## 4 Discussion
### General Discussion
#### 4.2.4 Are Threshold Levels Required? At reservoir scale, the Lacq-Rousse CCS pilot has proven its integrity throughout the injection period [90]. Data from surface monitoring also suggest site integrity is preserved: any thresholds that were exceeded were attributed to surface processes referring to gas source attribution techniques [59; 85; 86]. At least, no leak event is reported as having occurred near monitoring locations. The threshold values were defined after completing the baseline acquisitions, and were the only references that were used throughout storage activity. It must be stressed that the definition of the thresholds was not under the responsibility of the scientific team performing the site acquisitions. Therefore, other ways to compute thresholds, such as calculating one threshold for each geological formation, were beyond the scope of the monitoring actions. The thresholds were then redefined for the closure and post-closure phases. The option chosen by the site operator was to define warning levels using the whole baseline database and to interpret it using statistical mean value and standard deviation. This procedure was approved by the French administration, and \"vigilance\" and \"anomaly\" thresholds were defined and referred to throughout the \"low season\" injection periods. Feedback from local authorities and inhabitants suggests that the literal sense of the word 'threshold' may induce unnecessary fears. A better way would have been to refer only to the sole mathematical definition. From that perspective, the use of \"statistical difference from baseline values\" and \"significant or notable difference from baseline values\" for levels 1 and 2, respectively, would have been more suitable. The question of the sole use of wintertime monitoring data is also open to debate. From March 2010 to March 2013, \"low season\" data acquisitions used the first definition of threshold levels, and CO\({}_{2}\) concentrations and CO\({}_{2}\) flux were considered separately (Figure 7). This led to few occurrences of level exceeded for concentrations. The situation was different for soil flux, with high soil degassing in March 2012 and March 2013, the hottest \"low season\" months of the entire dataset (Figure 2). In addition, the confrontation between relatively high air temperatures and biological processes in soils that have just begun to reactivate after winter causes normal CO\({}_{2}\) concentrations and high CO\({}_{2}\) flux. From December 2013 to December 2015, \"low season\" data were acquired using the redefined threshold levels. The change in CO\({}_{2}\)-flux threshold levels had almost no impact on threshold exceeding, but the situation was greatly different for CO\({}_{2}\) concentrations in soil. Numerous cases of threshold exceeding were noted during the field work, covering both \"vigilance\" and \"anomaly\" levels, which would not have occurred when using only baseline levels. Fortunately, the coupling of soil-gas and flux data did not lead to the exceeding of warning levels, but this could have occurred as well. A consequence of these observations is that the redefinition of threshold levels during the activity on a site may lead to an exceeding of warning levels, thus requiring additional measurements to ensure that no adverse event affects the storage. However, such a change of threshold values may also be seen as a proactive approach to gain more benefit from earlier acquired data. As a CCS site generally is scheduled to operate over decades, any opportunity to adapt the monitoring scheme during the life of the site will be meaningful for considering the potential effects of climate change [85]. The downside is that such a modification of threshold values can have a financial impact on the project budget, because additional measurements may be needed. Such a change in monitoring activity is likely to be questioned so as to avoid triggering warning levels whose origin is only related to surface- or near-surface processes.