2.1.1. The Study Site

As illustrated in Figure 1, H2 is venting from a 600 × 500 m depression that is barren of vegetation. One possibility is that the reduced conditions caused by hydrogen venting have killed the vegetation; another is that periodic flooding of the small depressions is he cause. To be neutral, we label the area *Abarren*. The study area is the large barren area shown in Figure 1 with *Abarren* = 300,000 m2. It is venting hydrogen at an estimated rate of between 7000 and 178,000 m<sup>3</sup> *H*2 per day (Prinzhofer et al., 2019) [11]. Assuming the concentration of the source is 50% *H*2, the venting rate at depth, *QH2*, is between 14,000 and 356,000 m<sup>3</sup> per day. If the flow is uniform across the vent area, the gas flux is *Vdeep* = QH2/Abarren = 0.05 to 1.2 m d−1. The average concentration of *H*2 in the discharge measured at ~1 m depth is ~100 ppm (Prinzhofer et al., 2019 their Figure 2) [11].
