Fracture permeability and water–rock interaction in a shallow volcanic groundwater reservoir and the concern of its interaction with the deep geothermal reservoir of Mt. Amiata, Italy

• New data are gathered from continuous coring within the Mt Amiata extinct volcano.
• Older volcanic units have been discovered under the known cropping out sequences.
• The volcanic reservoir is characterized by fracture permeability.
• The water table depth is in disagreement with previous hydrogeological models.
• Alteration minerals on fractures indicate low-T fluids and gas inlets in aquifer.

The study of core samples and in-hole data of a 545 m deep well drilled in Mt. Amiata extinct volcano allowed a better characterization of the shallow volcanic reservoir, which is exploited for domestic utilization. The new discovery is that: 1) the water table level is at a depth of 302 m b.g.l. (783 m a.s.l.), in agreement with recent magnetotelluric surveys, and in disagreement with previous hydrogeological models; 2) there is no evidence of present or past interaction with geothermal fluids, the alteration minerals being present only in fractures within the volcanic rocks and indicating fluids of low temperature and relatively low pH due to gas inlets in the volcanic reservoir; and 3) the volcanic reservoir is characterized by fracture permeability, as shown by the fracture system along the well. On the base of these new data the previous geological and hydrogeological models of Mt. Amiata should be revised. In particular, the hypothesis of a catastrophic lowering of the water table in a short time span is unlikely.