1 ANNALS OF GEOPHYSICS, 64, 4, HS438, 2021, doi:10-4401/ag-8536 OPEN ACCESS A hydrogeochemical approach to the characterization of low-enthalpy geothermal systems: the Scordia – Lentini graben (Sicily, Italy) Gloria Maria Ristuccia*, Pietro Bonfanti, Salvatore Giammanco (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania - Osservatorio Etneo, Italy Article history: received July 30, 2020; accepted May 29; 2021 Abstract We describe the geochemical characteristics of groundwater samples collected in 23 water wells located on the northern margin of the Hyblean plateau (East Sicily). This area, mostly made of highly permeable carbonate rocks, is rich in low temperature (T < 50° C) hydrothermal groundwaters, distributed in an active sismogenetic zone, with several ENE-WSW-directed tectonic structures that drove magma to the surface during Upper Pliocene and Pleistocene. The chemical features suggest complex mixing between rainwater, CO 2 -rich groundwater, steam-heated groundwater and geothermal brines, as highlighted by Principal Component Analysis (PCA). Some parameters, however, indicate widespread pollution of the aquifers from human activities. Stable isotopes analysis confirms the meteoric origin of groundwater and supports the origin of dissolved CO 2 mostly from mantle degassing through deep tectonic faults. Geothermometric estimates, mostly based on quartz and Saturation Indexes geothermometers, suggest minimum reservoir temperature between 100 and 120° C. Keywords: Hydrothermal systems; Hyblean plateau; Water geochemistry; Stable isotopes; Principal Component Analysis; Geothermometry. 1. Introduction In Italy, most of the energy used for industrial and domestic purposes is produced through the use of fossil fuels, which are mostly imported. Italy’s energy dependence from foreign producers, combined with the commitments undertaken by the Italian government at international level about the reduction of green-house gas emissions into the atmosphere, makes it essential to promote scientific studies finalized to finding and exploiting alternative and renewable energy sources. In this context, geothermal energy plays a fundamental role, due to its broad availability on the Italian territory and its favorable cost-benefit balance [Lo Russo et al., 2009]. Sicily is probably one of the Italian regions with the highest geothermal potential, due to the many areas of active or recent volcanism and tectonism as well as of anomalous heat flow [Barberi et al., 1974; Bianchi et al., 1987; ENEL et al., 1988; Cataldi et al., 1995; Dall’Aglio et al., 1995; Schmincke et al., 1997; Della Vedova et al.,