Water-rock interaction processes and hydrogeological pathways in seismically active areas as revealed by a multi-isotopic (C, S, O, H, B, Sr) approach

Waters circulating in the seismically active (up to 6.4 Mw) Pesaro-Urbino province (central Italy) were investigated to understand water–rock interaction processes and groundwater circulation patterns through a multi-isotopic approach. The investigated waters showed different geochemical facies: Ca-HCO3, Ca-SO4, Ca-HCO3-SO4, and Na-HCO3. Water geochemistry and isotopic composition suggest that Ca-HCO3 waters are related to the dissolution of carbonate-rich rocks and, subordinately, Al-silicate minerals and are generally characterized by shallow or fast hydrogeological circuits. In contrast, Ca-SO4, Ca-HCO3-SO4, and Na-HCO3 waters relate to longer water–rock interaction and/or deeper circulation patterns within the aquifers. All the waters show biogenically derived δ13C-TIDC values and δ2H- and δ18O-H2O signatures indicative of meteoric origin. The combination of δ34S-SO4 and 87Sr/86Sr values suggest that the Ca-HCO3-SO4 and Ca-SO4 waters interact with the evaporitic anhydrite-rich rocks of the Triassic Burano formation that constitute the regional basal aquiclude, making these waters interesting to be monitored for seismic tracers, being likely able to carry possible deep seismic signals (e.g., deep-sourced gases inflow, enhanced metals mobility), similarly to what found in comparable geological contexts. Contrarily, Na-HCO3 waters show 87Sr/86Sr ratios and δ11B values approaching those of the siliciclastic Marnoso Arenacea Fm, consistent with long-lasting interactions with Na-bearing silicates. Our results suggest that the hydrogeochemical and multi-isotopic approach provided paramount information to detect sites more prone to record possible geochemical variations during the build-up phase of seismic events, making it suitable for application to other seismically active areas.