GNGTS 2013 - Atti del 32° Convegno Nazionale

of these structures, remarkable hydrothermal alteration phenomena that generated kaolin and alum deposits, which are known since ancient times, are present. According to the geophysical study of Bruno et al. (2000) the investigated thermal spring of San Calogero is located close to a direct fault that juxtaposed two distinct lithologies characterized by different permeability, i.e. 1) an andesite lava flow and 2) a pyroclastic deposit. According to historical chronicles (Cavallaro, 1954) the temperature recorded at the spring decreased, as it was 93 °C in the year 1872, ca. 60 °C in the 1950. The temperature decrease has been coupled with a decrease in the discharge that is currently varying between 40 and 90 liters per hour. Geochemical characterization of the thermal water of San Calogero. The waters have been sampled during a period of 2 years (August 2012 - April 2013) to evaluate the compo- sitional variations. In situ measurements of samples (Sg) collected in the proximal point of emission of the spring showed that temperature and pH range between 40-50°C and 7.0-7.6, respectively, whereas the electric conductivity varies between 2.4 and 2.9 milliSiemens/cm. Slightly higher pH (7.6-7.9) and lower temperature are observed in samples (Th) collected in the old thermal building (named tholos) located 20 m far from the emission point, in relation to CO 2 escape and CaCO 3 precipitation that is observed as diffuse carbonate concretions. The water samples were subsequently investigated in the laboratories of the Department of Physics and Earth Science of the University of Ferrara using an ion-chromatograph DIONEX ICS-1000 for the analysis of anions and an inductively coupled plasma mass spectrometer (ICP-MS; X Series, Thermo-Scientific) for the analysis of major cations and trace elements. Certificated standards were periodically used to calibrate measurements and to verify analyti- cal precision and accuracy. Results, reported in Tab. 1 and Fig. 1, indicate that the TDS varies between 2050 and 2200 mg/l, with a hydrochemical facies characterized by sulphate as pre- dominant anion, and sodium and magnesium dominating among cations. Tab. 1 – Chemical composition (expressed in mg/l) of the San Calogero thermal water sampled during the period August 2012 – April 2013. The label Sg refers to the samples taken in the proximal point of emission of the spring, while the label Th refers to samples taken in the old thermal building named tholos. Note that arsenic concentration often exceeds the WHO standard for drinkable water. sample Sg Th Cl 250-282 263-283 Br 0.5-0.7 0.4-0.5 NO 3 18.1-23.6 15.7-29.3 SO 4 895-1110 932-1018 HCO 3 450-600 435-510 Ca 87.2-142 54.5-87.6 Mg 83.6-96.3 68.7-95.0 Na 240-283 240-272 K 23.4-27.2 21.8-29.3 Li 0.06-0.10 0.06-0.09 Rb 0.02-0.04 0.02-0.04 Sr 2.36-3.72 1.3-2.3 B 0.05-0.20 0.03-0.11 As 0.01-0.02 0.01 Noteworthy, the studied waters display a constant SO 4 /Cl (3.4-3.6) ratio very similar to that recorded in the historical hydrochemical investigations carried out in the fifties (Cavallaro, 1954). This is probably an indication of a constant long term supply of juvenile fluids. The Cl - /Br - molar ratio with values near 1200 suggests a non marine origin of the groundwater, 274 GNGTS 2013 S essione 1.3