## 3 Results
### Soil Gas Concentrations--Spot Data  
The distribution of data (CO\({}_{2}\), O\({}_{2}\) in soil gas) follows neither a normal nor a lognormal distribution. The use of Kolmogorov-Smirnov or Khi2 statistical tests always gives p-values \(<\)0.0001. The only exception to this rule is for CO\({}_{2}\) fluxes that are likely to have a lognormal distribution (p-value of 0.271 with the Kolmogorov-Smirnov test and a p-value of 0.157 with the Khi2 test). Therefore, the use of arithmetic mean or median values, used here although not ideal, is preferred as soil-gas studies usually refer to the use of such averaging methods. CO\({}_{2}\) concentration changes with time are shown on Figure 3. O\({}_{2}\)-concentration variations will be discussed later in the Discussion section, together with CO\({}_{2}\)-concentration variations. A total of 710 single measurements is available for the entire monitoring period. CO2 concentrations varied from near atmospheric levels (0.03 to 0.04%) up to 16.7% (Point 24-A, September 2008). Mean value is 2.1 \(\pm\) 2.2% for the whole dataset and the median value is close to 1.4%. Oxygen soil-gas concentrations ranged from 1 to 21% with a mean value of 17.8 \(\pm\) 3.5% and a median value of 19%. Figure 2: Air temperature, soil water content and atmospheric pressure measured during each field session at each monitoring location. Data are shown as box plots presenting median value, quartiles and outliers. The inset on the right details the evolution of soil temperature measured at each point and measured at Point 24 using a Barasol probe (in black), and the mean air temperature measured at Pau-Uzein airport (in blue; [[http://french.wunderground.com/](http://french.wunderground.com/)]([http://french.wunderground.com/](http://french.wunderground.com/))). Figure 3: From top to bottom: soil CO\({}_{2}\) concentration, soil-gas flux at the soil/atmosphere interface, \(\xi^{13}\)C isotope ratios of the CO\({}_{2}\) in soil gas (all data), and \(\xi^{13}\)C isotope ratios of CO\({}_{2}\) in soil gas (excluding CO\({}_{2}\) <0.2%). Data are shown as box plots presenting median value, quartiles and outliers. Mean values indicated by the red line correspond to the mean values calculated for each pilot phase (pre-injection, injection and post-injection). There are some differences between \"low season\" and \"high season\" measurements. During summer, CO\({}_{2}\) concentrations tended to be higher (mean value 2.7 \(\pm\) 2.7%; median value 1.7%; 252 data) as a result of biological and microbial activity in soils. This statement has to be balanced by reference to the temperatures and especially the soil-water contents (Figure 2). In September 2009 and 2010, soil-water contents were at their lowest levels (<30%). Soil dryness has adverse effects on soil organisms and reduces their activity. This may have led to a decrease of CO\({}_{2}\) production in soil so that soil CO\({}_{2}\) concentrations had low mean and median values, mimicking those measured during winter. The highest mean and median CO\({}_{2}\) concentrations were measured when soils were warm and humid, which was mainly encountered during June acquisitions. On the contrary, winter-time acquisitions do not necessarily imply a strong decrease in CO\({}_{2}\) soil-gas concentrations. The mean value calculated for the whole dataset is lower (1.8 \(\pm\) 1.9%; 468 data), but the median value (1.3%) is relatively close to that calculated for summer months (only a 0.4% decrease in the median value). This suggests that soil biological activity decreases during winter, but that there is still enough activity to produce significant CO\({}_{2}\) gas concentrations in soil. The influence of soil humidity on soil CO\({}_{2}\) production was less obvious as no water stress was found during winter months. Soil temperature influenced the measurements; when they dropped below 10 \({}^{\circ}\)C (e.g., December 2009, March 2010 and December 2012), the soil CO\({}_{2}\) concentrations were at very low median levels, well below 1% in volume.