New insights on the meteoric recharge at the Le Biancane area (Larderello geothermal system) from fluid geochemistry

Geothermal energy represents an important aspect of the ongoing green transition; therefore, geothermal reservoirs have to be properly investigated and studied in many and different aspects. In this frame, the Larderello geothermal field represents an important energy resource for the Tuscan region, and the first geothermal reservoir to be used for energy production. It is a vapor-dominated reservoir producing superheated steam and characterized by areas where the permeable formations of the shallower reservoir outcrops, with thermal manifestations at the surface, such as fumaroles and steaming-ground. In particular, the Le Biancane area, one of the main outcropping zones, represents a potential recharge point where meteoric water can infiltrate through the carbonate-anhydrite formations of the Tuscan Nappe.

Although the Larderello geothermal system has been studied since the beginning of the last century, a detailed and systematic investigation of the recharge in the infiltration potential areas is still missing. Indeed, only few chemical and isotopic data of meteoric waters and geothermal fluids in the Le Biancane area are available. These are not enough to give information on the main infiltration areas and origin of the geothermal fluids, considering also the re-injection of spent fluids that has been introduced since the ’70 in some of the wells. Therefore, with the aim to analyse in detail the recharge in the Le Biancane area, 11 fumaroles and 37 cold and thermal springs were sampled. In order to be able to well define the isotopic marker of this area, the investigated springs were selected on a wider area surrounding Le Biancane, on a regional scale. Two sampling campaigns have been carried out, one after the rainy season in May/June 2023 and one after the dry season in October 2023.

From isotopic analyses on fumarole condensates, differences in δD and δ¹⁸O were evident highlighting the possibility that these fluids undergo different processes before reaching the surface. Furthermore, fumaroles were analysed, among the many components, also for COS, which represents a new potential geo-indicator. On the other hand, regarding the water recharge investigation, in this work we will present the results of the isotopic hydrological approach on the spring samples.