A three-step hydrogeochemical approach for groundwater monitoring networks in seismically active areas

Hydrogeochemical monitoring of groundwater is recognised as a powerful tool to investigate the preparation phases of earthquakes, particularly in tectonically active regions where fluid–rock interactions and structurally controlled fluid-flow systems may respond to crustal stress changes. However, the selection of suitable monitoring sites is a critical and often overlooked issue, as groundwater systems may display a relevant temporal variability or a limited resilience to seasonal forcing, that could mask potential seismo-hydrogeochemical signals. In this study, we propose and apply a three-step hydrogeochemical strategy designed to identify groundwater sites with the highest potential sensitivity to earthquake-related processes. Such approach was tested in the northern Marche Region (central-eastern Italy), being characterized by a moderate-to-high seismic activity in past and recent years and includes: (i) large-scale characterization of groundwater chemistry and dissolved gases, aimed at identifying dominant geochemical processes and those sites potentially influenced by deep fluid circulation; (ii) isotopic assessment (δ³⁴S–SO₄, δ¹⁸O–SO₄, δ¹¹B, ⁸⁷Sr/⁸⁶Sr, δ¹³C–TDIC) to provide insights on circulation depth and water–rock interaction pathways; (iii) high-frequency monitoring (monthly/quarterly, up to two years) to evaluate temporal stability and resilience of physico-chemical parameters, major and trace elements, and water isotopes. The integrated analysis reveals that a few sites exhibit groundwaters affected by the combination of (i) temporal stability and geochemical resilience, (ii) deep circulation and structurally controlled pathways and (iii) proximity to active seismogenic structures, making then optimal candidates for the inclusion in a long-term hydrochemistry monitoring network. By contrast, other sites, despite showing favourable characteristics are context-dependent, being affected by shallow flow paths or moderate seasonal variability, thus making them unfit for the research purposes.

The proposed methodology is scalable, reproducible, and readily transferable to other geodynamic settings. By providing a transparent, data-driven workflow for site selection, this approach strengthens the robustness of hydrogeochemical monitoring networks and enhances their capability to detect earthquake-related anomalies, thereby offering a practical framework for seismic surveillance initiatives worldwide.