Carbon dioxide, methane and heat emissions of the Monterotondo Marittimo and Sasso Pisano geothermal areas (Tuscany, Italy)

In the central part of Tuscany region (Italy), is located the Larderello – Travale geothermal system which is a large-scale steam dominated system with reservoir temperatures that can exceed 350°C (Bellani et al. 2004). The characteristic high heat flow in this particular area is due to the presence of a thermal anomaly caused by the intrusion of a big Pliocene batholith into the upper crust (Musumeci et al. 2002). This work is aimed at investigating the relationships between carbon emissions and heat, particularly to analyse the distribution of CO₂, CH₄ and soil temperatures in the Monterotondo Marittimo and Sasso Pisano areas. Three hundred measurements of gas fluxes from the soil have been performed using the accumulation chamber method. CO₂ fluxes range from 0.1 gm^-2d^-1 to about 20,000 gm^-2d^-1, while CH₄ fluxes, available for a lower number of points, vary between 0 and 637 gm^-2d^-1. Soil temperatures were also measured at each location and ranges from 8.0 °C to 100 °C, with an average of 39.8 °C.

CO₂ fluxes show a polymodal statistical distribution with (i) a background population characterised by an average CO₂ flux in the order of 16.0 g m^-2 d^-1 and (ii) anomalous populations with an average CO₂ flux of 400 g m^-2 d^-1 and 1600 g m^-2 d^-1   for Sasso Pisano and Monterotondo Marittimo respectively. Not null CH₄ fluxes were measured only at points with a CO₂ flux in the range of the anomalous CO₂ flux population. The statistical distribution of the CH₄ resulted more complex with two populations characterized by an average value of 0.8 g m^-2d^-1 and 174 g m^-2d^-1 respectively, probably reflecting differences in the gas transport mechanism in the soil and/or soil permeability, which is largely variable in the areas with anomalous flux.

The areas characterized by anomalous soil gas fluxes, show also an evident soil temperature anomaly (reaching values close to 100 °C), suggesting that soil degassing is accompanied by a significant process of steam condensation. In the anomalous areas, the CO₂/CH₄ ratios by weight vary between 1.6 x 10^-4 to 1.0 x 10^-1 and fall in the range of variation observed for the geothermal fluids of the Larderello-Travale region (Truesdell & Nehring, 1978; Chiodini et al., 1991; Chiodini & Marini, 1998).

Assuming that the soil is heated by steam condensation, a thermal energy release associated to the degassing process of about 200 MW is estimated for Monterotondo Marittimo, an energy release >15 MW is estimated for Sasso Pisano, where the measurements are still in progress.