A peculiar case of coastal springs and geogenic saline groundwater_ the Santa Cesarea Terme thermal springs (Southern Italy)

Polemio, M. Limoni, P.P. Liotta, D. Palladino, G. Santaloia, F. Zuffianò, L.E., 2014, A peculiar case of coastal springs and geogenic saline groundwater_ the Santa Cesarea Terme thermal springs (Southern Italy), SWIM 2014, 23rd Salt Water Intrusion Meeting, Husum (Germany), 16-20/06/2014,
URL: http://www.cnr.it/prodotto/i/350291

Carbonate aquifers, located in foreland tectonic settings, could represent important thermal water resources outside the volcanic areas, supplying spas or geothermal installations. Thermal springs constitute the discharge areas of deep marine and continental groundwaters flowing within these carbonate aquifers whose hydraulic conductivity and the relevant geothermal fluid migration are strictly controlled by both the discontinuity network and the karst processes involving the foreland environment. An example of these springs occurs along the south-easternmost portion of the Apulia region (Southern Italy) where some sulphurous and warm waters (22-33 °C) flow out in partially submerged caves located along the coast, thus supplying the spas of Santa Cesarea Terme. These springs are known from ancient times (Aristotele in III century BC) and the physical-chemical features of their thermal waters resulted to be partly influenced by the sea level variations. Some hypotheses about the origin of these warm waters were proposed up to now by previous researches but some uncertainties still exist. For this reason, the area has been selected in order to define the conceptual model of the geothermal resources related to the thermal springs and, as a consequence, the origin of the thermal springs. It is one of the pilot site of the Vigor Project (Evaluation of the geothermal potential of Regions of Convergence), promoted by the Italian Ministry of Economic Development and National Research Council. Santa Cesarea Terme zone is located within the Apulia carbonate platform, the foreland of the southern Apennines, which consists of Jurassic-Cretaceous limestones, thick more than 5 km in the study area and affected by intense karst processes, resting above the Late Triassic evaporite (Burano Fm) and, unconformably, overlaid by Cenozoic calcareous successions. Belonging to a coastal area, the studied groundwater, whose top is located almost to the sea level, is involved in saltwater intrusion and therefore the salt-fresh water interface occurs at some meters below the sea level moving inland. Geological and hydrogeological surveys, including geo-electrical prospecting, and chemical and isotopic analyses of both groundwater and seawater have been carried out. Stable isotopes (?18O, ?D) were used to define the origin of the thermal waters and the recharge mechanism of the geothermal systems while the unstable isotope (3H) was determined for estimating the age of the thermal waters and to define the conceptual model of this low temperature geothermal resource. All the data have been analysed to improve the knowledge of the groundwater flow system, thus assessing the possibility of using low-temperature geothermal fluids to fulfil the thermal needs of the town of Santa Cesarea Terme. In this narrow area, the source of geogenic salinization of spring groundwater was referred to ascending very deep groundwater, more saline than current sea water.The geochemical composition and the physical features of the sampled waters suggest that thermal waters should be moving from ancient seawaters subjected to intense evaporation processes, infiltrated at great depth within the seabed substratum. Afterwards, these thermal fluids should flow up through the almost vertical structures, related to the transtensional structures, identified within a narrow sector of the studied territory.

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