week following the injection of dissolved hydrogen.

These cycles of variations in concentrations does not appear at any of the piezometers located further downstream (Figure 23). This shows that the aquifer has already returned to its natural calco-carbonic equilibrium at a distance of 10 m downstream from the injection well. These cycles of variations in concentrations does not appear at any of the piezometers located further downstream (Figure 23). This shows that the aquifer has already returned to its natural calco-carbonic equilibrium at a distance of 10 m downstream from the injection well.

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

(**c**) (**d**) **Figure 23.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and at the

mg·L−1 before dropping again and stabilizing at 26.0 mg·L−1.

We observed a quasi-stability of the cumulative concentration of alkaline ions (Na+ and K+) at around 18.0 mg·L−1, while the concentration of Cl− increased very significantly shortly after the injection of the water with tracers (Figure 24): it rose from 25.0 mg·L−<sup>1</sup> before injection to 32.5 mg·L−1 from the moment of tracer injection and then peaked at 44.5

The first sample in which an increase is detected is taken before the injection of hydrogen, but 0.50 h after the injection of the tracers. It is therefore an artifact caused by the presence of an ionic tracer, the lithium ion, in the form of lithium chloride (LiCl). This tracer was added to the water in the first tank in order to achieve a Li+ ion concentration of 10 mg·L−1, which also represents an additional Cl− ion concentration of 51.4 mg·L−1.

downstream piezometers: (**a**) PZ3 (5 m); (**b**) PZ4(20 m); (**c**) PZ5(30 m); and (**d**) PZ6 (70 m).

*3.8. Chlor-Alkali Ions (Cl−, Na+, and K+)* 

hydrogen.

tion well.

**Figure 23.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and at the downstream piezometers: (**a**) PZ3 (5 m); (**b**) PZ4(20 m); (**c**) PZ5(30 m); and (**d**) PZ6 (70 m). **Figure 23.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and at the downstream piezometers: (**a**) PZ3 (5 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (70 m).

We observed a quasi-stability of the cumulative concentration of alkaline ions (Na+ *3.8. Chlor-Alkali Ions (Cl*−*, Na<sup>+</sup> , and K<sup>+</sup> )*

*3.8. Chlor-Alkali Ions (Cl−, Na+, and K+)* 

and K+) at around 18.0 mg·L−1, while the concentration of Cl− increased very significantly shortly after the injection of the water with tracers (Figure 24): it rose from 25.0 mg·L−<sup>1</sup> before injection to 32.5 mg·L−1 from the moment of tracer injection and then peaked at 44.5 mg·L−1 before dropping again and stabilizing at 26.0 mg·L−1. The first sample in which an increase is detected is taken before the injection of hy-We observed a quasi-stability of the cumulative concentration of alkaline ions (Na<sup>+</sup> and K<sup>+</sup> ) at around 18.0 mg·L −1 , while the concentration of Cl− increased very significantly shortly after the injection of the water with tracers (Figure 24): it rose from 25.0 mg·L −1 before injection to 32.5 mg·L −1 from the moment of tracer injection and then peaked at 44.5 mg·L <sup>−</sup><sup>1</sup> before dropping again and stabilizing at 26.0 mg·<sup>L</sup> −1 . *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 21 of 27

drogen, but 0.50 h after the injection of the tracers. It is therefore an artifact caused by the

**Figure 22.** Comparative evolution of the concentrations of dominant ions and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week following the injection of dissolved

These cycles of variations in concentrations does not appear at any of the piezometers located further downstream (Figure 23). This shows that the aquifer has already returned to its natural calco-carbonic equilibrium at a distance of 10 m downstream from the injec-

**Figure 24.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week of experiment. **Figure 24.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week of experiment.

As previously, this excess of chloride ions is no longer observed at the piezometers located further downstream, i.e., beyond 10 m from the injection well (Figure 25). The first sample in which an increase is detected is taken before the injection of hydrogen, but 0.50 h after the injection of the tracers. It is therefore an artifact caused by the presence of an ionic tracer, the lithium ion, in the form of lithium chloride (LiCl). This tracer was added to the water in the first tank in order to achieve a Li<sup>+</sup> ion concentration of 10 mg·L −1 , which also represents an additional Cl<sup>−</sup> ion concentration of 51.4 mg·L −1 .

Nitrates and sulfates are oxidized ions which are potentially reactive to the presence of hydrogen: they are in fact liable to be reduced to nitrites or ammonium ions for the former and sulfides or sulfites for the latter, particularly in the presence of metal catalysts. Figure 26 shows that the sulfates do not seem to have been affected during injection. At PZ2BIS, their concentration remained stable at 26.9 mg·L−1, a value corresponding to that of the PZ1 upstream piezometer (27.0 mg·L−1). The same behavior is observed at the other piezometers located further downstream of the injection well. On the other hand, there is no trace of sulfides or sulfites above their analytical detection limit of 0.01 mg·L−1. There is therefore no evidence of a reactivity of the sulfates to the injection of hydrogen under the conditions of the experiment. It is noted that the tubing of all the piezometers

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

(**c**) (*d*) **Figure 25.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and at the

downstream piezometers: (**a**) PZ3(10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (70 m).

*3.9. Nitrates, Sulfates, and Their Derivatives* 

(5 m downstream) during the week of experiment.

**Figure 24.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and PZ2BIS

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 21 of 27

As previously, this excess of chloride ions is no longer observed at the piezometers located further downstream, i.e., beyond 10 m from the injection well (Figure 25). As previously, this excess of chloride ions is no longer observed at the piezometers located further downstream, i.e., beyond 10 m from the injection well (Figure 25).

**Figure 25.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and at the downstream piezometers: (**a**) PZ3(10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (70 m). **Figure 25.** Comparative evolution of the concentrations of chlor-alkali ions and tracer at PZ1 (20 m upstream) and at the downstream piezometers: (**a**) PZ3 (10 m); (**b**) PZ4 (20 m); (**c**) PZ5 (30 m); and (**d**) PZ6 (70 m).

## *3.9. Nitrates, Sulfates, and Their Derivatives 3.9. Nitrates, Sulfates, and Their Derivatives*

Nitrates and sulfates are oxidized ions which are potentially reactive to the presence of hydrogen: they are in fact liable to be reduced to nitrites or ammonium ions for the former and sulfides or sulfites for the latter, particularly in the presence of metal catalysts. Figure 26 shows that the sulfates do not seem to have been affected during injection. Nitrates and sulfates are oxidized ions which are potentially reactive to the presence of hydrogen: they are in fact liable to be reduced to nitrites or ammonium ions for the former and sulfides or sulfites for the latter, particularly in the presence of metal catalysts.

At PZ2BIS, their concentration remained stable at 26.9 mg·L−1, a value corresponding to that of the PZ1 upstream piezometer (27.0 mg·L−1). The same behavior is observed at the other piezometers located further downstream of the injection well. On the other hand, there is no trace of sulfides or sulfites above their analytical detection limit of 0.01 mg·L−1. There is therefore no evidence of a reactivity of the sulfates to the injection of hydrogen under the conditions of the experiment. It is noted that the tubing of all the piezometers Figure 26 shows that the sulfates do not seem to have been affected during injection. At PZ2BIS, their concentration remained stable at 26.9 mg·L −1 , a value corresponding to that of the PZ1 upstream piezometer (27.0 mg·L −1 ). The same behavior is observed at the other piezometers located further downstream of the injection well. On the other hand, there is no trace of sulfides or sulfites above their analytical detection limit of 0.01 mg·L −1 . There is therefore no evidence of a reactivity of the sulfates to the injection of hydrogen under the conditions of the experiment. It is noted that the tubing of all the piezometers is made of PVC or HDPE and no metallic item likely to act as a catalyst came into contact with the aquifer.

Figure 26 also shows that the nitrates do not seem to have been affected during injection. Their concentration at PZ2BIS remained stable at 33.3 mg·L −1 , a value corresponding to that of the PZ1 upstream piezometer (33.9 mg·L −1 ), and there is no trace of nitrites above their analytical detection threshold (0.01 mg·L −1 ). On the other hand, the ammonium ions exhibited cyclic fluctuations during monitoring, but this is detected at all the piezometers, including the one located upstream of the injection well (Figure 27). Before the injection of hydrogenated water, therefore, there are already traces of ammonium ions at all piezometers at an average concentration of about 0.10 mg·L −1 . These ions could be either the residues from the application of ammoniacal fertilizers or the result of a weak natural denitrification. It is recalled that, since the well's tubing is not metallic, it cannot act as catalysts for a possible denitrification linked to hydrogen injection.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 22 of 27

is made of PVC or HDPE and no metallic item likely to act as a catalyst came into contact

is made of PVC or HDPE and no metallic item likely to act as a catalyst came into contact

**Figure 26.** Comparative evolution of the concentrations of sulfates, nitrates, and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week following the injection of dissolved hydrogen. **Figure 26.** Comparative evolution of the concentrations of sulfates, nitrates, and tracer at PZ1 (20 m upstream) and PZ2BIS (5 m downstream) during the week following the injection of dissolved hydrogen. residues from the application of ammoniacal fertilizers or the result of a weak natural denitrification. It is recalled that, since the well's tubing is not metallic, it cannot act as catalysts for a possible denitrification linked to hydrogen injection.

**Figure 27.** Comparative evolution of the ammonium ion concentrations at all piezometers before and during the dissolved hydrogen injection experiment. **Figure 27.** Comparative evolution of the ammonium ion concentrations at all piezometers before and during the dissolved hydrogen injection experiment.

## **4. Discussion 4. Discussion**

with the aquifer.

with the aquifer.

**Figure 27.** Comparative evolution of the ammonium ion concentrations at all piezometers before and during the dissolved hydrogen injection experiment. **4. Discussion**  Panfilov [3] pointed out in 2015 that few articles have been published on the scientific aspects of hydrogen behavior in geological structures. Since then, studies on the impact Panfilov [3] pointed out in 2015 that few articles have been published on the scientific aspects of hydrogen behavior in geological structures. Since then, studies on the impact of hydrogen leaks in an aquifer are still very rare. These include the experimental labora-Panfilov [3] pointed out in 2015 that few articles have been published on the scientific aspects of hydrogen behavior in geological structures. Since then, studies on the impact of hydrogen leaks in an aquifer are still very rare. These include the experimental laboratory work of Berta et al. [22] and an ongoing experiment by the same team at the University of Kiel (D) on a shallow aquifer, the results of which have not yet been published. The results obtained by Berta et al. [22] are however not extrapolatable to our case: they concerned a long contact period (six months) between hydrogen under pressure (2–15 bars) and a reconstituted aquifer medium. Under these specific conditions, the reduction of sulfates, the production of acetate, the precipitation of calcite, and, consequently, an increase in pH and a decrease in electrical conductivity were observed. These parameters were considered as potential targets of a monitoring network covering a hydrogen gas storage site. However, it has been shown that these modifications resulted from the development of a hydrogenotrophic microbial community including sulfate-reducing bacteria.

> of hydrogen leaks in an aquifer are still very rare. These include the experimental labora-In our experiment, several physicochemical and hydrogeochemical phenomena occurred in the hydrogen saturation tank, and then, following the injection of the hydrogenated water, at the piezometers near downstream to the injection well. Some of these phenomena are the result of the injection of hydrogenated water (drop in the redox po-

tential dissolved hydrogen in water), but some others are the results of the preliminary dissolution of helium and tracers (mainly, deoxygenation of water and degassing of CO2). At PZ2BIS, the main monitoring piezometer located 5 m downstream to the injection well, such variations are synchronous with the passage of the first uranine peak (fluorescent tracer), which signaled the arrival of the hydrogen-free (helium-saturated) water from the first tank. Once injected into the aquifer, this first water therefore caused the same phenomena as those observed in the tank, but at a lower intensity as a result of its dilution: A drop in the oxidation-reduction potential, the dissolved O<sup>2</sup> concentration, the electrical conductivity and the concentration of dominant ions (Ca2+, Mg2+, HCO<sup>3</sup> −). In addition, the water from this tank also contained an excess of Cl− ions, linked to the presence of LiCl as an ionic tracer: it thus caused an increase of Cl− ions in the aquifer. Since this ion is particularly conservative, it makes it possible to calculate that the dilution factor of the first injected plume must have reached 60% during the passage of the tracer peak at this piezometer.

The dissolution of hydrogen in the second tank also resulted in the same phenomena related to the degassing of dissolved O<sup>2</sup> and CO2: the dissolved O<sup>2</sup> concentration dropped and the pH increased as a result of the degassing of dissolved CO2. This must have also induced the precipitation of some of the dominant ions (Ca2+, Mg2+ and HCO<sup>3</sup> −) in the water of the tank. The presence of dissolved hydrogen at a concentration close to saturation, additionally caused a significant drop in the oxidation-reduction potential, which made the water in this tank highly reducing: this is the only real impact directly linked to the saturation of water with hydrogen. Shortly after its injection into the aquifer, the oxidationreduction potential of the groundwater thus fell at PZ2BIS, making reductive the initially oxidizing groundwater. This drop coincides with the passage of the second tracer peak induced by the injection of water from the second tank. Compared to the baseline state, the concentration of dissolved O<sup>2</sup> also decreased at that time, as well as those of the dominant ions (Ca2+, Mg2+, and HCO<sup>3</sup> −), while the pH increased slightly. In the piezometers located further downstream, i.e., at a distance of 10 m or more from the injection well, no impact of this type is measured in the aquifer: only the tracers show a weak presence which clearly reflects the passage of the plumes of water injected from the tanks. In addition, no piezometer showed significant variations, other than natural ones, in the concentrations of the oxidized and potentially reactive NO<sup>3</sup> − and SO<sup>4</sup> <sup>2</sup><sup>−</sup> ions or in any of their expected metabolites (NO<sup>2</sup> −, NH<sup>4</sup> + , SO<sup>3</sup> <sup>−</sup>, and S2−). It should be specifically noted that no element likely to play a catalytic role (especially metals) came into contact with the groundwater on the site.

The behavior of the aquifer during the passage of the third tracer plume is also interesting. This plume corresponds to the slow release of the water stock that has remained trapped in the poorly permeable zone at the base of the injection well PZ2 or in the porous matrix of its immediate surroundings. Thus, at PZ2BIS, the oxidation-reduction potential remained low throughout the first day following injection, although it regularly increased at an average rate of +59 mV·day−<sup>1</sup> due to three possible mechanisms: (i) the dilution of the plume injected into the water table; (ii) the presumed degassing of the hydrogen; and (iii) the potential chemical or biochemical reaction of the hydrogen with some elements present in the aquifer rock or in the groundwater. Because the hydrogen is injected under conditions of undersaturation with respect to the aquifer (hydrostatic pressure > saturation pressure of the 5 m<sup>3</sup> tank), and no impact of the injection of hydrogen on the concentration of nitrates and sulfates could be demonstrated, the dilution of the injected plume seems to be the principal mechanism. Again, at PZ2BIS, the dissolved O<sup>2</sup> concentration of the aquifer remained low until at least 2.80 days after the start of the injections. During this same period, the pH and the concentration of dominant ions dropped below their initial values. Regarding the conductivity, the decreases observed during the passage of each injected plume of water are followed by brief increases before returning to the initial values. The decreases correspond to the passage of undersaturated water due to the precipitation of calcite and dolomite within the tanks, linked to the degassing of CO<sup>2</sup> due to the bubbling

of He or H2. The observed increases however could represent a brief renewal of the carbonaceous aquifer rock dissolution following the passage of more aggressive water due to its lower concentration of dominant ions (Ca2+, Mg2+, and HCO<sup>3</sup> −). cipitation of calcite and dolomite within the tanks, linked to the degassing of CO2 due to the bubbling of He or H2. The observed increases however could represent a brief renewal of the carbonaceous aquifer rock dissolution following the passage of more aggressive water due to its lower concentration of dominant ions (Ca2+, Mg2+, and HCO3−).

at an average rate of +59 mV·day−1 due to three possible mechanisms: (i) the dilution of the plume injected into the water table; (ii) the presumed degassing of the hydrogen; and (iii) the potential chemical or biochemical reaction of the hydrogen with some elements present in the aquifer rock or in the groundwater. Because the hydrogen is injected under conditions of undersaturation with respect to the aquifer (hydrostatic pressure > saturation pressure of the 5 m3 tank), and no impact of the injection of hydrogen on the concentration of nitrates and sulfates could be demonstrated, the dilution of the injected plume seems to be the principal mechanism. Again, at PZ2BIS, the dissolved O2 concentration of the aquifer remained low until at least 2.80 days after the start of the injections. During this same period, the pH and the concentration of dominant ions dropped below their initial values. Regarding the conductivity, the decreases observed during the passage of each injected plume of water are followed by brief increases before returning to the initial values. The decreases correspond to the passage of undersaturated water due to the pre-

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 24 of 27

According to lithium breakthrough curves and the corresponding calculated dilution factors, the theoretical hydrogen concentration should have reached a maximum of around 0.70 mg·L <sup>−</sup><sup>1</sup> at PZ2BIS and 0.002 mg·<sup>L</sup> <sup>−</sup><sup>1</sup> at PZ3. The values observed of 0.63 and 0.002 mg·L −1 , respectively, correspond well with this hypothesis. During this experiment, the hydrogen therefore behaved mainly as a conservative tracer (Figure 28). It can also be estimated that, at the more distant piezometers, the maximum concentration of dissolved H<sup>2</sup> must have been lower than the analytical detection threshold, which meant that this element could not be detected, nor could its possible impact on the aquifer. According to lithium breakthrough curves and the corresponding calculated dilution factors, the theoretical hydrogen concentration should have reached a maximum of around 0.70 mg·L−1 at PZ2BIS and 0.002 mg·L−1 at PZ3. The values observed of 0.63 and 0.002 mg·L−1, respectively, correspond well with this hypothesis. During this experiment, the hydrogen therefore behaved mainly as a conservative tracer (Figure 28). It can also be estimated that, at the more distant piezometers, the maximum concentration of dissolved H2 must have been lower than the analytical detection threshold, which meant that this element could not be detected, nor could its possible impact on the aquifer.

**Figure 28.** Comparative evolution of the ratio of maximum concentrations (Cmax) to the injection concentration (Cinj) of tracers and hydrogen up to 20 m downstream of the injection well. **Figure 28.** Comparative evolution of the ratio of maximum concentrations (Cmax) to the injection concentration (Cinj) of tracers and hydrogen up to 20 m downstream of the injection well.

### **5. Conclusions 5. Conclusions**

The hydrogen leak simulation experiment consisted of extracting water from the aquifer, saturating it with hydrogen gas in a tank, and then reinjecting it into the aquifer. The saturation of the water with gaseous hydrogen initially caused several physicochemical and hydrochemical phenomena within the tank and, consequently, in the aquifer: The hydrogen leak simulation experiment consisted of extracting water from the aquifer, saturating it with hydrogen gas in a tank, and then reinjecting it into the aquifer. The saturation of the water with gaseous hydrogen initially caused several physicochemical and hydrochemical phenomena within the tank and, consequently, in the aquifer:


These variations are primarily observed at the main PZ2BIS monitoring piezometer located 5 m downstream of the injection well and, to a lesser extent, at the PZ3 piezometer located 10 m downstream of the injection well. The other piezometers, located between 20 and 60 m downstream of the injection well, are not significantly affected. Our monitoring results show that, under the experimental conditions, the impact is only significantly measurable up to 10–20 m downstream of the injection well. This demonstrates the utility of closely monitoring the immediate surroundings of a future hydrogen injection well. This surveillance must be applied not only to the groundwater aquifer, but also to all major aquifers located deeper.

These results, however, are only valid under the experimental conditions of this test, which consisted of injecting a limited amount of dissolved hydrogen (9 g or 100 L STP) for a short time to simulate a sudden leak. Thus, in the case of a larger and/or longer leak, it is

likely that the physicochemical and hydrogeochemical impacts would be greater across both space and time. In addition, bacterial growth could take place and induce biochemical reactions that may consume some dissolved species (sulfates, hydrogen), as observed by Berta et al. [22].

It is also shown that, during this experiment, the rapid transfer of hydrogen through the aquifer and its significant dilution beyond 10 m downstream of the injection well did not allow the development of significant chemical or biochemical reactions: hydrogen behaved here as a predominantly conservative element and is not (or not very) reactive.

This experiment is a first test of the impact of a hydrogen leak in a shallow unconfined aquifer. It made it possible to show that there are direct and indirect impacts of the arrival of dissolved hydrogen even in low amounts in an aquifer and, therefore, to recommend the implementation of a monitoring of future underground hydrogen storage sites that takes all of these physicochemical and hydrogeochemical parameters into account: concentrations of dissolved H2, O2, and CO2, pH, electrical conductivity, and oxidation-reduction potential. In the case of long-term leakage, the concentrations of sulfates, nitrates, and bicarbonates must also be monitored.

This short time experiment should be considered as a first test intended to fit the injection and monitoring protocols for the effective monitoring of the impacts of a hydrogen leak in an aquifer under geological storage conditions. To be more representative of a real hydrogen leak, it should be supplemented in the future by a continuous leak simulation, lasting several days, the monitoring of which should focus on the relevant parameters previously identified.

**Author Contributions:** Conceptualization and methodology, P.G., S.L., Z.P. and E.L.; validation, P.G., S.L., Z.P., E.L., P.d.D. and N.J.; data curation, P.G. and S.L; writing—original draft preparation, P.G. and S.L; writing—review and editing, P.G., S.L., Z.P., E.L., P.d.D. and N.J.; visualization, P.G. and S.L.; project administration, P.G.; supervision and funding acquisition, P.G. and S.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the French Scientific Interest Group GEODENERGIES in the framework of the ROSTOCK-H project (Risks and Opportunities of the Geological Storage of Hydrogen in Salt Caverns in France and Europe).

**Institutional Review Board Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
