### Risk of Contamination of Drinking Water Aquifers  
Monitoring the aquifers would therefore be a way of detecting an ongoing hydrogen leak, for alerting purposes, especially since this gas is not generally present in groundwater. In the hydrogeological context of the Paris Basin, two aquifers could fulfill the role of barrier and surveillance zone: the deep Albian-Neocomian aquifer and the shallow Cretaceous chalk aquifer. The latter contains a generally unconfined water table which is used to supply drinking water to a part of Paris and almost all of the neighboring towns. When it is close to the surface, its water is oxygenated, thus under oxidizing conditions, and often enriched with nitrates from anthropogenic surface activities, as well as, locally, with sulfates from the overlying Tertiary formations. The arrival of hydrogen in such an aquifer has the immediate effect of increasing the dissolved hydrogen content of the water and, as a result, reducing its oxidation-reduction potential and possibly its content in natural dissolved gases (mainly N\({}_{2}\), O\({}_{2}\), and CO\({}_{2}\)). The hydrogeochemical impacts expected in the aquifer zone affected by the presence of hydrogen could be as follows [12; 13; 15; 16; 17; 22; 23]:  
1. Reduction of nitrates (NO\({}_{3}\)\({}^{-}\)) to nitrites (NO\({}_{2}\)\({}^{-}\)), or even to ammonium (NH\({}_{4}\)\({}^{+}\)), and then to gaseous nitrogen (N\({}_{2}\))
2. Reduction of sulfates (SO\({}_{4}\)\({}^{2-}\)) to sulfides (SO\({}_{3}\)\({}^{-}\)), or even to hydrogen sulfide (H\({}_{2}\)S)
3. Reduction of iron III to iron II
4. Dissolution of metallic trace elements potentially present in the aquifer rock following the drop in the redox potential  
The consequences of a possible reduction in ion oxides such as NO\({}_{3}\)\({}^{-}\) can be significant because the standards for drinking water destined for human consumption stipulate 50 mg\(\cdot\)L\({}^{-1}\) for nitrates against 0.50 mg\(\cdot\)L\({}^{-1}\) for nitrites and 0.10 mg\(\cdot\)L\({}^{-1}\) for ammonium [24]. Thus, allowing for a mean concentration of nitrates of 33 mg\(\cdot\)L\({}^{-1}\) in the groundwater at the Catenoy experimental site, during the baseline [6], it would be sufficient to reduce only 2% of these ions into nitrites or 0.5% into ammonium to render the water unfit for human consumption in the first case or to trigger health warning measures (information, reinforced surveillance, treatment) in the second case. However, the literature shows that, under normal pressure and temperature conditions, the reduction of these nitrates and sulfates cannot take place except in the presence of a catalyst such as iron, nickel, copper, or platinum. Nevertheless, the frequent use of iron \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Impacted** & **H\({}_{2}\) Partial** & **Temperature** & **pH** & **Catalyst** & **Resulting Species** & **Reference** \\
**Species** & **Pressure (bar)** & **(\({}^{\circ}\)C)** & **pH** & **Catalyst** & **Resulting Species** & **Reference** \\ \hline SO\({}_{4}\)\({}^{-}\) & 4–16 & 250–300 & \(\leq\)5.6 & None & H\({}_{2}\)S & [13] \\ Fe5\({}_{2}\)\({}^{*}\) & 8–18 & 90–180 & 6.9–8.7 & None & Fe5\({}_{1+\
u}\) H\({}_{2}\)S & [14] \\ Fe5\({}_{2}\)\({}^{*}\) & 3–30 & 90–250 & 7.8–9.8 & None & Fe5\({}_{1+\
u}\) H\({}_{2}\)S & [12] \\ NO\({}_{3}\)\({}^{-}\) & 0.1 & 22 & 7.0–8.7 & Fe & NO\({}_{2}\)\({}^{-}\) & [15] \\ NO\({}_{3}\)\({}^{-}\) & 0.05 & 200 & \(\sim\)6 & Fe & NH\({}_{4}\)\({}^{+}\) & [16] \\ NO\({}_{3}\)\({}^{-}\) & 7.5 & 90–180 & 4–9 & Carbon steel & NH\({}_{4}\)\({}^{+}\) & [17] \\ NO\({}_{3}\)\({}^{-}\) & 0.2–7.5 & 90–150 & 4–9 & Stainless steel & NH\({}_{4}\)\({}^{+}\) & [18] \\ NO\({}_{3}\)\({}^{-}\) & 0.1–0.5 & 7–25 & 5–11 & Pd, Cu & N\({}_{2}\) & [19; 20] \\ \hline \hline \end{tabular} \({}^{*}\) Pyrite transformed into pyrrhotite (Fe5\({}_{1+\kappa}\) with 0 \(<\) x \(<\) 0.125).  
\end{table} 
Table 1: Some examples of abiotic reductions due to H\({}_{2}\) under experimental laboratory conditions.