Geothermal potential of Marche region: Acquasanta thermal area

The Adriatic area has relatively normal continental lithospheric thickness (about 70-90 km), thick and deformed foredeep sequences and low heat flow, also caused by a fast recent sedimentary deposition [1]. In this framework, many thermal areas are present, and one of these is Acquasanta Terme, located in the southern part of Marche region (Central Italy). This area is characterized by a major asymmetric anticlinal structure, trending about N 170°, about 25 km-long, and plunging both to the north and to the south. Along this structure, the upper part of the Umbria – Marche calcareous sequence crops out within the Laga Basin deposits, just in front of the Sibillini Thrust [2]. The general aim of this study is to investigate the hydrogeological pattern and the hot waters ascent path, and to estimate the maximum temperature, the geothermal features and the possible sustainable exploitation of the area. Preliminary studies to achieve this aim include structural analyses, water chemical analyses of major and secondary elements, and quantification of water stable isotopes of oxygen and deuterium. First structural data allowed to distinguish the major fracture sets related to the complex structural history of the area. This pattern allowed a preliminary outline of the possible preferential directions of groundwater flow. Isotopic data obtained so far seem to show a common origin for surface and deep waters, probably with the same infiltration area, but with two different paths, a shallow and a deep one. Chemical data of cold and thermal waters outline variations in temperatures, pH, conductivity and concentrations, revealing a possible mixing between the two circuits. On the basis of correlation straight lines, we can estimate the altitude of water infiltration at approximately 1500 m s. l. m.. This is compatible with well defined recharge areas. Additional chemical and isotopic data will provide certainties about the degree of mixing and water paths at depth.