## 2 Materials and Methods  
### Gas Analyzer  
IN-SITU IR: Infrared spectra of gases were recorded in-situ using a portable infrared spectrometer ALPHA provided by BRUKER (Bruker Optik GmbH, Ettlingen, Germany) with high humidity optics and zinc selenide (ZnSe) beamsplitter. The spectra were acquired with 16 scans in the mid-infrared range (5000-600 cm\({}^{-1}\)) with a spectral resolution of 4 and 1 cm\({}^{-1}\) and a zero-filling factor of 2. A stainless-steel gas cell with a short path length (5 cm) with ZnSe windows was placed in the sampling module. The low-power NDIR sensor (home-made engineered) without moving part allows the continuous monitoring of solely CO\({}_{2}\) was also employed. In situ samples were also analyzed with a gas chromatography analyzer (CP-4900 Micro-Gc, Varian BV, Netherlands) to determine the soil gas composition, mainly CO\({}_{2}\) and CH\({}_{4}\). A more detailed explanation regarding the calibration technique of the primary gas analyzer (FTIR) is given below.  
EX-SITU FTIR: Because of the low resolution used (1 cm\({}^{-1}\)) for IR measurements, calibration curves for CO\({}_{2}\) and CH\({}_{4}\) analyses must be determined in the laboratory [43]. Two reference gases (CO\({}_{2}\): 99.7\(\%\)\(\pm\) 2\(\%\) and 5000 ppm \(\pm\) 5\(\%\)) were used to create a CO\({}_{2}\) calibration curve, and one reference gas (CH\({}_{4}\): 5000 ppm \(\pm\) 5\(\%\)) was used to develop a CH\({}_{4}\) calibration curve. Furthermore, the reference gases were mixed using AlyTech GasMix(tm), an on-site customized gas standard preparation system with a relative uncertainty of less than 2\(\%\) for various mixtures such as CO\({}_{2}\) and Ar, CO\({}_{2}\) and N\({}_{2}\), CH\({}_{4}\) and Ar, and CH\({}_{4}\) and N\({}_{2}\). The gas mix instrument is based on mixing, diluting, and injecting up to 12 gas standards at different concentrations that can be used for instrument calibration purposes. The gas standard was controlled in the software interface allowing the user to define the gas concentration, flow rate, and pressure. A G2201-I gas analyzer (Picarro Inc., Santa Clara, CA, USA) based on Cavity Ring-down Spectroscopy (CRDS) was used to verify the concentration indicated by the gas mix coupled with Gas Chromatography.  
During the calibration procedure, the gas pressure inside the cell was measured using a built-in piezoresistive-based pressure gauge manufactured by Keller, ranging from 1 mbar to 1 kbar with an accuracy of 0.01-1\(\%\). A [PERSON] vacuum model MVP 015-2 was used during the measurement of references. The calibration was conducted at a constant room temperature of 22 \({}^{\circ}\)C and pressure of 1 \(\pm\) 0.002 bar considering the total pressure most likely encountered ranges from 0.950 to 1.050 bar. Besides, the gas's temperature in the borehole is relatively stable, especially at \(-\)6 m, which was around 10 \({}^{\circ}\)C. The leak testing was carried out where this configuration managed to maintain its pressure of 0.02 bar within 15 min of the timeframe. The subtraction of atmospheric CO\({}_{2}\) was done for each recorded infrared spectrum to eliminate the ambient air background spectrum. The different spectral areas of CO\({}_{2}\) and CH\({}_{4}\) were calculated, as shown in Figure 1. As for the CO\({}_{2}\) concentration, the analysis is performed using the fundamental band located at 2350 cm\({}^{-1}\) (v\({}_{3}\)) [2400; 2220] and for CH\({}_{4}\) using the fundamental bands at 1400-1200 cm\({}^{-1}\) (v\({}_{4}\)) and 3200-2800 cm\({}^{-1}\) (v\({}_{3}\)).