Response of the shallow aquifer of the volcano-hydrothermal system during the recent crises at Vulcano Island (Aeolian Archipelago, Italy) Giorgio Capasso, Cinzia Federico ⁎, Paolo Madonia, Antonio Paonita Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy abstract article info Article history: Received 28 June 2013 Accepted 8 January 2014 Available online 30 January 2014 Keywords: Hydrothermal system Vulcano Island Fluid pressure Thermal wells The shallow thermal aquifer at Vulcano Island is strongly affected by deep volcanic fluids. The most significant variations were observed during the 1989–1996 crisis due to a large input of steam and acidic gases from depth. Besides chemical variations related to the input of deep fluids, the record of the water-table elevation at monitored wells has provided remarkable insights into the pressure conditions of the volcano-hydrothermal system. After the pressure drop due to the extensive vaporization of the hydrothermal aquifer, occurred after 1993, the volcano-hydrothermal system has been re-pressurized since 2001, probably because of the contribu- tion of volatiles from the hydrothermal-magmatic source. The increase in fluid pressure may have caused reopening of fractures (which had self-seated during the previous period of cooling) and the onset of a phase of higher vapor output in the fumarole field later in 2004. The fracture opening would have promoted further vapor separation from the deep fluid reservoir (hypothesized at 0.5–1.5 km depth) and finally the drainage of S-rich fluids into the shallow thermal aquifer (found out at few tens of meters of depth). The monitoring of both the water chemistry and the water-table elevation provides insights into the eventual pressurization of the volcano-hydrothermal system that precedes the fracture opening and the extensive drainage of deep fluids. The findings of this study could represent crucial information about the stability of the volcano edifice, and lead to reliable techniques for determining the risk of or even predicting phreatic explosions. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Fluids circulating in volcanic edifices have attracted increasing interest from scientists, mostly because their role in triggering flank instability, phreatic explosions and eruptions has been documented in dozens of cases worldwide (Newhall et al., 2001). The pressurization of fluids in porous rocks, favored by conditions of low permeability as frequently encountered in hydrothermally altered volcanic rocks (from 10 -14 to 10 -17 m 2 ; Reid, 2004), reduces the shear stress of rocks and soils, thus leading to instability phenomena. This effect is produced either by pres- surization of a shallow meteoric aquifer (e.g., due to increased rainwater recharge coupled to sluggish water flow) or, more often, by internal pres- surization of magmatic volatiles and hydrothermal systems (Day, 1996; Reid, 2004; Thomas et al., 2004). In the latter cases the pressurization and flashing of hydrothermal systems can lead to dramatic steam blasts and, eventually, to an eruption (Hill et al., 2002). As a representative case, the periodic bradyseisms at Phlegrean Fields (Italy) have been interpreted as fluid pressure variations inside the geothermal system (Caliro et al., 2007 and references therein; Todesco et al., 2010). The fluid pore pressure can also be changed by nonvolcanic causes such as variation of the stress field and, more generally, variation of the porosity or permeability of volcanic rocks. Static or dynamic stress changes, as well as loading or unloading of volcanic edifices, exert a significant control on fluid patterns and eventually on the upward movement of melts (Newhall et al., 2001; Hill et al., 2002; Mortimer et al., 2011). This work aimed at elucidating the feedback mechanism involving fluid pressuriza- tion, enhanced fluid output from fumaroles, and consequent pressure drop at Vulcano Island (Aeolian Archipelago) occurred from 2001 onward, through the monitoring of the shallow volcanic aquifer. Vulcano Island lies on a NNW-to-SSE-trending fault that forms part of a transpressive belt (Argnani et al., 2007) where the dominant mech- anism is right-lateral shear (Mattia et al., 2008). The volcano has fre- quently displayed explosive activity (spanning from phreatomagmatic to magmatic), emitting volcanics with compositions ranging from calc-alkaline to shoshonitic (Barberi et al., 1974, 1988). The magmatic chamber has been identified at a depth of 2–3 km b.s.l. (Clocchiatti et al., 1994; Nuccio and Paonita, 2001), while a hydrothermal system is hypothesized as being present at a depth of 0.5–1.5 km (Carapezza et al., 1981; Chiodini et al., 1992; Nuccio et al., 1999; Alparone et al., 2010). Since the last eruption occurred during 1888–1990, the active volcanic center of La Fossa cone displays fumarole activity, character- ized by periodic phases of increased output flux and temperatures of emitted fluids. Fumarolic fluids emitted during periods of increased fumarolic activity have chemical and isotope compositions indicating that the contribution of magmatic volatiles prevails over that of the Journal of Volcanology and Geothermal Research 273 (2014) 70–80 ⁎ Corresponding author. Tel.: +39 0916809493; fax: +39 0916809449. E-mail address: cinzia.federico@ingv.it (C. Federico). 0377-0273/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2014.01.005 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores