## 4 Discussion
### Relations between Parameters
#### 4.1.2 Relationships between Gas Species  
As part of a \"process-based approach\" [60], the relationship linking CO\({}_{2}\) and O\({}_{2}\) soil-gas concentrations (Figure 5) is the most frequently used. There is a strongly inverse correlation between CO\({}_{2}\) and O\({}_{2}\) (O\({}_{2}\) = 20.7 \(-\) 1.32 x CO\({}_{2}\); R\({}^{2}\) = 0.724), a rise in CO\({}_{2}\) being balanced by a depletion in O\({}_{2}\), though the slope of the regression line is not equal to \(-\)1. The ratio of \(-\)1 would suggest that each appearance of 1 CO\({}_{2}\) mole induces the disappearance of 1 O\({}_{2}\) mole (pure respiration process: CH\({}_{2}\)O + O\({}_{2}\) = CO\({}_{2}\) + H\({}_{2}\)O; in green on Figure 5A). The regression line calculated for summer acquisitions (\"high season\") points to a slope close to \(-\)1 and thus points to a dominant influence of the respiration process in soil to account for the CO\({}_{2}\)/O\({}_{2}\) relationship. The CO\({}_{2}\)/O\({}_{2}\) relationship during winter (\"low season\") has a different slope of \(-\)1.6 (Figure 5A), closer to that of the CH\({}_{4}\) oxidation line (CH\({}_{4}\) + 2O\({}_{2}\) = CO\({}_{2}\) + 2H\({}_{2}\)O; in black, Figure 5A). Some \"high season\" data have CO\({}_{2}\) concentrations >2% vol. and plot above the respiration line (i.e., slope less than 1), in an area where the CO\({}_{2}\) concentration in gas rises with only minor depletion in O\({}_{2}\) (CO\({}_{2}\)/O\({}_{2}\) ratio <1). Such behaviour may correspond to the influence of exogenous CO\({}_{2}\). The presence of CO\({}_{2}\) from mantle/crustal origin is unlikely; such seeps from depth are not reported locally - mineral waters circulating at depth are N\({}_{2}\)-rich and depleted in CO\({}_{2}\). The presence of naturally occurring CO\({}_{2}\) from the Lacq reservoir, or of CO\({}_{2}\) resulting from the injection process, is not compatible with carbon-13 isotope data. The origin of data falling above the respiration line thus remains unclear, but may be due to specific processes occurring in soil in late summer, when soils are under hydric stress. Some data, especially during the \"low season\", may show some link with the CH\({}_{4}\) oxidation line, or with processes such as pyrite oxidation [63] that deplete the O\({}_{2}\) with a CO\({}_{2}\)/O\({}_{2}\) concentration changes ratio >1. Nevertheless, pyrite oxidation was discarded during the baseline acquisitions [35], as this mineral is uncommon within the detrital sedimentary formations. Information from the N\({}_{2}\)/O\({}_{2}\) ratio vs. CO\({}_{2}\) concentrations plot [59] given on Figure 5B also suggests CH\({}_{4}\) oxidation may have some contribution for some measurements, albeit the main driver of CO\({}_{2}\)/O\({}_{2}\) concentration changes in soil remains respiration processes. As mentioned before, no CH\({}_{4}\) flux was measured but some soil gas measurements did point to the presence of a few ppm of CH\({}_{4}\). The presence of CH\({}_{4}\) at concentrations over the atmospheric concentration is not common, as most soils in a temperate climate show CH\({}_{4}\) oxidation in the  
Figure 5: (**A**) Relationship between CO\({}_{2}\) and O\({}_{2}\) in soil gas and comparison to source attribution protocol defined by [59]; (**B**) Relationship between CO\({}_{2}\) and N\({}_{2}\)/O\({}_{2}\) in soil gas. Data fits are compared with control lines corresponding to CH\({}_{4}\) oxidation and respiration processes. soil column [64; 65]. Natural leakage from the former CH\({}_{4}\) reservoir may thus be the culprit, even though no isotope ratios were measured (\(\delta^{13}\)C\({}_{\text{CH4}}\) and \(\delta\)D\({}_{\text{CH4}}\)) to clearly attribute a CH\({}_{4}\) origin. Methane concentrations of a few tens of ppm are likely to be found in faulted areas (vertical migration of CH\({}_{4}\); [66]) but deep-rooted faults are not known to affect the thick sedimentary pile of molasse formations [31; 67].