Circulation path of thermal waters within the Laga foredeep basin inferred from chemical and isotopic (δ18O, δD, 3H, 87Sr/86Sr) data. (March 2017)

• Record Type: Journal Article
• Authors: Fusari, Alessandro; Carroll, Michael R.; Ferraro, Stefano; Giovannetti, Rita; Giudetti, Geoffrey; Invernizzi, Chiara; Mussi, Mario; Pennisi, Maddalena
• Is Part Of: Applied geochemistry. Volume 78(2017:Mar.)
• Page Start: 23
• Page End: 34
• Publication Date: 2017-03
• Subjects: Thermal waters -- Geochemistry -- Isotopes -- Fluid circulation -- Laga basin; Environmental geochemistry -- Periodicals; Water chemistry -- Periodicals; Geochemistry -- Social aspects -- Periodicals; Geochemistry -- Periodicals; 551.9
• DOI: 10.1016/j.apgeochem.2016.11.021
• Languages: English
• ISSNs: 0883-2927

Abstract

Abstract: The geochemistry of thermal spring waters in the Acquasanta Terme area, located on the Adriatic side of central Italy, has been investigated in order to characterize the geothermal feeding system. The springs discharge more than 100 L/s at temperature ranging between 20 and 30 °C. They occur in a tectonic window of Mesozoic limestones in the central sector of the Acquasanta anticline within the Laga foredeep Basin. Chemical and isotopic compositions of thermal and cold fluids were investigated, most of them monitored for one year, in order to understand the thermal fluid circulation paths. The chemistry of the major elements defines the existence of Na–Cl and Ca–Cl–SO4 2- hot discharging waters and permits the characterization of the thermal end-member hosted in carbonate-dominated reservoir (Burano Anhydrites Fm–Calcare Massiccio Fm). This deep fluid is well represented by the sample T1 and shows high temperature and electrical conductivity (EC), very stable over time, and not affected by mixing phenomena. This is also confirmed by tritium results (0 T.U.). Close to the surface at different depth, such water undergoes mixing or dilution processes with HCO3 − rich freshwaters, driven by the complex structural setting of the area and by diffuse karst caves. This is identified as the main reason for observed compositional variations of sampled springs, and three areas affected by different mixing phenomena have been defined at the surface. The concentrations of SO4 2− and H2 S suggest redox processes affecting sulfur after interaction with evaporitic formations, identified with the Burano Anhydrites, at the base of the Umbria–Marche sedimentary sequence (∼3500-m-deep). Contribution from this reservoir is also supported by characteristic Sr isotope signature. δ 18 O and δD values indicate a meteoric origin of the thermal waters and allow estimation of an average infiltration altitude ranging between 1500 and 1700 m a.s.l. This datum, supported by structural data, suggests the Laga Mountains as the recharge area of the system. Reservoir temperature inferred by SO4 2− –HCO3 − –F, Ca/Mg and SO4 2− /F geothermometers is about 80 °C, consistent with the geothermal gradient of the foredeep basin, and deserving further investigations for potential economic implications about this low-enthalpy geothermal area. Highlights: One deep thermal end-member has been identified to interpret the chemical results of samples. Acquasanta thermal water is hosted in limestone, dolomites and anhydrites aquifers. A geochemical model, water flow path and dilution are proposed. Thermal end-member shows meteoric origin and average infiltration altitude of 1500–1700 m a.s.l. Laga Mountains about 30 km south of Acquasanta are identifying as the recharge area.