Linking soil CO2 emissions to the structural features of a fault: insights from the Monterotondo-Sasso Pisano area (Larderello geothermal field, Italy)

Carbon dioxide (CO2) emission naturally released from geothermal systems is a timely theme, being the object of scientific research since decades. Establishing the diffuse degassing processes and pathways provide valuable information for exploration and exploitation purposes of geothermal reservoirs. Areas with high CO2 emissions are indeed able to reveal major upflow zones from deep reservoirs through deep-reaching permeable fault zones. In this work, a high-resolution (3 x 3 m grid) CO2 flux (up to 2927 g m-2 d-1) and soil temperature (up to 98.8 °C) survey was carried out in a selected area of the Monterotondo-Sasso Pisano fault zone (Larderello geothermal system, Tuscany, Italy) to define the architecture of the fault system and estimate the effective CO2 emission from the fault zone. The presence of multiple populations of CO2 flux, supported by the carbon isotopic content measured in the interstitial carbon dioxide, suggest that three different transport mechanisms control the emissions: i) purely diffusive, ii) mixed diffusive-advective, and iii) purely advective, characterized by efflux values of <20, between 20 and 300 and >300 g m-2 d-1, respectively. The spatial distribution of these fluxes well agrees with the fracture distribution of Jurassic radiolarite (Diaspri Fm) dissected by NNE-striking faults. The interaction between fractures and faults enhances, locally, the secondary permeability of rocks as highlighted by the correlation between Discrete Fracture Network modelling and advective flux. Eventually, by normalizing the CO2 output to the fault strip (1350 m2), we came up with the released of CO2 equal to ~155 t d-1 km-2, although when the area characterized by advective flux (460 m2) was considered the efflux rose up to 326 t d-1 km-2, confirming the pivotal role of the structural features in controlling the fluid emissions.