We review the methods based on the measurement of CO2 emissions for the computation of geothermal heat flow, both at a local (hydrothermal sites, a few km2) and regional scale (hundreds km2). At the local scale, we present and discuss the cases of the Latera caldera and Torre Alfina (Italy) geothermal systems. At Torre Alfina and Latera, the convection process sustains a CO2 emission of ~1 kg s–1 and ~4 kg s–1, and heat flows of 46 MW and 130 MW, respectively. At the regional scale, we discuss the case of the central Apennine (Italy), where CO2 mass and enthalpy balances of regional aquifers highlights a wide and strong thermal anomaly in an area of low conductive heat flow. Notably, the CO2/heat ratios computed for the central Apennines are very similar to those of the nearby geothermal systems of Latium and Tuscany, suggesting a common source of CO2-rich fluids ascribed to the Tyrrhenian mantle.

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<p>The French Massif Central (central-southern France) and the Eifel region (central-western Germany) are both young volcanic systems and considered dormant. They are part of the European Cenozoic Rift System (ECRIS) and show similar surficial manifestations of ongoing hydrothermal activity. For example, both areas exhibit numerous low flow rate CO<sub>2</sub>-rich springs, mainly occurring in concomitance of faults and fractures inherited from the Variscan orogeny.</p><p>Here, the chemical and isotopic characterization of different fresh water bodies (springs, wells, rivers and volcanic lakes) has been provided. The composition of dissolved gases and the isotopic signatures of dissolved carbon indicate that meteoric water infiltrated and then interacted with a CO<sub>2</sub>-rich, mantle-related, component. The majority of studied water samples exhibit pCO<sub>2</sub> between 0.3 and 1 bar and the total dissolved inorganic carbon (TDIC) is of the order of 0.01 mol/kg. At surface, most spring water samples are oversaturated with calcite, dolomite, chalcedony and quartz and are in equilibrium with amorphous silica. The correlation between the TDIC and its isotopic composition (δ<sup>13</sup>C<sub>TDIC</sub>) suggests that part of the analysed water samples experienced a degassing process prior to or immediately after emergence. The computed CO<sub>2</sub> flux transported by groundwaters is of the same order of magnitude of the global baseline theorized for geothermal areas. This indicates that passive rifts systems contribute to the atmospheric CO<sub>2</sub> content and highlights the importance of taking into account each carbon source in the study of the global carbon cycle.</p>

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