Hydrogeochemistry and stable isotopes of thermal springs: earthquake-related chemical changes along Belice Fault (Western Sicily)

Three geothermal systems, Montevago, Castellammare-Alcamo and Sciacca, are located along the main seismogenetic structures in Western Sicily. Concentr ations of dissolved species including the gases CO2, N-2 He and the results of stable isotope measurements delta O-18, deltaD and delta C-13(TDIC) in water samples collected from six thermal springs and 28 cold discharges wer e used to characterise their feeder aquifers and to reveal the relationship s between water chemistry and regional seismicity. The Sciacca thermal spri ngs differ chemically and isotopically from those of Montevago and the Cast ellammare-Alcamo areas. The inferred deep end-members of the thermal waters of Montevago and Castellammare-Alcamo are almost similar? suggesting that both systems are fed by carbonate waters and selenite waters. A slight cont ribution (1-3%) of seawater, during groundwater ascent it is also present. The Sciacca thermal springs are fed by a deep reservoir comprising a mixtur e of 50% carbonate water and 50% seawater. During ascent towards the surfac e, these waters interact with NaCl-rich evaporite layers. By combination of published and present data significant temporal variations of temperature and some chemical parameters in the thermal waters of Western Sicily have b een recorded. These variations were mostly between 1966 and 1969. Although the data are discontinuous it is still possible to reveal a direct link bet ween physico-chemical changes in the Acqua Pia and Terme Selinuntine spring s and the 1968 Belice Valley earthquake. Within the studied springs, two ki nds of geochemical behaviour have been recognised. The chemistry of the Mon tevago thermal springs was permanently changed in response to changes in th e groundwater system. Water temperature and dissolved SO4, Cl, Na, and TDS showed minimum values before the earthquake and maximum values after the ev ent. Almost constant values substantially higher than before, were recorded after the seismic event. Conversely, the temporal variations observed in t he waters of the Terme Selinuntine spring, from 1965 to 1991, exhibit a tra nsient increase most probably caused by a temporary contribution of deep CO 2-rich fluids caused by the strain release during the 1968 earthquake. (C) 2000 Elsevier Science Ltd. All rights reserved.