## 4 Discussion
### General Discussion  
#### 4.2.1 Monitoring Strategy--The Aims  
Geochemistry, including surface and near-surface investigations, is one of the many tools available for CCS site monitoring [37; 80] and was extensively used at the Lacq-Rousse CCS pilot [23]. Among these investigations, soil-gas monitoring-concentration and soil flux measurements--was implemented under a scheme approved by the French administration. Soil-gas monitoring is a major geochemical tool for detecting gas micro-seepage from geologic sequestration [81] and should be included in MMV protocols [18]. A key parameter about the impact of leakage from deep storage formations, commonly discussed in recent years, was highlighted by [PERSON] et al. [82]: \"The impacts from many lower level fault--or well--related leakage scenarios are likely to be limited spatially and temporarily and recovery may be rapid\". However, [PERSON] et al. [83] stated that \"leakage detection can be complicated by the natural background of near-surface gas geochemistry and by its variability as a consequence of temporal, seasonal, geological, biological... changes\". Consequently, each near-surface strategy for gas monitoring is site specific and must be thoroughly evaluated in order to provide reliable information during the injection and post-injection periods. The definition of baseline monitoring is important [37]. Once injection takes place, no reverse process exists for determining background levels of soil gas. Even if no leakage occurs, detractors may argue that the storage formation could have experienced some changes after which it has become difficult to prove site integrity. The controversy about the Weyburn field and the allegations of leakage after gas injection illustrate the importance of correct baseline measurements, even though in this case seepage allegations were dropped in the light of data and methods that were not specifically considered during baseline, if any, data acquisitions [57; 84]. The soil-gas monitoring of the Lacq-Rousse field relies on these facts. The initial ideas in 2006 for this work focused on studying the feasibility of the pilot itself, but the importance of a correct design of the baseline monitoring and its strategy became rapidly clear and was adopted in early 2007 when the European Directive 2009/31/EC was not yet operative. This matched French regulatory requirements as well as requirements of the site operator. Compared to, for instance, the Weyburn area [84], the Lacq-Rousse site has a hilly relief (slopes of 15% are common), even though hilltops culminate at only +360 m above sea level. This had a direct effect on monitoring: valleys are more accessible and have a thinner sedimentary succession than hills. A third of the monitoring points were thus located in valleys as they may be earlier affected by CO\({}_{2}\) leakage. The monitoring locations covered the whole surface projection of the storage reservoir and were equally distributed around the CO\({}_{2}\) injection well. Their number is higher near the surface projection of the reservoir and includes locations on topographic highs. Six wells (locations 1 to 6, Figure 1) were drilled for seismic monitoring. As they might represent potential leakage pathways [82], some monitoring points were located close to these geophysical boreholes. Similarly, the injection well (point 16) and a former CH\({}_{4}\) exploitation borehole (point 17) were surveyed as well. In order to consider the geological structure of the Pyrenees forefront, some points covered different geological units, such as to the south of the reservoir. Although their number is low, these points are near outcrops of deep-rooted geological units whose internal discontinuities may affect horizons lying 2000 to 3000 m above the storage reservoir at a depth of 4500 m.  
Finally, the baseline characterization was done at high frequency albeit not continuous [82]. It is established that leakage is easier to detect during winter, when biological activity is low [55].