Interazione acqua-roccia nell'acquifero superficiale del Monte Amiata

The volcanic rocks of Monte Amiata host a major shallow regional aquifer exploited for the domestic water supply of a wide area of Southern Tuscany. The volcanic aquifer lies over a thick sequence of flysch units (approx. 300 - 600 m), consisting of shale and limestone to the south, and shale and sandstone to the north of the volcanic edifice. An exploration well was drilled in 2010 by the Tuscan regional authority to monitor the water table level and improve the hydrogeological model of the Monte Amiata volcanic aquifer. The well is located close to Poggio Truzzolo at 1086 m a.s.l. It crossed the whole volcanic sequence, found the water table of the volcanic aquifer at 783 m a.s.l. and reached the flysch substratum at 556 m a.s.l. In this chapter, we describe evidences of water-rock interaction in the volcanic rocks, both above and below the present-day water table, in order to study its variations with time. Therefore, we studied the mineralogy and petrology of the rock samples, the location, type and mineralization of the in-hole fractures. The results are interpreted considering the existing geological, geochemical and hydrogeological data. The main outcomes of this well are: 1) the location of the water table level at a depth of 302 m b.g.l. (783 m a.s.l.), in agreement with the results of recent magneto-telluric surveys, and in contrast with previous hydrogeological models; 2) the absence of present or past interaction between volcanic rocks and hydrothermal fluids, the alteration minerals being present only in fractures and indicating fluids of low temperature and low pH due to gas inflow in the volcanic reservoir; 3) the volcanic reservoir is characterized by fracture permeability, as shown by the fracture system found along the well.