Carbon dioxide soil emission monitoring as a tool for the assessment of environmental impact of geothermal plants
Authors: Lenzi Alessandro , Cinci Marcello , Alemanni Annalisa , Mazzuca Nicoletta , De Terrasson Pierre , Parisi Luigi , Sbrana Alessandro , Sbrana Michele , Sironi Selena , Invernizzi Marzio 
Published: February 2023

&lt;p&gt;Volcanic and Non-Volcanic geothermal areas are often interested by&amp;#160; surface emission of high temperature or cold vents of gases and steam.&lt;/p&gt;
&lt;p&gt;The aim of the study was to evaluate the relative contribution of carbon dioxide emissions from geothermal power plants compared to ground emissions from geothermal areas. The techniques adopted required the use of the accumulation chamber for diffuse measurements of CO&lt;sub&gt;2&lt;/sub&gt; emissions from the ground and volumetric measurement systems in areas with localized emissions and analysis of the emitted gases.&lt;/p&gt;
&lt;p&gt;In the traditional area of Larderello, a different approach was used by reconstructing the gas and steam emissions associated with the ancient &#039;lagoni&#039; from which boric acid was extracted since the early 19th century. The analysis of the gas and boric acid content emitted by the endogenous fluids and the historical boric acid production data have allowed to reconstruct the emission of incondensable gases and steam. These emissions have completely disappeared during the deployment of the endogenous steam by deep drilling of wells for geothermal plants. The data obtained estimate the DEPLETION of natural emissions in this area. The case of the Monte Amiata area where the wide-spread soil emission of gases, and the focused emission of gases have been measured, highlight a complex panorama in which surface emission have high CO2 rates also in areas without geothermal plants (e.g. Bagni San Filippo). The total soil emission rates, both wide-spread and focused areas, of the areas where geothermal plants are present are significantly smaller compared to the emission rate of the natural undisturbed geothermal areas. In our&amp;#160; study the degree of depletion of the emission rate have been calculated quantitatively compared to gas emission in the developed and undisturbed areas demonstrating that the difference in the emission rates between these two areas are of the same order of magnitude of the emission rates of carbon dioxide emitted by the geothermal plants. This phenomenon has been represented in the Larderello area where natural soil emission have quantitatively disappeared after geothermal development as shown by historical documentations. The main result of the study is a quantitative estimation of carbon dioxide emission putting in evidence that both the focused and wide-spread soil emission have to be measured together with the emission from the plants in order to estimate the total mass balance of CO&lt;sub&gt;2&lt;/sub&gt; emission in geothermal areas and also that the natural emission in the undisturbed area are equivalent to the sum of the soil and plant emission reaching an equivalent substitution in the entire system. To do that it is fundamental to plan an adequate measurement approach that considers the special requirements in terms of method and technology for the investigation of the entire geothermal area in order to acquire measures that permit to evaluate the relative contribution of geothermal plants respect to the emission of the entire geothermal area. Moreover, the focused and wide-spread emission of carbon dioxide flow rate measurement are potentially an interesting and very easy tool to estimate the presence and extension of geothermal fields for future developments.&amp;#160;&lt;/p&gt;