*2.1. Overview of the Site Context and Equipment*

The experimental site is located at Catenoy (Oise), about 50 km north of Paris [6]. It is situated in the unconfined chalk aquifer of the Paris Basin. The geology corresponds to a few meters of Quaternary deposits and Tertiary formations, lying over a hundred meters of Senonian chalk that is only visible in the thalwegs (see also Figure 1). Under the site, the underground geology of the first 25 m is: 3 m of colluvium, 4 m of Thanetian sands, and 18 m of chalk. The chalk encloses an aquifer with a static level at a depth of 13 m, which flows in the WSW–ENE direction. The storage coefficient is from 1.1 <sup>×</sup> <sup>10</sup>−<sup>2</sup> to 6.5 <sup>×</sup> <sup>10</sup>−<sup>2</sup> and the hydraulic conductivity is from 6.4 <sup>×</sup> <sup>10</sup>−<sup>4</sup> to 1.4 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>m</sup>·<sup>s</sup> −1 . *Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 5 of 27

**Figure 1.** Location and geological context of the experimental site. **Figure 1.** Location and geological context of the experimental site.

The experimental site has eight piezometers which are 25 m deep and aligned in the direction of the aquifer flow (Figure 2), over a distance of 80 m. The PZ1 piezometer, which functions as a control, is located 20 m upstream of the PZ2 injection well. The PZ3, PZ4, PZ5, and PZ6 piezometers are, respectively, located 10, 20, 30, and 60 m downstream of the injection well. There are also two close monitoring piezometers: the PZ2BIS located at 5 m, which is the main monitoring piezometer for this study, and the PZ2TER located at 7.5 m, which contains Infrared and Raman spectrometers (the specific results from these are the subject of another article in preparation). The site is also equipped with a technical

The monitoring protocol for the saturated zone was detailed by Lafortune et al. [6] and is based on the preliminary injection of helium, because hydrogen and helium have a comparable physical behavior, in particular a very low solubility and a high diffusion co-

1. Two physicochemical probes measure temperature, pH, electrical conductivity, oxidation-reduction potential, and dissolved oxygen. One is installed in the PZ2bis while the other is mobile in order to take measurements at all the piezometers. It should be noted that this last probe had to be replaced on 19 November 2019 at 14:00, which is visible on the graphs as a deviation due to the new calibration (just before

2. Two field fluorometers allow on site measure of the water fluorescence. As before, one is installed in the PZ2bis, while the other is mobile in order to take measurements

3. Six submerged electric pumps are installed at a depth of 16 m at the PZ1, PZ2BIS, PZ3, PZ4, PZ5 and PZ6 piezometers. They are used to regularly sample the groundwater in order to perform laboratory analyses of tracers, dissolved gases (CH4, He,

As a reminder, the monitoring protocol includes (Figure 3):

*2.2. Overview of the Monitoring Protocol* 

the injection of dissolved hydrogen).

in all the piezometers by groundwater sampling.

efficient in water.

**Figure 1.** Location and geological context of the experimental site.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 5 of 27

room and a meteorological station. The data are transmitted in real time to the dedicated e.cenaris cloud monitoring system [25,26].

**Figure 2.** Detailed plan view of the experimental site. **Figure 2.** Detailed plan view of the experimental site.

*2.2. Overview of the Monitoring Protocol*  The monitoring protocol for the saturated zone was detailed by Lafortune et al. [6] and is based on the preliminary injection of helium, because hydrogen and helium have a comparable physical behavior, in particular a very low solubility and a high diffusion coefficient in water. As a reminder, the monitoring protocol includes (Figure 3): 1. Two physicochemical probes measure temperature, pH, electrical conductivity, oxidation-reduction potential, and dissolved oxygen. One is installed in the PZ2bis The hydrodynamic characteristics of the aquifer were determined during a pumping test carried out previously [10]. Depending on the piezometer considered, the porosity varies from 1.1 <sup>×</sup> <sup>10</sup>−<sup>2</sup> to 6.5 <sup>×</sup> <sup>10</sup>−<sup>2</sup> and the hydraulic conductivity from 6.4 <sup>×</sup> <sup>10</sup>−<sup>4</sup> to 1.4 <sup>×</sup> <sup>10</sup>−<sup>3</sup> <sup>m</sup>·<sup>s</sup> −1 . The groundwater flow velocity, measured during a tracing test, varies from 3 m·day−<sup>1</sup> at PZ3 and PZ5 to 10 m·day−<sup>1</sup> at PZ4, which is situated in a preferential flow path (probably a fissured zone). This highlights the dual porosity of the aquifer studied, which may be reflected hydrogeologically by contrasts in flow velocity and variations in transit times from one piezometer to another.

### while the other is mobile in order to take measurements at all the piezometers. It should be noted that this last probe had to be replaced on 19 November 2019 at 14:00, *2.2. Overview of the Monitoring Protocol*

which is visible on the graphs as a deviation due to the new calibration (just before the injection of dissolved hydrogen). 2. Two field fluorometers allow on site measure of the water fluorescence. As before, one is installed in the PZ2bis, while the other is mobile in order to take measurements The monitoring protocol for the saturated zone was detailed by Lafortune et al. [6] and is based on the preliminary injection of helium, because hydrogen and helium have a comparable physical behavior, in particular a very low solubility and a high diffusion coefficient in water.

in all the piezometers by groundwater sampling. As a reminder, the monitoring protocol includes (Figure 3):


5. There are specific analyzers for measuring the gas concentration in the piezometer head spaces and in the gas mixture released from the water extracted from the aquifer. A DRÄGER multigas analyzer equipped with a catalytic cell (resolution of 0.1% vol.) and a portable Biogas analyzer equipped with an electrochemical cell (resolution of 1 ppm) are used for hydrogen, while an ALCATEL ASM 122D transportable mass spectrometer is used for helium. 5. There are specific analyzers for measuring the gas concentration in the piezometer head spaces and in the gas mixture released from the water extracted from the aquifer. A DRÄGER multigas analyzer equipped with a catalytic cell (resolution of 0.1% vol.) and a portable Biogas analyzer equipped with an electrochemical cell (resolution of 1 ppm) are used for hydrogen, while an ALCATEL ASM 122D transportable mass spectrometer is used for helium.

H2, and H2S), major elements (Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, SO42−, and NO3−, with detection limits of 0.01–0.05 mg·L−1), and minor elements (SO3−, S2−, NO2−, and NH4+,

4. Raman and Infrared (IR) spectrometers installed at the PZ2TER [25,27] make it possible to analyze the concentration in the groundwater of mononuclear diatomic molecules (H2, O2, and N2 for Raman) and polar molecules (CO2 and CH4 for Raman and IR). It should be noted that the data acquired by these devices are not presented here but are the subject of another specific publication to come. Here, we selected Raman and IR technologies because they were already used in other studies, which ensures they suit our needs. New sensing technologies using optical fiber grating platforms are presently developed and tested on lab benches and would be promising for field

6. Device for extracting and degassing water by mechanical agitation serves to establish the dissolved gas concentration in the groundwater, in conjunction with the gas analyzers detailed above. 6. Device for extracting and degassing water by mechanical agitation serves to establish the dissolved gas concentration in the groundwater, in conjunction with the gas analyzers detailed above.

*Appl. Sci.* **2021**, *11*, x FOR PEER REVIEW 6 of 27

with detection limits of 0.01–0.02 mg·L−1).

studies in the close future [28].

The migration of the plumes injected into the aquifer is monitored automatically at PZ2BIS and at PZ2TER by means of devices installed in situ and manually at the other piezometers by sampling the water. All measuring devices or probes are installed between 15 and 16 m deep, i.e. in the most productive zone of the aquifer under study [10]. The migration of the plumes injected into the aquifer is monitored automatically at PZ2BIS and at PZ2TER by means of devices installed in situ and manually at the other piezometers by sampling the water. All measuring devices or probes are installed between 15 and 16 m deep, i.e., in the most productive zone of the aquifer under study [10].

During the first week, the monitoring was relatively frequent with 4–15 samples per day depending on the piezometer. During the following weeks, a follow-up with a more spaced measurement frequency was set up, at a rate of 1–3 samples per piezometer and per week. In total, this monitoring made it possible to take 130 water samples to analyze the tracers and chemical elements and to conduct 104 physicochemical measurements. In During the first week, the monitoring was relatively frequent with 4–15 samples per day depending on the piezometer. During the following weeks, a follow-up with a more spaced measurement frequency was set up, at a rate of 1–3 samples per piezometer and per week. In total, this monitoring made it possible to take 130 water samples to analyze the tracers and chemical elements and to conduct 104 physicochemical measurements. In addition, 127 water samples were taken to measure in situ the helium (tracer gas) and hydrogen contents using the method of partial degassing by mechanical agitation.
