## 2 Materials and Methods
### Preparing the Test  
The helium was injected with the aim of testing and optimizing a future hydrogen injection device using an inert gas, and to configure the monitoring protocol (types of measurement and time intervals) depending on the piezometer being monitored. The objective of this test is to create a plume of dissolved helium in the aquifer, comparable to the future plume of dissolved hydrogen, and to monitor its propagation in the saturated zone. Before this test, the propagation of the dissolved He plume was modeled in 1D using PHREEQC. Modeling parameters were determined using the results of previous CO\({}_{2}\) injection tests [4; 5]. The result is shown in Figure 7 and shows a maximum dissolved helium concentration between 1.46 mg\(\cdot\)L\({}^{-1}\) and \(8\times 10^{-21}\) mg\(\cdot\)L\({}^{-1}\) from PZ2 BIS to PZ6, and a peak arrival time between 100 min and 23 days. Peak values at PZ5 and PZ6 are expected to be below 1 mg\(\cdot\)L\({}^{-1}\) and thus it will not be possible to detect helium in these two piezometers. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Parameter** & **NO\({}_{\mathbf{2}}\)\({}^{-}\)** & **NH\({}_{\mathbf{4}}\)\({}^{+}\)** & **SO\({}_{\mathbf{3}}\)\({}^{2-}\)** & **S\({}^{2-}\)** & **Total Fe** & **Total Mn** & **Total (N + S)** \\ \hline Average & \textless{DL} & 0.10 & 0.01 & \textless{DL} & 0.99 & 0.11 & 0.11 \\ SD & - & 0.13 & 0.03 & - & 1.58 & 0.22 & 0.13 \\ CV & - & 123\% & (490\%) & - & 160\% & 193\% & 114\% \\ \hline \hline \end{tabular} Legend: DL = Detection Limit; SD = Standard Deviation; CV = Coefficient of Variation; Total (N+S) = Total of sulfur and nitrogen ions.  
\end{table} 
Table 4: Main characteristics of minor ions analyzed during baselining. Figure 6: Evolution of the cumulative concentrations of major and minor ions and their extreme limits during the establishment of the baseline in 2019: (**a**) Sum of major ions; (**b**) Sum of minor ions. The solid and dashed lines correspond respectively to the upper and lower extreme limits calculated according to the [PERSON]’s formula: Q1 \(-\) 1.5 (Q3 \(-\) Q1) and Q3 \(+\) 1.5 (Q3 \(-\) Q1) where Q1 and Q3 are the 1 st and 3 rd quartiles. Then, the water from the aquifer was extracted beforehand by pumping in the PZ2 piezometer (future injection well) to fill two HDPE tanks (Figure 8a): a first 1 m\({}^{3}\) tank which contains the tracers to help determine the arrival of the plume of dissolved gas and precisely quantify its kinetics, and a second 5 m\({}^{3}\) tank in which the water will be saturated with helium by bubbling it. It was decided not to incorporate the tracers in the tank of water saturated with helium to avoid the risk of reducing the solubility of this gas. In the first 1 m\({}^{3}\) tank, two types of tracers were used:  
* fluorescent organic tracers that exhibit no analytical interference between them, to allow in situ detection in real-time of the arrival of the injected plume thanks to the installation of a GGUN FL-30 field fluorimeter; these are uranine or fluorescein sodium (green dye), sulforhodamine B (red dye) and Amino G Acid (a colorless tracer emitting in blue); however, previous experiments with these types of organic tracers with a long carbonaceous molecule (C\({}_{20}\) to C\({}_{40}\)) have shown  
Figure 8: Views of the two tanks and the helium saturation device: (**a**) View of the 1 m\({}^{3}\) tracers tank [A], the 5 m\({}^{3}\) He saturated tank [B] and the compressed He cylinder [C]; (**b**) View from the manhole of the bubbling device in the 5 m\({}^{3}\) He tank (20 m of PVC pipe pierced with 200 needles of 0.5 mm diameter). Figure 7: Model of the propagation of the dissolved He plume using PHREEQC.