Abstract:
Diffuse degassing in Italy: a contribution to the estimation of gas hazard in the Larderello geothermal field
V5 project (INGV-DPC), “Diffuse degassing in Italy” point to evaluate the global flux of CO2 in atmosphere from diffuse degassing of soil. The main tasks of the project are to identify areas characterized from CO2, anomalous fluxes and to draw isoflux maps. Moreover, a file spreadsheet reporting the main gas manifestations active in Italy has to be developed. In this context, the IGG- CNR of Pisa working area is Southern-Western Tuscany and, in particular, the Larderello geothermal field.
This degree thesis, included in the V5 project, aims to recognize all thermal zones, to characterize the thermal manifestation and to acquire information in ordered to support civil defence strategy. For this purpose, satellite images and air-photos have been collected and analyzed to identify anomalous thermal areas. Furthermore, field work has been made to check the existence of those thermal areas mentioned above.
In the whole area, many types of thermal manifestation may be detected such as: diffuse emission areas, steam vents, steaming ground, hot springs. The most important manifestations and those whose morphology allows for pounding of gasses have been chosen to collect samples. Some new IR images have been realized to better delimitate local thermal anomalies which are generally associated to strong gas and vapour fluxes. Moreover, 26 water samples and 15 gas samples have been collected and analyzed on November 2005 and July 2006. Analysis have been carried out in the IGG-CNR laboratory of Pisa for the concentration of principal element in water (Na, K, Ca, Mg, alkalinity, SO4, Cl, NO3), some minor ones (B, Li, SiO2, NH4, F, Al) and metals in traces (Fe, Mn, Zn, Cu, Cr, Cd, Pb, As, Sr, Ni). The concentration of CO2, H2S, H2, CH4, CO, N2, Ar, He e O2 have been analysed in the gas samples. Langelier-Ludwig diagrams have been utilized to identify the hydrochemical types and the existence of mixing phenomena; component principal analysis and multiple linear regression individuate the leakage indicators (T, PCO2, B e NH4); geothermometer techniques on solutes and a software (EQ3/6) helped in studying chemical speciation and to verify the existence of equilibrium conditions between water and the principal mineralogical phases.
Gases data support the application of principal geothermometer and geobarometer techniques.
Finally, results have been enclosed in a specific data form spreadsheet which is now available on the website of the V5 project.
In conclusion, data collected during this work represent an essential contribution to the development of the gas manifestations archive of Italy. Moreover, the obtained results contribute to a better knowledge of Larderello geothermal field and related areas.
