Assessing CO2 emissions in diffuse degassing areas – a valuable tool for land-use planning

Carbon dioxide (CO₂) may constitute a hazard not only associated with active volcanic environments and ongoing eruptions, but also during periods of dormancy. CO₂ may be released in a silent and permanent way through the volcanic soils and, if accumulated in high concentrations, may act as an asphyxiant. Soil CO₂ surveys highlight the association between anomalous CO₂ zones and tectonic structures, as well as show the topographic and lithological control on gas emissions. Gas released on diffuse degassing areas is also highly affected by environmental parameters, such as the barometric pressure, air temperature, rainfall and wind speed, which can cause significant variations in the gas flux and result in seasonal trends on the CO₂ emissions.

Hazardous CO₂ concentrations have been detected in several areas of the world (e.g., DR Congo, France, Italy, Portugal, Spain, USA), and the gas may introduce in buildings and accumulate in such concentrations that reaccommodation of residents is requested. In the Azores archipelago, indoor CO₂ concentrations as high as 90 vol.% have been measured, even during quiescent periods of activity.

CO₂ susceptibility maps have been performed for several diffuse degassing areas in the Azores, and some villages have several buildings classified with high risk of CO₂ exposure. This study not only aims to discuss the criteria used for the definition of maps but also evaluates the adequacy of the defined strategy as well as the limitations of the proposed methodology, highlighting the relevance of performing surveys with high density of points. Other critical aspects are the existence of thermal anomalous zones and the need to account with the topography to characterize the area. The adequacy of the maps will be complemented with indoor and outdoor CO₂ measurements carried out in the study areas. The adequacy of the maps is supported by indoor and outdoor CO₂ measurements carried out in the study areas.