Aspetti idrogeologici delle vulcaniti nel Monte Amiata

The Monte Amiata volcanics host one of the most important groundwater body of Tuscany. The aquifer is mainly unconfined (Doveri et al., 2012) and its impervious substratum is principally made up of shale belonging to the Ligurian Units. Groundwater is drained by several springs (more than 150 according to Barazzuoli et al., 1994) that are distributed all around the volcanic edifice, generally close to the contact between volcanic rocks and substratum. Two major groups of springs are located in the southern (close to Santa Fiora) and northern (close to Vivo d'Orcia) parts of the volcanic complex. The first group is represented by Galleria Nuova, Galleria Bassa, Carolina and Peschiera springs, and it has an average flow rate higher than 700 L/s (Dini et al., 2010; Doveri et al., 2012). The second group is essentially represented by the spring named Ermicciolo, which has an average flow rate of about 100 L/s. Most springs are taped for supplying drinking water over a wide and densely populated area that encompasses the Siena and Grosseto districts and part of the Arezzo and Viterbo districts. The shaly substratum plays also the role of cap-rock respect to a regional evaporitic-carbonate reservoir, which at places hosts thermal waters. Downward, the hydrogeological succession ends with a Palaeozoic basement, which is mainly impermeable, excluding fractured zones that are exploited by deep geothermal wells (deeper geothermal fields, at Bágnore and Piancastagnaio). In this chapter, a discussion on hydrogeological aspects of the Monte Amiata volcanics is performed, mainly basing on published data. In the past, the hydrogeological studies of this area essentially consisted of water budgets. In relation to a hydrologic period of more than fifty years, the meteoric recharge and the total output at springs resulted very similar and of the order of 50-55E06 m3/yr (Celico, 1987; Barazzuoli et al., 1994, 2014). The elaboration of the piezometric surface for the entire aquifer extension was performed by Calamai et al. (1970) and Manzella (2006) and was principally based on geophysical data. The lack of direct measurements for the geophysical data calibration limits the representativeness of these piezometric surfaces and this is likely the main cause of the remarkable differences between the results achieved by these two studies. The recent construction of six piezometers in the inner part of the volcanic body enabled to carry on tests and continuous monitoring of the water level, with consequent production of hydraulic and hydrodynamic data. By performing hydraulic tests Doveri et al. (2012; 2013a; 2013b) achieved value of K in the range 5.0E-06 ÷ 4.6E-05 m/s for the volcanics of the Bágnore System (as defined in the Chapter 4). Moreover, by comparing hydrographs of springs and piezometers, and by performing piezometric measurements at selected points along a principal hydrogeological section (Galleria Nuova spring - Monte Amiata ridge - Ermicciolo spring), Doveri et al. (2012) elaborated the first piezometric profile steered by experimental data and elaborations of the hydrogeological type. Again, by elaborating data from piezometers recently constructed, in this chapter we point out, for the first time, the existence of very different hydrodynamic conditions between the volcanics of the Bágnore System and those of the Monte Amiata System (as defined in the Chapter 4). Furthermore, we confirmed the velocity of about 50 m/d, already calculated by Doveri et al. (2012), regarding the downstream propagation of hydraulic head variations (or flow rate variations) in the aquifer. These results underline the importance of the monitoring activities that have been enhanced during last years, and that should be continued, and possibly further reinfor