## 2 Materials and Methods
### Establishing the Baseline  
Hydrogen is a very mobile gas that can leak towards the surface and accumulate in the groundwater and the soil. A complete monitoring protocol could interest the saturated zone, the unsaturated zone, the soil, and the surface because hydrogen can be detected in all these compartments as it can be seen in natural hydrogen emission areas [15]. However, the leakage simulation protocol (see under) is based on the injection of water saturated with dissolved gas (helium or hydrogen) directly into the aquifer. The water will be previously saturated at atmospheric conditions, i.e., at a pressure of 0.10 MPa, and then injected from 2 m to 11 m under the water table, where the hydrostatic pressure is between 0.12 and 0.21 MPa. Thus, the water will always remain undersaturated and nor helium nor hydrogen will degas. In these conditions, the only way for the dissolved gas to propagate is to follow the groundwater flow. In this study, the monitoring system has thus be designed for the saturated zone, with a light control of eventual weak degassing in the internal atmosphere of the piezometers but only for safety purposes. Prior to setting up a monitoring system for the hydrogen injection test, a baseline of the initial piezometric, chemico-physical and hydrogeochemical values of the aquifer was established over 388 days starting on 27 October 2018. On all of the 7 main piezometers of the site (PZ1, PZ2, PZ2 BIS, PZ3, PZ4, PZ5, and PZ6), the baseline corresponds to more than 200 measurements of each of the main chemico-physical parameters of the water: pH, temperature, electrical conductivity (EC), oxidation-reduction potential (ORP) and dissolved O\({}_{2}\) (Table 1). The water has a neutral pH (pH = 7.25), is moderately mineralized (EC = 562 \(\upmu\)S\(\cdot\)cm\({}^{-1}\)), and oxygenated (O\({}_{2}\) = 5.44 mg\(\cdot\)L\({}^{-1}\)) due to its proximity to the soil surface, and is thus globally oxidative (ORP = +103 mV). These chemico-physical parameters are quite stable over space and time. Figure 3 represents the boxplots of the dissolved oxygen and the oxidation-reduction potential at each piezometer during the baseline. Figure 4 represents the evolution of these main chemico-physical parameters over time and seems to show a certain sensitivity to the depth of the aquifer, which varies from 13.06 m to 13.94 m (Figure 3). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **O\({}_{2}\)** & **pH** & **T** & **EC** & **ORP** \\ \hline
**Unit** & **(mg\(\cdot\)L\({}^{-1}\))** & **-** & **(\({}^{\circ}\)C)** & **(\(\upmu\)S\(\cdot\)cm\({}^{-1}\))** & **(mV)** \\ \hline Number & 208 & 223 & 224 & 223 & 221 \\ Average & 5.4 & 7.3 & 12.1 & 562 & 103 \\ SD & 1.7 & 0.3 & 0.6 & 66 & 89 \\ \hline \hline \end{tabular} Legend: O\({}_{2}\) = dissolved oxygen; T = Temperature; EC = Electric Conductivity; ORP = Oxidation-Reduction Potential; SD = Standard Deviation.  
\end{table} 
Table 1: Baseline of the chemico-physical parameters. Figure 3: Boxplots of dissolved oxygen concentration and oxidation-reduction potential along the experimental site during the baseline: (**a**) Dissolved oxygen concentration; (**b**) oxidation-reduction potential. The colored boxes represent the 1 st and 3 rd quartiles (respectively Q1 and Q3), the black line is the median line, the dots are the measured values, the whiskers are the upper and lower extreme limits calculated according to the [PERSON]’s formula: Q1 + 1.5 (Q3 - Q1) and Q3 - 1.5 (Q3–Q1). During the acquisition of the baseline data, 94 water samples were taken to analyze the major ions (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42-, NO3-) and the main minor ions liable to be modified by a hydrogen-water-rock interaction (NO2-, NH4+, SO32-, S2-, Fe, Mn). To comply with the storage conditions for all the elements, the analyses were carried out within 24 h of each sample being taken, using the methods presented in Table 2.  
Regarding the major elements, Table 3, Figures 5 and 6 show that their behavior is also very stable throughout the baselining.