**1. Introduction**

Geothermal energy is the energy stored as heat within the upper crust of the Earth. Due to magmatism and hydrothermal circulation, rocks at high temperatures, in the 150–350°C range, can be found in specific areas at reasonably shallow depths. Because rocks are poor heat conductors, geothermal heat energy is most commonly extracted by circulating fluids in the pores and fractures of the hot rock using water injection and production (withdrawal) wells.

Geothermal energy is commonly perceived as environmentally friendly in Italy [1–3]. However, at Amiata Volcano in Southern Tuscany (**Figure 1**), geothermal energy exploitation has quantifiable and substantial local and more widespread environmental impacts, which suggest that this form of energy production needs substantial technical improvements before it may be considered eco-friendly.

At Amiata, there are six flash-type 20-MWe power-plants units, three for each of the two geothermal fields of Bagnore and Piancastagnaio (**Figure 1**). Briefly, the produced geothermal vapour (**Figure 2**) is used to spin a turbine to produce electricity. Immediately after, the vapour is condensed within a shower of cold geothermal fluid, and separated into two streams, a liquid brine condensate and a non-condensable gas, that are treated as follow:


Because the geothermal fluid contains also a significant amount of ammonia, to mitigate the emissions, sulphuric acid is added to the geothermal fluids, if the SO2 produced by the AMIS is insufficient, to convert ammonia into ammoniumsulphate salts. In the case that residual SO2 is present, soda (Na2CO3) is added to the liquid stream. In short, the gaseous pollutants are converted to salts that are solubilised in the liquid stream and for the most part, reinjected into the reservoir. However, because of the substantial evaporation occurring in the cooling towers, an unknown fraction of the geothermal fluid droplets evaporate completely leaving fine particles that, with the fine particles contained in the non-condensable gasstream [9], are emitted as aerosols to the atmosphere with the air-vapour flow. In addition, the majority of the non-condensable gases and smallest size-fraction of droplets of geothermal fluid are emitted to the atmosphere carried by the upward airflow.

In this paper, we present data and explanation to show how, because of this specific type of geothermal energy exploitation, some significant impacts occur at Amiata as follows: (I) decline in the water table of the volcanic freshwater aquifer *The Geothermal Power Plants of Amiata Volcano, Italy: Impacts on Freshwater Aquifers… DOI: http://dx.doi.org/10.5772/intechopen.100558*

#### **Figure 1.**

*(a) Structure of the Amiata volcano area, after Borgia et al. [4]; the three geothermal areas are circled in blue; brown areas = permeable rocks, blue areas = impermeable rocks; black lines = normal faults, yellow lines = basal compressive structures, red lines = anticlines, green lines = synclines. The blue line is the cross-section of (b). Purple line is cross section of Figure 8; numbers are piezometers: 1 = Enel inferno, 2 = Lazzaretti, 3 = Enel4, 4 = Enel Castagno, 5 = Enel Valle, 6 = Galleria Nova drainage tunnel. Inset is the location of Amiata volcano in Tuscany (black line), Italy. (b) Cross-section through Amiata volcano and the Piancastagnaio geothermal field (after [4]); section trace is in (a). Note the doming structures of the anhydrites (AB-TU-1) that constitute the superficial geothermal field; TMC is the Tuscan metamorphic complex, VO are volcanic rocks, LT-TU2 are Tuscan units, LI is Ligurian units, M-P-Q are Miocene–Pliocene-Quaternary marine sediments. Numbers 1, 2, and 3 are the locations of local stress fields (indicated on top of the figure) consistent with volcanic spreading, which allow for activation of normal, strike-slip and thrust [5] faults, respectively.*

because of geothermal fluid production and depressurization, (II) increase in induced seismicity mainly because of fluid reinjection, and (III) decrease of air quality because of the atmospheric emissions of gases and aerosols. Other environmental impacts may arise because of overall geothermal-field depressurization, such as subsidence and soil gas emissions, but are not discussed here.

#### **Figure 2.**

*(a) The PC5 (left-hand side of the photo) and PC4 (right-hand side of the photo) power plants at the Piancastagnaio geothermal field. Note the plumes emitted from the cooling towers and the steam pipes in the foreground. (b) Schematic of a flash geothermal power plant such as those at Amiata volcano (modified from [6]); the cooling towers are composed of three cells exhausting air with vapour, gases, droplets of water and fine particles to the atmosphere. See text for further explanation.*

#### **2. Geology**

The geology of the Amiata Volcano and the surrounding areas was originally studied by Calamai et al. [8], Ferrari et al. [10], Brogi [11] and references therein. Borgia et al. [4, 12] presented a volcanic spreading model for the volcano-tectonic evolution of Amiata (**Figure 1**), expanding on the idea originally suggested by Ferrari et al. [10] and Garzonio [13], and that relates the deep-seated gravity deformation of the volcanic edifice with the formation of the geothermal fields. More recently, Principe and Vezzoli [14], prefer a three-phases volcano-tectoniccollapse model for the origin of the numerous faults that cut the volcanic edifice of

#### *The Geothermal Power Plants of Amiata Volcano, Italy: Impacts on Freshwater Aquifers… DOI: http://dx.doi.org/10.5772/intechopen.100558*

Amiata, a model originally proposed by Mazzuoli and Pratesi [15] and later by Calamai et al. [8]; these authors, however, fail to recognise the numerous compressive structures and anhydrite and shaley diapirs that are found around the base of the volcano [8, 16–19] and that tectonically balance the collapse and spreading of the volcanic edifice [4]. They also fail to recognise the faulting and grabens in the basement below the volcano [8] that do not correspond to their collapse structures within the volcano but are in good agreement with the volcanic spreading model. Active compressive tectonics away of the eastern base of Amiata Volcano are also shown by recent thrust and strike-slip focal mechanism solutions [20].

Aside from the details of the different interpretations of the volcanotectonic evolution of Amiata, there is a general agreement that a relatively large number of recent and active, normal faults cut the volcanic edifice and its basement at least down to the anhydrites at the base of the Tuscan Formation (in the case of the volcanic spreading model [4]), or to deeper levels above the plutons (in the case of the volcano-tectonic collapse model [14]), or even deeper into the crust (in the case of the regional tectonic model [11]). These faults and the volcanic conduits, in addition to the sandstones of the Ligurian Formations and limestones of the Tuscan Formations, constitute the permeable pathways that connect the shallow freshwater aquifer contained in the volcanic rocks with the regional-scale hydrothermal aquifer [4, 8, 12, 21–23]. This freshwater aquifer is often referred to as the superficial aquifer.
