Exploring cave systems as natural analogues for studying CO2 leakage risk

The investigation of the potential risk associated to gas leakage in geological carbon storage sites (CCS) is a crucial challenge for preventing environmental impacts on ecosystems and to ensure an agile approach towards the sustainable energy transition prospected for the near future. Karst systems are the expression of the high secondary porosity in heterogeneous carbonates and represent useful natural analogues where investigate how carbon dioxide (CO2) is transported within the Earth’s critical zone. This study explores the natural CO2 emissions in a karst vadose zone in NW Sardinia (Italy) with the aim to provide a surrogate of the geological sequestration sites for the investigation of the behaviour of gas leakage and for its early detection at the surface. For this purpose, standard micrometeorological techniques have been applied in a 100 m-deep shaft (where high CO2 concentration was previously detected) in order to investigate the gas permeable structure of the karst critical zone and to identify the carbon flux at the geosphere-atmosphere interface. The cave atmosphere was examined at -10 m-deep from the entrance with multiple meteorological sensors that recorded continuously air CO2 and other micrometeorological parameters (temperature, relative humidity, barometric pressure, direction and intensity of cave air masses movement). The cave atmosphere monitoring, adapted to site-specific conditions, has shown that the temporal variation in gas concentration can be explained by advection/convection processes related to atmospheric variables. The barometric pressure pumping and, with minor contribution, the thermal instability have been recognized as the main drivers in the upwards migration of substantial amounts of CO2. During the year, fluctuations in barometric pressure drove the advective transport of CO2 from the deep vadose zone to the atmosphere on a semi-daily time scale, while the seasonal variability in atmospheric temperature led to phases of thermal instability, more frequent in winter rather than in summer. The results also suggest that near-surface measurements inside caves could help in developing, testing and optimizing different monitoring platforms, also equipped with low-cost probes, capable of detecting gas emissions even at concentration close to atmospheric values.