West-East transect of fluids geochemistry across the Umbria-Marche Apennine (Italy): from thermal waters to highly saline fluids

In central Italy, the Apennine fold-and-thrust belt constitutes the boundary between the peri-Tyrrhenian extensional area and the Adriatic compressional-foredeep domain. The different tectonic settings of the two sides of the Apennine promote different geochemical features of the circulating fluids. While the western side  is affected by thermal anomalies, geothermal areas and CO₂ degassing sites, in the easternmost side, the presence of foredeep basins promote the presence of hydrocarbon reservoirs, mud volcanoes, and highly salty mineral waters. An extensive database of chemical-isotopic compositions of groundwater across the central Apennine has been recently compiled in the framework of the EMOTION INGV project which was devoted to the geochemical characterisation of geothermal manifestations in central-northern Italy. About 1000 groundwater analyses were retrieved from the available scientific literature and other data sources. Starting from these data, basing on thermal anomalies and other relevant geochemical-isotopic indicators, ~40 thermal/mineral springs and wells were selected and sampled for a wide-spectrum geochemical analyses including major ions, trace elements, dissolved gases and stable isotopes (H2O, C and He). The thermo-mineral waters of the western sector are generally slightly saline (TDS 1-9 g/L), with temperatures from 15 to 30°C, compositions spanning from Ca(Mg)-HCO3-SO₄ to Na-Cl and affected by the input of deeply derived CO₂. In addition to thermal waters, a slight temperature anomaly of large flow rate springs reveals geothermal heating of the waters corresponding to relevant heat flux. Moving from the “axial” to the easternmost sector, mineral waters show chemical compositions from Ca-SO₄ to Na-Cl, the latter of which reaches very high salinities (TDS up to 183 g/L) and Br^- and I^- relevant contents. In these sectors, mineral waters don’t show significant thermal anomalies and show low to null input of deeply derived CO_2, while showing a relative enrichment in dissolved CH₄. The only exceptions are Triponzo and Acquasanta Terme systems with temperature of ~30 °C and higher CO₂ contents. In this regional trends, local peculiarities are under further investigation. The produced dataset is also valuable for investigating natural resources. For instance, Li occurs at highly variable concentrations (0.01–3 mg/L) but remains of negligible economic significance, with no appreciable differences between the western and eastern sectors. In contrast, other elements of potential interest, such as B, Br, Sr, and Mn, are enriched in the high-salinity waters of the eastern sector, locally attaining potentially useful concentrations.