GNGTS 2013 - Atti del 32° Convegno Nazionale

Carbon isotopes point out a biogenic origin of the dissolved carbon for the bulk of the groundwaters (Cerling et al. , 1991), while a trend toward the equilibriumwith deep carbonates was showed by thermal waters of Caronte. This process is possible only hypothesizing a slow deep circulation in the geothermal system, characterized by a prolonged WRI. Equilibrium with carbonates is favored by both the presence of well developed fracture network (Tansi et al. , 2007) in the Mesozoic complex and by the relatively high thermal condition existing at depth. Geothermometric considerations on the collected waters were carried out with extreme care, due to the geological, lithological and mineralogical features of the Lamezia Terme area. The use of the classical Na/K and K/Mg ionic solute ratios (Giggenbach and Corrales, 1992) on collected waters evidenced their immaturity, emphasizing their unsuitability for any estimate of deep temperatures. The evaluation of deep temperatures was carried out by applying selected geothermometer equation. Due to the geological, lithological and mineralogical features of the Lamezia Terme area, we selected and applied the SO 4 /F 2 geothermometer (Marini et al. , 1986), mostly used in carbonate-evaporite environment in Italy that indicated a realistic deep temperature estimate of about 50-60°C, confirming the low enthalpy of the Caronte hydrologic system. Contursi Terme area. Basing on their outlet temperature, waters were classified as i) thermal (Contursi Bagni, T > 35°C), ii) hypothermal (20 < T < 35°C) and iii) cold (T < 20°C), following the classification of Marotta and Sica (1933). The relative abundances of major elements allowed us to identify three geochemical families. Waters have the following chemical composition: i) Ca-HCO 3 , characterizing the cold and hypothermal water and one thermal water (sample 1). Their chemical composition is typical of the interaction with the carbonate lithologies (that characterize the studied area) and surface sediments (that characterize local geology). These processes differ both in timing and intensity, justifying the different position of the samples in the Langelier-Ludwig diagram. Generally, the cold water seem to be characterized by a fast and shallow circulation and seem to have a limited interaction time with the lithologies. Hypothermal waters have lower ratios of HCO 3 /Cl, HCO 3 /SO 4 and (Ca+Mg)/(Na+K) than cold waters, feature most noticeable in thermal sample (sample 1). Probably, this water is characterized by a slower and deep circuit, with a more intense leaching of the Triassic evaporite rocks; ii) Na (Ca-Mg) – HCO 3 (Cl) , represented by thermal spring (sample 10). This sample, together with sample 11, represents the end- member of the deep circulation of the study area. These waters circulate in deeper geological units (dolomites, limestones and evaporites) and acquiring their thermal character and salt content. These waters are more aggressive against the rocks because of the presence of dissolved carbon of deep origin and, consequently, the water-rock interaction processes are more intensive; iii) Na (Ca) – Cl (HCO 3 ) , represented by thermal spring (sample 11, for discussion on its chemical composition, please consult the ii )). The investigate area is characterized by an intense and diffuse degassing of CO 2 in the groundwater, especially in the hypothermal and thermal waters. The measured values are extremely high (up to 650 cc/l) comparable with other geo-tectonic contexts in Italy (both in volcanic areas that not volcanic) and in adjacent areas (Telese, Mefite d’Ansante, Irpinia). There is a positive correlation between electrical conductance and CO 2 ; it means that the presence of CO 2 in solution (i.e., an acid environment) favors the leaching of rocks/sediments, increasing the salinity of water. O and H isotopes (Fig. 3) point out a common meteoric origin of the sampled waters. All waters fall between the Global Meteoric Water Line (Craig, 1961) and the Regional Meteoric Water Line (RMWL, defined by Longinelli and Selmo, 2003). Environmental isotopes minimizing the existence of high temperature (> 150°C) at depth due to the lack of any oxygen- shift; however, there are clear isotopic exchange between CO 2 and groundwater ( negative oxygen shift ), especially in the gaseous bubbling springs. In order to individuate the elevation 260 GNGTS 2013 S essione 1.3