Groundwater and Dissolved Gases Geochemistry in the Pesaro-Urbino Province (Northern Marche, Central Italy) as a Tool for Seismic Surveillance and Sustainability

Abstract

The Pesaro-Urbino province (PUP) (northern Marche, central Italy) is one of the most seismically active areas in Italy, with the most recent earthquakes (5.2 and 5.5 Mw) having occurred on 9 November 2022 with an epicenter located in the Adriatic Sea. A detailed geochemical and isotopic characterization of 87 groundwaters (and dissolved gases) circulating in the PUP was carried out to (i) unravel the geochemical processes controlling the water circulation, (ii) investigate the interplay between deep originated fluids and shallow aquifers, (iii) evaluate the reliability of specific geochemical parameters as tracers for seismic activity, and (iv) select the most suitable sampling sites to deploy a monitoring network to highlight possible compositional changes related to the regional and local tectonic activity. The geochemical dataset includes waters showing five different hydrochemical compositional facies: (i) calcium bicarbonate with low Total Dissolved Solids (TDS); (ii) calcium bicarbonate with relatively high concentrations of sulfate (>200 mg/L); (iii) sodium bicarbonate with pH > 8.8; (iv) calcium sulfate; (v) sodium chlorine. Two distinct groups of dissolved gases can be recognized: (a) N2-dominated gases with N2/Ar ratios similar to those of Air-Saturated Water (ASW); (b) CO2- and CH4-rich gases associated with high TDS and springs rich in S-bearing reduced species. The isotopic values of δ13C-CO2 and δ13C-CH4 suggest a predominant biogenic origin for both species with a negligible contribution from deep-seated fluids. The Ca-HCO3(SO4), Ca(Na)-SO4(Cl), and Na-HCO3 waters, being likely related to deep hydrological pathways, are the best candidates to be included in the monitoring network in the Pesaro-Urbino province. This will be of paramount importance in addressing the challenge of unravelling fluid geochemical precursors of earthquakes, thus increasing and improving seismic surveillance practices and hazard mitigation.