**3. Impact on Amiata volcano freshwater aquifer**

The Amiata Volcano freshwater aquifer—the name derives probably from Latin "ad meata", which means "at the springs"—is one of the most important sources of potable water in southern Tuscany and Latium serving about 700 thousand people, particularly during the summers, in the provinces of Grosseto, Siena, Arezzo and Viterbo. The richness of this resource is due not only to the freshness and abundance of the water (with productions on the order of 1 m<sup>3</sup> /s) but also to the fact that it is located at a relatively high altitude (most springs are between 600 and 900 m asl) in such a way that the aqueducts from Amiata can deliver water to the surrounding country by gravity without the need of pumping.

The first comprehensive study of this aquifer about the development of geothermal power plants is the one by Calamai et al. [8] (**Figure 3a**). In this study, they used all available data from before geothermal exploitation, including an electrical resistivity survey, calibrated with deep boreholes, that had up to 10-km-long survey lines to define the water table in addition to the top of the shaley and sandstone basement (the so-called "impermeable" layer below the volcanic rocks), and the top of the carbonatic rocks (Tuscan Formation) where the geothermal fields are located. It is seen that the water table rises from the lower elevations toward the top of the volcano reaching 1200 m above sea level where at least one spring was known to exist. The maximum gradients of the water table are relatively high at around 10%.

Numerical models of the Amiata Volcano freshwater aquifer have been developed by Delcroix et al. [25] and Caparrini et al. [26]. They find that for adequate permeability of the volcanic rocks and recharge, the water table indicated by Calamai et al. [8] is appropriate.

After the first years of exploitation of the geothermal fields (the first wells were drilled in 1959), the pressure of the geothermal fields was reduced by about 15 bar [21] (**Figure 4**). Around the same time, many fresh-water cold springs had substantially decreased their flow rate or had dried out, without any comparable decrease in rainfall (**Table 1**) [27]. During the same period, several water-drainage

#### **Figure 3.**

*Piezometric surface of the Amiata volcano freshwater aquifer (a) before geothermal exploitation started in 1959 (redrawn from [8]), and (b) after the beginning of geothermal exploitation (redrawn from [24]). The grey-coloured extent of the drawing is the outcrop of volcanic rocks as in (a). The decrease in elevation of the volcanic water table after the beginning of geothermal exploitation is evident and in the range of 100–300 m. note also in (b) the minimum in the water table that indicates how the water from the superficial aquifer is drained down to the rocks below the volcano, where the hydrothermal system is located.*

tunnels had to be constructed to increase the amount of water put into the aqueducts to deliver to the water users. The flow rate of at least one of these drainage tunnels ("Galleria Nova") at the beginning of geothermal production showed an inverse relation with the production of geothermal fluids (**Figure 5**). Namely, as the flow rate of geothermal fluid produced increases, the flow rate of the spring decreases and vice versa.

To study the water table among other things, ENEL carried out an electric resistivity survey [24] to detect changes in the phreatic surfaces of the superficial *The Geothermal Power Plants of Amiata Volcano, Italy: Impacts on Freshwater Aquifers… DOI: http://dx.doi.org/10.5772/intechopen.100558*

#### **Figure 4.**

*Decrease in the shut-in pressure of the geothermal wells in the reservoirs after the first years of geothermal exploitation (redrawn from [21]). Note that the initial pressure of the wells changes from about 22 bar in 1959 to about 7 bar in 1964. B.# (closed blue dots) are the names of the various wells first drilled at Bagnore. Open dots are non-producing wells.*


#### **Table 1.**

*Flow rates in l/s of major springs SW of Amiata volcano that decreased their flowrate or dried out after the beginning of geothermal exploitation (after [27]).*

aquifer. This survey (**Figure 3b**) shows a very different water table from the original one (**Figure 3a**) and effectively indicates that the superficial freshwater aquifer was drained through the main faults and the eruptive chimneys that connect the volcano to the geothermal system. It also shows a major minimum in the water table, which the same authors indicate as a potential drainage at depth [24]. This minimum in the water table was later measured also with an electric resistivity survey by the Province of Grosseto [29] and its time-evolution was monitored from August 2003 to April 2006 twice a year using magnetotelluric measurements by the Tuscan Region [30] showing oscillations in the phreatic surface that varied between 700 and 600 m asl.

Delcroix et al. [25] find that fluid production from the geothermal field above 0.5 m<sup>3</sup> /s can create critical conditions in the superficial freshwater aquifer because many areas of the aquifer could dry out. Also, Caparrini et al. [26] notice a direct correlation between the level of the monitored water table and the pressure below the superficial volcanic aquifer, that is the pressure of the geothermal field.

Additional evidence of the impact of geothermal exploitation on springs comes from the observations of the flow rates at the "Poggetto" hot spring (**Figure 6a**),

#### **Figure 5.**

*Correlation between vapour extraction at Bagnore geothermal field (pink line, left-hand side scale increasing upward) and water flow rate at the "galleria Nova" (dark-blue line, right-hand side scale increasing downward). The minimum water flow rate at the "galleria Nova" during the years 1962–1964 corresponds to a maximum in vapour extraction from the geothermal field in the same period. Conversely, as the vapour extraction decreased in the following years, the water flow rate at "galleria Nova" increased again (vapour production from [28]; galleria Nova flowrate data are from a personal communication from Regione Toscana to a. Borgia).*

located about 5 km from the geothermal fields northeast of Amiata Volcano (Poggio Zoccolino geothermal area, **Figure 1a**). The flow rate of this spring increased when the Piancastagnaio geothermal field was closed, and decreased again when the geothermal field was reopened. Similarly, the spring flow rate increased when the Bagnore geothermal field was closed and it dropped to zero when the geothermal field reopened with power production tripled. A parallel change in flow rate can be also observed in the flow rate of the "Galleria Nova" drainage tunnel (**Figure 6b**). Similar changes are also observed in the various piezometers.

To monitor the time evolution of the minimum in the water table, the local government and ENEL installed within the volcanic edifice a set of piezometers that continuously measure the water table, in addition to groundwater salinity, conductivity and temperature. Five of these piezometers are indicated in **Figure 1a**, which form the highest to the lowest elevations are: "Enel Inferno", "Lazzaretti", "Enel4", "Enel La Valle", "Enel Castagno". In addition, the draining tunnel of "Galleria Nova" constitutes the lower point of emergence of the water table. **Figure 7** shows the elevations of the water table for these piezometers in December 2018 projected along the purple line in **Figure 1a**. Two observations may be made:

1.From the original (before geothermal exploitation) elevation, the water table has dropped about 200–250 m in the two piezometers found at the higher elevations. Mineral precipitates (mainly goethite) in the fractures of the lavas of the cores of the "Lazzaretti" piezometer are found at about 200 m above today's water table (direct observations by the authors; cf. also [31, 32]). Because these precipitates can form only below the water table, this finding confirms that, since their precipitation, the water table has dropped by a similar elevation. Also, today's water table elevation at the "Lazzaretti" piezometer is about 50–100 m higher than the elevation measured by Compagnia Mediterranea Prospezioni [24], Marocchesi [29], and Manzella [30].

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

#### **Figure 6.**

*(a) Flowrate versus time of the spring "Poggetto" between 2003 and 2016. The spring flow rate varies inversely with geothermal fluids production of the Mt. Amiata fields. Data courtesy of Ing. Pagano. (b) Flow rate versus time of the galleria Nova drainage tunnel from 1990 to 2020. From 1990 to 2009, the flow rate has clear cycles with a 3–4 year periodicity. The anomalous 30% increase in flow rate that begins in the summer of 2010 (blue arrow) was just before the installation of the first piezometer (Figure 1a) and corresponded to the closure of the Piancastagnaio geothermal field. Also, the closure of the Bagnore geothermal field corresponds to a significant increase in flow rate, while the reopening of the fields matches decreases in flow rate. Data from Regione Toscana—Centro Funzionale Monitoraggio Idrologico-Idraulico.*

2.The water table has a strong inflexion in the gradient with the water table sloping toward the interior of the volcano—as determined from the piezometers Enel Castagno/Enel Valle toward Enel 4—.

These observations should be analysed in light of the following simplified conservation laws:

#### **Figure 7.**

*Sketch of water table levels measured in December 2018. Red line is the approximate original water table from Figure 3a. The elevation of the water table inferred by projected piezometer data from "ENEL Valle" and "ENEL Castagno" is the linear interpolation between their two elevations. Note: (1) the reduction in the water table and (2) the flow of water that is from the Valle-Castagno piezometers (located at lower topographic elevations) toward the ENEL4 piezometer (located at a higher topographic elevation) shows an inversion in the original direction of water flow in the aquifer. Data from Regione Toscana—Centro Funzionale Monitoraggio Idrologico-Idraulico.*

$$\frac{\partial v\_x}{\partial \mathbf{x}} + \frac{\partial v\_x}{\partial \mathbf{z}} = \mathbf{0} \quad \text{2D}-\text{mass conservation},\tag{1}$$

$$v\_{\mathbf{x}} = -\frac{k}{\mu} \frac{\partial p}{\partial \mathbf{x}} \quad \mathbf{x}-\text{momentum conservation},\tag{2a}$$

or

$$
\upsilon\_{\mathfrak{x}} = -K \frac{\partial H}{\partial \mathfrak{x}},
\tag{2b}
$$

where

$$K = \frac{k\rho\text{g}}{\mu} \tag{2c}$$

and *vx* and *vz* are the groundwater Darcy's velocities in the *x* and *z* directions respectively, *k* is the rock permeability, *m* and *r* are respectively the water viscosity and density, *<sup>K</sup>* is the hydraulic conductivity, *<sup>∂</sup><sup>p</sup> <sup>∂</sup><sup>x</sup>* and *<sup>∂</sup><sup>H</sup> <sup>∂</sup><sup>x</sup>* are respectively the pressure and head gradients in the *x* direction, and *g* is the acceleration of gravity.

Mass conservation (Eq. (1)) states that if the flow velocity changes in one direction, there must be an opposite change in the other direction. Momentum

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

conservation (Eq. (2); Darcy's law) states that the flow is in the negative direction of the pressure or head gradient, that is from high to low pressure or head.

From these observations and Eq. (2), we may see that, for constant hydraulic conductivity *K*, if the hydraulic gradient decreases in the *x*-direction (horizontal) so does the velocity. Therefore, from Eq. (2c), if the velocity decreases in the *x*direction it must increase in the *z*-direction. That is, due to gravity, the groundwater can only flow to the minimum and than downward toward the geothermal system (indeed, there are no pumping wells in the area). This conclusion is made even more evident by the positive gradient between the piezometers Enel n.4 and Enel Castagno/Enel Valle that forces the groundwater to flow toward the interior of the volcano.

In addition, it can be observed that the temperature and salinity of the water at the ENEL "Castagno" piezometer substantially increases if the water table drops below about 748–757 m asl. In the first of these events, the salinity drops from 285 to 185 ppm as the water table rises from 754 to 767 m asl (**Figure 8a**). In the second event, the salinity rises from 170 to 210 ppm as the water table drops below 748 m asl; on the contrary, as the water table rises again in elevation to 755 m asl the salinity drops again to lower values. The changes in temperature during these events are even more pronounced (**Figure 8b**). In the first event, the temperature increases by 2°C as the water table drops from 762 to 749 m asl decreasing to the original temperature as the water table rises again. In the second event as the water table drops below 754 m asl the temperature increases by about 1.0°C, but when it drops below 746 m asl, there is a temperature increase of about 4.5°C. As the water table recovers to rise above 746 m asl, the temperature drops again by 2°C.

Both the salinity and temperature variations analysed in conjunction with the water table elevation changes indicate that as the water table falls below a given elevation, the pressure at the bottom of the aquifer decreases and, as suggested by Caparrini et al. [26], the hot saline fluids rise into the freshwater aquifer decreasing its quality. We point out that the piezometer ENEL "Castagno" is located at the intersection of two relevant faults (the "Le Mura" and "Poggio Pinzi" faults) created by the volcanic spreading processes (**Figure 1a**). These faults create particularly high-permeability pathways connecting the anhydrite and carbonate rocks of the geothermal field with the volcanic rocks of the freshwater aquifer.

Future work will attempt to quantify the actual volume of water that is drained from the superficial freshwater aquifers to the geothermal system. This calculation is at the moment hindered because flow rates from the various wells and the producing pressures and vapour/liquid water ratio in both geothermal systems are unknown.

### **4. Induced seismicity**

An earthquake occurs when the shear stress (*τ*) accumulated on a fault plane exceeds its shear strength (*τf*), which opposes the relative motion along the fault and is mainly dependent on lithology, roughness of faults'surface and normal stress acting on it (e.g., [33]). The effective value of the normal stress (*σn*) acting on a fault surface is controlled by the local stress field (i.e., values and relative directions of principal stresses *σ*1, *σ*<sup>2</sup> and *σ*3) and by the pore pressure (*pp*) in the neighbourhood of the fault.

$$
\mathfrak{r} > \mathfrak{r}\_{\mathbf{f}} = \mathbb{C} + \mu \left( \mathfrak{a}\_{\mathbf{n}} - \mathfrak{p}\_{\mathbf{p}} \right), \tag{3}
$$

#### **Figure 8.**

*Piezometer Castagno. (a) Water table elevation and salinity versus time. (b) Water table elevation and temperature versus time. Note the rapid changes in salinity and temperature as the water table drops to the lower values. Data from Regione Toscana—Centro Funzionale Monitoraggio Idrologico-Idraulico.*

where *C* is cohesion and μ is friction coefficient [34]. When values of *pp* increase—for example, due to anthropogenic operations, such as fluid injection, in the vicinity of the fault—pore pressure acts against *σn*, lowering the value of *τ<sup>f</sup>* and thus allowing for the generation of a seismic event at relatively low values of shear stress (for *τ* > *τf*).

Seismicity is said to be "induced" when a pore-pressure-increase brought about by underground human activities (e.g., fluid injection) reaches a fault plane (possibly at some distance from the injection well) increasing the value of *pp* and allowing the fault to slip, thereby releasing seismic energy accumulated on the fracture plane in the form of elastic strain [33]. The literature presents many past examples of seismicity induced by anthropogenic operations (e.g., [35, 36]) and, focus of this chapter, geothermal energy exploitation [37, 38]. In general, induced events are

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

usually of low intensity, because fault planes are not reactivated for their full extent [39], with hypocenters commonly located at relatively short distances from the injection well [37, 40, 41].

Seismicity is termed 'triggered' when the fault intersected by the pressure increase, due to gravity or tectonic loading, is already in a critical state in terms of shear stress (close to failure, i.e., critically stressed). In this case, *τ* is almost equal to *τ<sup>f</sup>* on the fault plane and even a small increase of *pp* activates a slip that interests the whole surface, releasing the entire stress accumulated on the structure in the form of seismic waves [39, 42]. A prerequisite for this to happen is the optimal orientation of the fault with respect to the principal stresses, with the normal to its surface lying in the plane *σ*<sup>1</sup> *σ*3, where the differential stress is higher [38].

Previous studies (e.g., [33, 39, 43–45]) suggest that faults when in critically stressed conditions, can be more conductive to fluids (and to poroelastic stress changes). Barton et al. [45] present strong evidence that, in crystalline rocks, faults that are optimally oriented for shear failure and critically stressed have increased permeability and conduct fluid along their planes. Non-critically stressed faults appear to provide no fluid migration pathways. The concept of periodic fluid flow along growth faults (within sedimentary basins) was introduced by Sibson [43] and Hooper [45] who show how fluid motion along fault planes is restricted during periods of fault activity. At Paradox Valley, Colorado, USA, injection for disposal of high-salinity water induced seismicity (with several events of *ML* > 4) which separated in two distinct zones: a principal one (>95% of events) asymmetrically surrounding the injection well and to a maximum radial distance of 3 km, and a second zone covering an area of about 10 km<sup>2</sup> and centred 8 km northwest of the injection [46]. Active faults and fractures at the edges of the valley allow for the stress change caused by the injection to reach the secondary seismic zone.

During the period 1982–2009, ENEL has recorded a large number of earthquakes with magnitudes in the range between 0 and 4 (**Figure 9**) [47]. These earthquakes, which appear to be induced by geothermal fluid production/

#### **Figure 9.**

*Earthquakes epicentres recorded by ENEL from 1982 to 2009 (modified after [47]). The volcanic rocks are pink coloured; triangles are seismic stations. Note the large number of earthquakes that concentrate in and around the geothermal fields, most of which seem to be induced by geothermal exploitation; red star is the epicentre of the 1st April 2000* M *= 3.9 earthquakes.*

reinjection, are concentrated mainly within the geothermal fields and most of them are close to the production and injection wells. Some earthquakes even of significant magnitudes are located in proximity to extensional structures within the volcanic edifice.

Mazzoldi et al. [48] described the microseismic activity recorded in 2000–2001 around the Piancastagnaio geothermal field. They show how long-time geothermal fluid production with strong depressurization of the geothermal fields (see **Figure 4**) could have augmented the effective shear strength of faults and the potential magnitude of triggered or induced earthquakes. Perhaps this could have been the mechanism for the April 1st 2000 triggered earthquake in the Piancastagnaio Geothermal Field that had *ML* > 3.9, damaging buildings in the Piancastagnaio and Abbadia San Salvatore municipalities and alarming the population [40, 41, 49].

During the last century, the strongest earthquake around Amiata Volcano occurred in 1919, with an epicentre at Piancastagnaio and with an estimated magnitude between 5.1 and 5.4. Two other events of magnitudes 5 and 4.6, with epicentres at Mt. Amiata (1948) and near Radicofani (some 10 km E of the volcano, 1958), respectively, have been reported. The latter is also the last event of *M* > 4.5 recorded in the area after the beginning of the geothermal exploitation (1959) [48].

During 14 months, Mazzoldi et al. [48] recorded about 600 seismic events of ML between 1 and 3. The recorded events could be split into two groups: (a) tectonic events and (b) hydraulic-fracturing events. The largest part of the record (about 3/4 of them) were microseismic events (*ML* < 1) belonging to group (a) with epicentres located within an area of 5 km in radius, centred within the geothermal field (around the PC16 injection well). Among microearthquakes, events with higher energies tended to be located within the area on the side of the volcano at very shallow hypocenter depths and close to the extensive structure of the edifice (cf. **Figure 1**). Hypocenter depths increase from the volcano toward the geothermal field where earthquake hypocenters are at and a few kilometres below exploitation depths (2–3 km). A small but significant part of the record (about 5%) consisted of group (b) events, which could not be accounted for by a pure brittle-fracture mechanism but rather seem indicative of hybrid events, generally related to fluidfilled fracture dynamics, typical of volcanic areas and geothermal fields [50]. Hypocenters of these events were tracked down with a grid-searching method based on the maximum energy distribution at the four stations [51] and were mainly concentrated within the geothermal field, on the south-eastern side of Amiata volcano still at exploitation depths.

Previous seismic analyses of recorded events [52, 53] and seismic observation by Mazzoldi et al. [48] show that the SE base of Mt. Amiata has the highest density of tectonic events in correspondence of the geothermal field location, the extensional structures that dissect the volcanic edifice (SE of the edifice) and the compressive structures at the base of the volcano—the last two formed by the volcanic spreading process [4].

Sorgenia [20] recorded microseismicity at the Eastern base of Amiata Volcano for 6-months. They located two distinct thrust-fault events (with a four-month delay between them) of approximate *M* = 2 and hypocenter depth of 8 km. These seismic events show active compression within the Siena-Radicofani Graben, which is consistent with the work of Bonini and Sani [18] and Borgia et al. [4].

Because of active volcanic spreading, after major earthquakes, faults tend to recover in time their critically stressed conditions according to the local Maxwell time (*t*) [54]. Using for the evaporites a Young's modulus *λ* = 10<sup>9</sup> Pa [55] and a viscosity *μ* = 1018 Pa s [4], this recurrence time is found to be:

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

$$\mathbf{t} = \frac{\mu}{\lambda} = \mathbf{10}^{9} \text{ s} = \mathbf{31}.7 \text{ a.} \tag{4}$$

Given the approximations in Eq. (4) and using the Gutenberg-Richter curve given by Mazzoldi et al. [48], this recurrence time corresponds to earthquakes magnitudes between 4 and 5. These values of volcanic-spreading earthquake magnitudes, although on the smaller side, are comparable to the magnitudes given by Mazzoldi et al. [48]. Assuming our approximation for recurrence time is correct, and considering that the latest earthquake with a magnitude of about 4 occurred in 2000, we may suggest that a similar earthquake is expected to occur in the next few decades. Experience suggests that geothermal exploitation could perhaps trigger such an earthquake.
