Hydrogeochemical characterization of the waters circulating in the seismically active area of the Pesaro-Urbino province (northern Marche, central Italy)

The Province of Pesaro-Urbino (northern Marche, central Italy) represents one of most seismically active areas in Italy, since it is interested by the presence of two major composite seismogenic sources: i) the first one is located in the Umbria-Marche Apennines; ii) the second one is along the Adriatic coast from Cattolica to Ancona cities. This area has recently experienced an intense seismic activity, e.g., the 1781 “Cagli Earthquake” with a magnitude of 6.4 Mw, and the 1930 “Senigallia Earthquake” of 5.8 Mw. The last earthquake (5.5 Mw) occurred on November 9, 2022, with its epicenter located in the Adriatic Sea, 35 km away from the city of Pesaro. Since the geochemical knowledge of this area is limited, a large-scale sampling survey was carried out during spring and autumn 2022. A total of 87 samples were collected from different types of emergencies (i.e., cold springs, wells, mineral springs, sulfur springs and ditches) and various geological and tectonic-structural contexts. The study area is dominated by a complex sedimentary structure (e.g., limestones, clays and alluvial deposits) and by climatic and topographic conditions that may influence the chemical and isotopic composition of the investigated fluids. A detailed geochemical characterization is thus of paramount importance in order to define a geochemical background. The aim of this study was to (1) understand the possible interaction of deep-originated fluids and shallow aquifers and (2) evaluate the use of selected geochemical parameters as possible seismic tracers. The results showed the presence of five different geochemical facies: (i) calcium-bicarbonate waters with low TDS (<500 mg/L); (ii) calcium-bicarbonate waters with a strong enrichment in sulfate (up to 200 mg/L); (iii) waters with extreme sodium-carbonate composition and an alkaline pH (>8.8); (iv) calcium-sulfate waters; and (v) sodium-chloride waters. The water isotopic composition showed a clear meteoric origin for all the investigated samples. The composition of major dissolved gases showed two different compositional clusters: (a) N2-dominated gases with N2/Ar ratios similar to those of air and ASW (Air Saturated Water); (b) CO2- and CH4-rich gases pertaining to mineral and sulfur springs. The origin of Ca-HCO3 waters is almost exclusively related to the dissolution of carbonate minerals. On the contrary, Ca-HCO3(SO4) waters are probably originated by deep circulation pathways and interactions with the Upper Triassic Burano Formation, composed by anhydrite layers. The Ca-SO4 waters should be considered as the product of ongoing flows within Miocene gypsum formations, whilst Na-HCO3 waters as the consequence of long-lasting interactions between meteoric waters and silicate rocks (containing albite) in saturation/oversaturation conditions for carbonate-bearing minerals. Finally, the Na-Cl waters probably derive from mixing processes between meteoric and highly saline connate waters trapped into the foredeep clayey deposits. Therefore, the Ca-HCO3(SO4) and Ca-SO4 waters can be regarded as the most interesting fluids to be monitored for a geochemical network aimed at recognizing chemical and isotopic variations related to seismic activity. They are indeed showing a deeper hydrogeological pathway and appear to be less influenced by surface processes.