SUMMARY

Exhalation flux of CO2 was measured by the accumulation method within a sedimentary basin in central Italy (Siena Basin). The area is characterized by underground pressurized gas linked to low enthalpy geothermal systems. The exhalation level of CO2 appeared to be controlled by the fracturing (i.e. tectonization) degree of subsoil and was consistent with the distribution of high concentrations of CO2, Rn and He in the soil air. The CO2 flux from the whole area (about 200 km2) would be of the order of 10 Mt y-1, about one order of magnitude higher than the mean flux normally related to the soil respiration. This excess flux has clearly an endogenous component, related to underground gas domains and deep faults, acting as preferential pathways for outgassing. The results show how geologic factors influence the CO2 level at the surface and how exhalation surveys, by identifying invisible surface CO2 leakage, are useful for environmental characterization.

INTRODUCTION

High flows of CO2 from subsurface are generally associated with volcanic activity or hydrothermal circulation. In these conditions the CO2 discharge at the surface is commonly evidenced by punctual manifestations such as gas vents. In these cases it is easy to sample the gas-phase and to study its variation over time or its effects on the surrounding environment. However, in most cases CO2 leakage does not produce visible effects. Even in non-volcanic areas high CO2 leakage in soil may be continuous or intermittent and generally pervasive (e.g. Hermansson et al., 1991; Etiope & Lombardi, 1995), bearing profound effects on the level of the biological activity in large areas.