## 1 Introduction
### Historical Background on the Larderello Area  
Boron is one of the important constituents of geothermal fluids and can be observed as boric acid (H\({}_{3}\)BO\({}_{3}\)). Its concentration spans from few to hundreds of mg/L depending on water-rock interaction and potential deep magmatic inputs. Boric acid is naturally contained in Larderello geothermal steam, and due to its high affinity with the liquid phase, it reaches high levels of concentration in steam condensation (up to 600 mg/L).  
In Italy, the extraction of boric acid, discovered in 1702 by [PERSON] [26], began at the end of the 18 th century, following its discovery in the 'Lagone Cerchiaio' in Montertono Marittimo, a few kilometers south of Larderello, by [PERSON] [27]. Boric acid was, at the time, known as the sedative Homberg salt [28]. Boric acid was collected in natural ponds containing muds and mixtures (called 'lagoni') and from geysers, whilst natural fumaroles were initially used as an energy source to evaporate waters enriched with boric acid (Figures 4 and 5). The 'lagoni' where first described by Italian naturalist [PERSON] [29]; their size and dimension ranged from a few meters up to more than 30 m. It is probable that each 'lagone' was the result of steam rising from the deep geothermal reservoir and condensing in the very few meters below the surface to form a shallow pool (perched aquifer) hosted in the geothermal system cover, made up of flysch facies containing sandstone and carbonic rocks.  
Figure 3: The first described ‘lagone’ called ‘lagone bizzarro’ (bizzarre lake—**left**) from [PERSON] and a picture of the author (**right**).  
With the introduction of industrial utilization of geothermal fluids, starting from 1818, the natural 'lagoni' were surrounded by a restricting wall and limited in order to enrich the descending saline waters. After the invention of a 'lagone coperto' (covered  
Figure 4: The ancient ‘lagoni’ in Larderello (outlined in circle) from a topographic map dated before 1827.  
Figure 5: The first ‘lagoni’ (natural lakes and ponds) were limited and connected to allow brines to be collected for further evaporation.  
geothermal lake, Figures 6 and 7) in 1827 by [PERSON], the natural saline waters were evaporated by the steam entrapped by the concrete and brick dome-shaped lids, which give the pools the name 'lagone coperto'. The geothermal fluid was collected and conducted into pipelines to use the steam as a source of energy and boric acid. In this sense, 1827 can be considered a milestone in the history of renewable energy sources, insomuch as it saw the first use of steam as an energy source.  
After 1836-1838, some very shallow (20-30 m) [30] hand-driven drilling wells started to enhance the steam output from the natural 'lagoni' and their surrounding area. The first deep-drilling system used to reach steam in the first reservoir (200-700 m) started at the  
Figure 6: The ‘lagone coperto’ (covered lake) used to collect steam and hot waters containing boric acid. (A) boric water exit; (B) steam exit; (C) hot water inlet.  
Figure 7: A section of the ‘lagone coperto’ (covered geothermal lake).  
beginning of the 20 th century, when the electrical use of steam was applied to machinery. With this modern drilling practice, the natural 'lagoni' underwent a progressive decline, and the last 'lagone' disappeared in 1922. In the Secolo Valley, the core of the Larderello field, also known as Devil's Valley, no surface manifestation exists anymore. The last of the remaining natural manifestations can still be seen in the Sasso Pisano and Monterotondo areas.  
A more detailed overview of the historical development can be found in [PERSON] et al. (2018).