[1] Carbon dioxide (CO 2 ) diffuse degassing structures (DDS) at Furnas volcano (São Miguel Island, Azores) are mostly associated with the main fumarolic fields, evidence that CO 2 soil degassing is the surface expression of rising steam from the hydrothermal system. Locations with anomalous CO 2 flux are mainly controlled by tectonic structures oriented WNW-ESE and NW-SE and by the geomorphology of the volcano, as evidenced by several DDS located in depressed areas associated with crater margins. Hydrothermal soil CO 2 emissions in Furnas volcano are estimated to be ∼968 t d . Discrimination between biogenic and hydrothermal CO 2 was determined using a statistical approach and the carbon isotope composition of the CO 2 efflux. Different sampling densities were used to evaluate uncertainty in the estimation of the total CO 2 flux and showed that a low density of points may not be adequate to quantify soil emanations from a relatively small DDS. Thermal energy release associated with diffuse degassing at Furnas caldera is about 118 MW (from an area of ∼4.8 km 2 ) based on the H 2 O/CO 2 ratio in fumarolic gas. The DDS also affect Furnas and Ribeira Quente villages, which are located inside the caldera and in the south flank of the volcano, respectively. At these sites, 58% and 98% of the houses are built over hydrothermal CO 2 emanations, and the populations are at risk due to potential high concentrations of CO 2 accumulating inside the dwellings.

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