## 1 Introduction and Aims
Potential risks include (i) the conversion of hydrogen to CH\({}_{\text{4}}\) and H\({}_{\text{2}}\)S due to microbial activity, (ii) chemical reaction of hydrogen with the minerals of the reservoir rock/cap rock and thus potential resulting porosity changes, and (iii) the loss of aqueous H\({}_{\text{2(aq)}}\) by diffusion through the cap rock. The first risk (i) arises from microbial activities in the underground. The two processes relevant here are bacterial sulfate reduction (2) and methanogenesis (3), where hydrogen is microbially catalyzed by bacteria and is converted to H\({}_{\text{2}}\)S\({}_{\text{(aq)}}\) or CH\({}_{\text{4(aq)}}\). The activity of the different bacteria depends on the availability of the different electron acceptors such as sulfate or carbon dioxide [1]. Other possible microbial processes are acetogenesis and iron(III)-reduction, summarized by [PERSON] et al. [13]. \[\text{Bacterial sulfate reduction (BSR): SO}_{4}{}^{2-}{}_{\text{(aq)}}+5\text{H}_{\text{2(aq)}} \rightarrow\text{H}_{\text{2}}\text{S}_{\text{(aq)}}+4\text{H}_{\text{2}}\text {O} \tag{2}\]  
\[\text{Methanogenesis: CO}_{\text{2(aq)}}+4\text{H}_{\text{2(aq)}} \rightarrow\text{CH}_{\text{4(aq)}}+2\text{H}_{\text{2}}\text{O} \tag{3}\]  
In aqueous anoxic environments, the sulfate-reducing bacteria (SRB) use sulfate as an electron acceptor to oxidize hydrogen and generate sulfide-S which could be available as aqueous S\({}^{2-}{}_{\text{(aq)}}\), HS\({}^{-}{}_{\text{(aq)}}\), and H\({}_{\text{2}}\)S\({}_{\text{(aq)}}\) and as gaseous H\({}_{\text{2}}\)S\({}_{\text{(g)}}\) as well. The sulfate is derived from the aqueous dissolution of mineral phases like anhydrite (CaSO\({}_{\text{4(g)}}\)). The energy gained from sulfate reduction is used by the SRB for cell growth [14]. SRB prefer temperatures around 38 \({}^{\circ}\)C [15] and near-neutral pH conditions [14], but are active even at extreme habitat conditions such as at great depths and temperatures, ranging from 0 \({}^{\circ}\)C to 60-80 \({}^{\circ}\)C [16; 17; 18] and in some cases up to 110 \({}^{\circ}\)C [19]. SRB activity was observed, for example, in hydrocarbon reservoirs [18] and in the underground storage of town gas [20]. The risk arising from bacterial sulfate reduction lies in the produced H\({}_{\text{2}}\)S, which is corrosive towards the storage facilities, toxic if inhaled, and can pose a risk to the environment. Typical environments for methanogenic bacteria are, for example, anoxic sediments and flooded soils [21]. However, the existence of methanogenic bacteria was also observed in town gas storages [22]. Methanogenic bacteria prefer temperatures of 30-40 \({}^{\circ}\)C for growth [10], but they also have been found at higher temperatures of 80 \({}^{\circ}\)C [23; 24] and up to 97 \({}^{\circ}\)C [10; 25]. With these bacteria, H\({}_{2(\text{aq})}\) is the electron donor and CO\({}_{2(\text{aq})}\) is the electron acceptor, which is reduced to form CH\({}_{4(\text{aq})}\). The methanogenic bacteria obtain their energy for cell growth from this conversion [21; 22]. The problem associated with methanogenesis lies in the loss of hydrogen and the related energy loss [2]. This phenomenon has already been observed in the underground storage of town gas. A well-known example is the Czech town gas storage at Lobodice, where the 54 vol. % injected H\({}_{2(\text{g})}\) diminished to 37 vol. % H\({}_{2(\text{g})}\). Concurrently, CH\({}_{4(\text{g})}\) increased from 21.9 vol. % to 40.0 vol. % within a time span of 7 months [22]. Another risk of underground hydrogen storage (ii) is the reaction of H\({}_{2(\text{g}/\text{aq})}\) with the minerals of the reservoir rock and the cap rock, which can lead to mineral dissolution and precipitation and resulting porosity changes as known from carbon capture and storage e.g., [26; 27; 28; 29]. Changes in the porosity of the cap rock can either improve or deteriorate its sealing capacity. Furthermore, precipitation of minerals at the well equipment may cause scaling [2].