Chemical and isotopic compositions of the shallow groundwater system of Vulcano Island, Aeolian Archipelago, Italy: an update

Cold and thermal waters were sampled on Vulcano Island from shallow wells in June 1995, June 1996 and January-February 1997; additional samples refer to a thermal spring, meteoric water from domestic cisterns and local seawater. The chemical and isotopic (hydrogen, oxygen, tritium and strontium in water, and sulfur and oxygen in sulfate ions) compositions measured in the samples showed that: 1) the shallow hydrothermal system is formed by the mixing of waters of meteoric origin and various ages with crater-type fumarolic steam and gases (phreato-volcanic waters), with water rising from the geothermal reservoir expected to exist at depth (sulfate-chloride waters), and with steam and gases released from the geothermal reservoir itself (steam-heated waters); 2) a major seawater contribution occurs only in water from a well exploited in the Togo-Togo Camping, that is located very close to the shoreline; 3) dissolved sulfate mostly derives from oxidation of fumarolic/magmatic sulfur species, with minor sporadic contributions from seawater or from dissolution of secondary anhydrite; and 4) major changes in the chemical and isotopic compositions of the bulk groundwater system were not observed during the study period, and the appreciable variations in waters from few wells may be interpreted as due to changes in the mixing proportions of the endmembers or to direct rainwater dilution. Based on the present study and previous ones from literature, an updated geochemical model is proposed for the groundwaters system at Vulcano and its relations with the crater and beach fumarolic fluids.