Hydrogeological and Geochemical Characteristics of the Coastal Aquifer of Stromboli Volcanic Island (Italy)

Abstract

Although groundwater is a strategic source in volcanic islands, most hydrogeochemical research on this topic has been focused on volcanic activity monitoring, overlooking general hydrogeological aspects. The same applies to one of the most studied volcanoes in the world, Stromboli Island (Italy). Here, we provide a hydrogeological scheme of its coastal aquifer, retrieving inferences about its potential use as a water supply source and for optimizing monitoring protocols for volcanic surveillance. Starting from the hydrogeochemical literature background, we analyzed new data, acquired both for volcano monitoring purposes and during specific surveys. Among these, there were saturated hydraulic conductivity measurements of selected rock samples and precise determinations of water table elevations based on GNSS surveys of wells. We identified a ubiquitous thin lens of brackish water floating on seawater and composed of a variable mixing of marine and meteoric components; inlets of hydrothermal fluids to the aquifer are basically gases, mainly CO2. Based on its hydrogeochemical character, the coastal aquifer of Stromboli could be used as a water supply source after desalinization by reverse osmosis, while the wells located far from the seashore are the most interesting for volcano monitoring, because they are less disturbed by the shallow geochemical noise.

Decadal Monitoring of the Hydrothermal System of Stromboli Volcano, Italy

“…The data set presented here suggests that some portions of the aquifer are flushed by a He-CH 4 -rich gas phase, derived from a volcanic gas modified during interaction with a multilevel hydrothermal aquifer, where the gases could separate and dissolve from one level to another (Capasso et al, 2005;Federico et al, 2008). The multilevel aquifer is probably hosted by the alternating levels of lavas and pyroclastics in the subsoil of the Scari area, as suggested by Madonia et al (2021). During gas-water interaction, different gas species fractionate according to their solubility in water, described in Appendix A.…”

“…Previous hydrogeological studies (Madonia et al, 2021;Revil et al, 2011) described the characteristics of the aquifer in this sector of Stromboli. Based on electrical resistivity measurements, soil CO 2 concentrations, temperature, and self-potential measurements, Revil et al (2011) provided insights regarding the position of shallow aquifers and the extension of the hydrothermal system.…”

“…The modeled initial CH 4 /CO 2 contents span from 5 × 10 −5 to 2 × 10 −4 and are higher than those measured in the SC5 fumarole (Capasso et al, 2005;Carapezza & Federico, 2000), suggesting a variable CH 4 production in different portions of the hydrothermal aquifer or different periods, besides an additional enrichment after vapor separation. Madonia et al (2021) suggested the existence of a stratified (multilevel) hydrothermal aquifer, with temperatures that could be progressively higher, in the deeper and inner parts of the island. The hypothesized fractionation upon dissolution in the shallow aquifer would produce a progressive enrichment in He and CH 4 and depletion of CO 2 , toward a gas composition, which in the studied aquifer is characterized by He contents as high as 2.5 × 10 −3 cc/L at STP and CO 2 contents as low as 10 cc/L at STP.…”

“…was assumed to be as low as −3 km in depth. More recently, Madonia et al (2021) indicated that there is a unique groundwater body in the form of a thin lens of fresh water floating above the infiltrating seawater and suggested the existence of a multi-level aquifer, consisting of alternating lavas and pyroclastic deposits.…”

“…Many geochemical studies on the thermal aquifer, which is accessible via privately drilled wells, have been carried out in the last two decades (Capasso et al., 2005; Carapezza & Federico, 2000; Carapezza et al., 2004; Federico et al., 2008; Grassa et al., 2008; S. Inguaggiato & Rizzo, 2004; Liotta et al., 2006; Madonia et al., 2021; Rizzo et al., 2008, 2009, 2015). These studies revealed (a) the main composition of thermal waters and gases dissolved therein, (b) the influence of shallow marine and meteoric waters (MWs) on the hydrothermal composition, (c) the presence and extent of gas‐water‐rock interaction processes, and (d) a magmatic signature concerning helium isotopes.…”