## 4 Discussion
### General Discussion
#### 4.2.4 Are Threshold Levels Required?
Further comments can be made on the above-defined thresholds whose use is restricted to winter times. Figure 7 well illustrates that CO\({}_{2}\)-flux threshold levels cannot be used during the 'high season' as most of the data would be anomalous or at least exceeding the \"vigilance\" level. In that case, if thresholds were to be used during summer times, then they should have been adapted to this time period (to a value at least two times greater). This is less true for CO\({}_{2}\) concentrations because these data had, during baseline acquisitions, a larger range of variation thus inducing a larger standard deviation (typically of the order of the mean value whereas flux data had only a SD of half of the mean value). This points out another drawback based on the univocal definition of thresholds considering the whole dataset: when the data are less related to phenomenological parameters such as temperature then the threshold definition may induce the calculation of levels that are too high. Such bias has been taken into account when the levels were re-evaluated at the end of the injection period (Figure 7). The thresholds for the flux had minor decrease (minus 25-30%) whereas the thresholds for the gas concentrations were more drastically reduced (minus 50-55%). As the CO\({}_{2}\) fluxes remained at low levels during winter acquisitions, the change of thresholds had limited influence onto level exceeding (1 or 2 exceeds with new \"vigilance\" threshold vs. 0 to 1 exceed with the former threshold). At the opposite, the CO\({}_{2}\) concentrations were high and thresholds were often exceeded (7 to 11 exceeds with new \"vigilance\" threshold vs. 4 exceeds with the former threshold). Without the help of other data (O\({}_{2}\) or C-isotopes) the attribution of so many exceeding thresholds would have been more complicated. In conclusion, it appears important to define threshold levels in connection with seasonal, meteorological and biological conditions, ensuring that the statistical definition of threshold levels covers as many parameters as possible. With our dataset, the influence of local geological conditions appears to be of secondary importance especially when considering CO\({}_{2}\) concentrations. Concentrations greater than 5% were indeed measured on half of the locations (18 points over 36) and these points were located over all the geological formations reported in the area and were indifferently set in valleys or top hill. Such a definition linked to other parameters will avoid a future occurrence of false positives that only result from normal climate-driven changes in near-surface formations. Nevertheless, the question of the validity of threshold levels for the entire period of activity of the storage remains open, as climate change and its potential long-term impact on soil-gas concentrations and soil flux cannot be controlled. Two options can thus be envisaged, either separately or complementarily. The first is to complement the data on soil-gas concentration and/or flux with the monitoring of other tracers. However, though such process-based methods are of great use, a conclusive detection of seepage may remain problematic [91]. The use of additional monitoring, such as of carbon-13 isotope ratios, is important as such ratios are not likely to change rapidly because of climate change, except if the consequence of climate change is a quick shift from photosynthetic C3 cycle to C4 cycle. Nevertheless [PERSON] et al. [92] reported that millions of years were necessary for CO\({}_{2}\) levels to decline to concentrations that were sufficiently low (from a few thousand to a few hundred ppm) to give an advantage to C4 photosynthesis over C3 photosynthesis. The reverse process (slight increase of the CO\({}_{2}\) concentrations) may thus have limited impact over short time scales. As a consequence, a change of carbon-13 isotope ratios will be rather linked to changes of the CO\({}_{2}\) sources (surface processes vs. deeper origin) which may have specific carbon isotope signature [75; 78].