Geochemical and mineralogical tracers of geothermal power plants input into the atmosphere: the case study of the Mt. Amiata geothermal field

Tuscany provides an outstanding example of a thermal perturbation caused by magma emplacement at shallow levels in the crust during Late Miocene-Quaternary times. This magmatism is responsible for extensive mesothermal fluid circulation and mineralisation and feeds a large-scale positive thermal anomaly that has resulted in the active geothermal areas of Larderello-Travale and Mt. Amiata. These geothermal fields are among the most important geothermal reservoirs in the world because of both their long-term industrial exploitation, dating back the first half of the 19th century, and their productivity. In Tuscany, the production of geothermal energy covers some 25-30% of the entire regional demand (ca. 20·10^9 kWh, yearly consumption) and certainly contributes to reduce the emission of greenhouse gases into the atmosphere owing to fossil fuel combustion. Despite being considered a renewable source of energy, geothermal power plants do not produce completely green energy. For example, considerable amounts of H2S, NH3, and CO2 are yearly delivered into the atmosphere, although modern technological advancements of geothermal power plant design (e.g. AMIS device) have greatly reduced their emissions. PATOS (Particolato Atmosferico in TOScana) is a joint research project with the Air Quality bureau of the Tuscan Region aiming at identifying robust geochemical and mineralogical tracers to assess the contributions of geothermal power plant emissions into the atmosphere. Accordingly, we have carried out a twelve months sampling of PM10 and air in the locality of Piancastagnaio, Siena, nearby one of the geothermal power plants of the Mt. Amiata area, along with a urban background locality as a reference site. Samples have been analysed for a wide range of geochemical parameters including inorganic major and trace elements, volatile organic compounds, radiogenic (Sr, Nd, Pb) and stable (S) isotopes along with mineralogical analyses to detect the phases occurring in the PM10. This study has revealed that the emissions into the atmosphere of the geothermal power plant of Piancastagnaio have a number of geochemical and mineralogical tracers, significantly distinct to other anthropogenic sources, that are liable to be used as specific markers, namely: Zn/Cu and the occurrence of mascagnite [(NH4)2SO4] and letovicite [(NH4)3H(SO4)2] in the PM10, along with C7H8/C6H6, in air samples. The outcome of this research project, if extended to the other geothermal power plants of Tuscany, is a potential powerful tool to be used in metereological models to asses the contributions of geothermal power plants to the total budget of anthropogenic emissions into the atmosphere, thus monitoring the fluxes of geothermal air masses in Tuscany.