Monitoring CO₂, CO and CH₄, affected or not by ventilation and anthropogenic activities, in the galleries of the Underground Research Laboratory (URL) of Meuse/Haute-Marne

The alternation between periods of ventilation and non-ventilation of the galleries in the Meuse/Haute-Marne Underground Research Laboratory (URL) within the Callovian–Oxfordian argillaceous rock affects the contents of CO₂, CO and CH₄ in atmospheric air. Monitoring the evolution of the gas composition in the galleries was conducted over a period of about 4 months. Two models of IR laser spectroscope (Picarro) were used, one to measure CO₂, CO and CH₄ concentrations and the other to measure CH₄ isotopic compositions. The results show a night and day periodicity with the highest concentrations at night-time and the lowest during the daytime in the URL. Long-term monitoring of these gases at the surface shows the same evolution, but with different amplitudes. Moreover, in the URL, a weekday and weekend or holiday periodicity were superimposed. The concentrations of CO₂ in the galleries are influenced by human activities and ventilation. During working days, the galleries are ventilated, and human activities increase the CO₂ concentrations. Conversely, during weekends and holidays, CO₂ concentrations decrease owing to the cessation of ventilation and human activities, and consumption of CO₂ by concrete carbonation. The data analysis confirms a CH₄ source from the rock and reveals compelling evidence of CO sources that probably originate from rock and/or gallery engineered structure materials. Despite the difficulties in quantifying local sources and sinks of these gases, this study demonstrates the feasibility of obtaining valuable insights into their variability. This study provides information on potential sources and sinks of CO₂, CO and CH₄ in this context.