## 5 Conclusions  
Measurements of soil-gas concentration and soil flux are reported for the TOTAL Lacq-Rousse CO\({}_{2}\) CCS pilot site. Baseline data, collected from September 2008 to December 2009, are compared with the data obtained during the injection (March 2010 to March 2013) and post-injection (February 2014 to December 2015) periods. Twenty separate monitoring sessions are available for the 8-year duration of the pilot project. Although not continuous, these methods did allow a correct evaluation of the integrity of the site while adequately attributing the origin of the gas signals. The French administration approved the planned strategy of monitoring (spot measurements strategy, number of sampling locations, frequency of monitoring and changes of this frequency). The main source of variations over time in soil-gas concentrations or CO\({}_{2}\) fluxes was found to be related to yearly seasonal changes. CO\({}_{2}\) soil-gas concentrations are roughly the same on an annual basis, but tend to be higher during summer and lower during winter. CO\({}_{2}\) flux is high compared to other sites, because of strong biological activity in humid and fertile soils under a temperate climate. Fluxes tend to be higher when the temperature is higher, well illustrating the thermal draw effect on local gas exchanges between shallow soil and the atmosphere. CO\({}_{2}\) and O\({}_{2}\) soil-gas concentrations are strongly reversely correlated and are greatly influenced by respiration processes in soil, especially during summer and for CO\({}_{2}\) concentrations \(>\)2%. The occurrence of few ppm of CH\({}_{4}\) in the soil by diffuse degassing from the former methane  
Figure 8: Log-probability plots for CO\({}_{2}\) concentrations, CO\({}_{2}\)-fluxes and \(\delta^{13}\)C\({}_{\text{CO2}}\) isotope ratios. **(A)** winter baseline data; **(B)** summer baseline data; **(C)** winter injection data. Mean value and standard deviation of the dataset are indicated in each plot. The threshold definition that was used during pilot activity is indicated for comparison. reservoir is yet unproven. The comparison between CO\({}_{2}\) concentration in soil and CO\({}_{2}\) flux shows no interdependence, advocating for a near-surface origin of the CO\({}_{2}\) gas and highlighting the poor vertical connectivity of the soils. Carbon-isotope ratios in CO\({}_{2}\) soil gas point also to a biological origin for this gas. Geochemical threshold values were used for defining \"vigilance\" and \"anomaly\" modes that were only valid for data acquisition during winter, as approved by the French authorities. This reference to winter times may question the validity of leakage detection during summer, and the ways of confirming or discarding the occurrence of possible leakage. Changes of the threshold levels when the site shifted to post-injection monitoring did not answer this point, but more information comes from the inclusion of carbon-13 isotope-ratio measurements. Threshold redefinition is a way to account for background variability over longer time scales but gas source attribution techniques may be better suited. The comparison of baseline and new thresholds reveals that the occurrence of threshold exceeding is more common for the new ones, even though no significant change occurred in the environmental parameters. From an operating viewpoint, this may indicate the need for additional measurements, to ensure that site integrity is preserved. This points out the difficulty of constructing monitoring scenarios and how to define the limits within which parameters can vary. It also points out the need for having complementary data, in this case carbon-isotope ratios of CO\({}_{2}\). From the viewpoint of a CCS site operator, the monitoring of gas phases that are not directly linked to CO\({}_{2}\), or to products injected with the CO\({}_{2}\), may seem to be of little interest because of cost/benefit considerations.