New insights on the water-recharge and circulation in the Larderello geothermal field: focus on the Le Biancane area

The Larderello geothermal system (Italy) is a well-known vapour-dominated field used for electricity production since the beginning of the last century. Its two reservoirs are recharged by meteoric water infiltrating in areas were permeable formations outcrop. Among these, the most significant are situated in the southern part of the field, in the area of Castelnuovo Val di Cecina and Le Biancane. These areas were identified as potential recharge zones in studies conducted between the 1970s and 1990s (Panichi et al., 1974; Calore et al., 1982; Ceccarelli et al., 1987; D’Amore et al., 1987; Celati et al., 1991). However, during the last decades of fluid extraction and re-injection, no further investigations have been carried out and a detailed study on recharge in this area is still lacking. This is of fundamental importance to ensure a sustainable use of the resource, based on a comprehensive knowledge of the system’s functioning and evolution. This work aimed to investigate the recharge at Le Biancane, an area characterized by thermal manifestations, such as steaming ground and fumaroles. To do so, fumaroles and springs were sampled during multiple campaigns, rain gauges were installed near Le Biancane, and continuous and monthly monitoring of springs and fumaroles was performed. Moreover, to obtain a broader overview of the system, core samples from two wells located in the north-easternmost part of the field, where direct meteoric infiltration is absent, were selected for fluid inclusion analysis. This gave the opportunity to characterize the hydrothermal fluids circulating in the deeper reservoir. Monthly sampling of rainwater enabled the characterization of the meteoric component and the definition of the Local Meteoric Water Line (LMWL), in good agreement with the Tuscan MWL (Natali et al., 2022). Integration of this dataset with cold perennial springs, representative of infiltrating meteoric waters, allowed the correlation of isotopic composition with mean infiltration altitude. From these results, the mean infiltration altitude of recharge waters was estimated to range between 440 and 800 m a.s.l., encompassing both Le Biancane and the area of Castelnuovo Val di Cecina. These findings thus confirm the recharge zones proposed by earlier studies and led to the detailed delineation of the infiltration area at Le Biancane. The meteoric component at Le Biancane was further assessed by monitoring two thermal springs. Both show rapid response to precipitation events, as recorded by variations in temperature and electrical conductivity. Monthly chemical analyses of these springs identified three components in this part of the system, namely the meteoric component, the Ca-SO4 meteoric component and the deeper Na-Cl thermal component (Duchi et al., 1992). Overall, both springs suggest a major contribution from the meteoric water infiltration in line with the observations on regional scale for the Larderello geothermal field. Fumaroles at Le Biancane were also sampled to investigate the influence of local meteoric infiltration and the source of discharged fluids. The isotopic composition of their condensates seems to indicate that the two superheated fumaroles are fed by fluids originating in a zone not directly influenced by recent meteoric water recharge, i.e. the Lago basin, situated westernmost Le Biancane. This interpretation is supported by tritium contents, which differ markedly from both the springs and the precipitation sampled in this study, and by higher arsenic concentrations. Nevertheless, although originating in a different area, these fluids are still affected by local meteoric infiltration, in particular in case of heavy rain events, as suggested by the variations in discharged temperature. This work has also been used to further analyse the COS content at the fumarole of Le Biancane. Through GC-ICP-MS analyses it was possible to use the detected concentrations for PCO2 estimations, based on the H2S/COS ratio. Values of PCO2reach up to 1.42 bar for the superheated fumaroles, slightly smaller than those reported by Granieri et al. (2023), but consistent with the lower COS concentrations in this work. To characterize not only the meteoric contribution, but also the deeper hydrothermal fluids, fluid inclusion analyses were conducted on quartz minerals from the deeper metamorphic reservoir. The results identified fluids representing a more recent stage of the system, in which the meteoric component was already present at depth, as well as an even younger fluid with PT conditions comparable to those measured at present. The oldest fluid reflects the presence of a deep magmatic component, likely originated during the final stages of magma exsolution, evidenced by the enrichment in elements as Li, B and W, while mixing between the deep hydrothermal fluid and a shallower fluid, circulating in the evaporitic units of the Tuscan Nappe was also recognized. Furthermore, the youngest fluid inclusions still show evidence of the magmatic component, which seems to continue circulating in the deepest parts of the system today. These investigations provide the first complete chemical analyses of fluid inclusions from the Larderello geothermal system using LA-ICPMS. Overall, this work provides the first detailed investigation of the recharge at Le Biancane, identified as one of the potential areas contributing to the geothermal system. Understanding meteoric recharge is essential to guarantee long-term productivity. The outcomes of this study can be further incorporated to update existing models of the Larderello geothermal system, which still rely on data from the 1970s-1990s. The determination of the LMWL and the correlation between altitude and isotopic composition of the precipitation represent powerful tools to further investigate the recharge in other areas of permeable outcrop, such as Castelnuovo Val di Cecina and Le Cornate. Additionally, the characterization of hydrothermal fluids in the deep reservoir sheds light on elemental transport in these fluids and supports the presence of a shallow hydrothermal fluid in the northern part of the system that contributes to the deeper fluids circulation.