Development of a water monitoring network in the Pesaro-Urbino province (northern Marche, central Italy) aimed at seismic precursors: a three-step hydrogeochemical approach to assess sampling sites’ suitability and sensitivity

In this Ph.D. thesis, a novel hydrogeochemical approach aimed at the identification of the most suitable and sensitive sampling sites to develop a permanent water monitoring network aimed at seismic tracers and surveillance in the seismically active Pesaro-Urbino (PU) province in Central Italy is presented. The proposed approach is divided in three step: (i) characterization, (ii) assessment and (iii) monitoring. The characterization phase involved a large-scale sampling campaign (87 samples), performed during Spring and Autumn 2022, covering as much as possible the entire study area with the aim of understanding geochemical composition and processes characterizing the groundwaters and evaluate the possible interplay between shallow aquifers and deep-seated fluids. Consequently, those sampling sites being characterized by peculiar geochemical composition (e.g.., Ca-SO4, Ca-HCO3(SO4) and NaHCO3) and a location close to major or local seismogenetic sources (e.g., faults, tectonic lineaments) were included into the assessment phase. During the latter, a detailed multi-isotopic approach (C, S, O, H, B and Sr) was carried out on a restricted number of sampling sites (21 sample) in order to better understand the water-rock interaction processes and to track the water hydrological pathways below the surface, two extremely important aspects that needs to be taken into account when dealing with seismic tracers. The combined application of the different isotopic systematics allowed to identify a restricted number of waters which are clearly interacting with the Triassic Burano evaporitic formation (TBf), hence indicating circulation patterns able to reach a depth of at least 600 m. Combing the results obtained in the first two steps, a restricted number of sampling sites were included in the final monitoring phase. The selected fifteen sampling sites were monitored periodically (on a monthly or quarterly basis) for at least one hydrological year, in order to highlight their variability and resilience with respect to seasonality, precipitation and external forcing (e.g., human activities). Those sampling sites showing the minimal variability and high resilience all over the monitoring period have to be considered the most suitable (and sensitive) to be included in a permanent water monitoring network since they are more likely to record any possible (hydro)geochemical change that may be related to an impending seismic event and to the seismic cycle. The proposed approach allowed us to identify 7 sampling sites that may be suitable for the research purpose. Furthermore, the results from two water monitoring network deployed in the Pesaro-Fano and Mt. Conero areas, respectively, following the November 9, 2022 Marche offshore seismic sequence to evaluate the influence of the event on water geochemistry are also reported, revealing in the case of Mt. Conero area a highly sensitive site that recorded strong variations before the seismic event.