Abstract
<p>A periodic sampling of the groundwaters and dissolved and free gases in selected deep wells located in the area affected by the May-June 2012 southern Po Valley seismic sequence has provided insight into seismogenic-induced changes of the local aquifer systems. The results obtained show progressive changes in the fluid geochemistry, allowing it to be established that deep-seated fluids were mobilized during the seismic sequence and reached surface layers along faults and fractures, which generated significant geochemical anomalies. The May-June 2012 seismic swarm (mainshock on May 29, 2012, M 5.8; 7 shocks M &gt;5, about 200 events 3 &gt; M &gt; 5) induced several modifications in the circulating fluids. This study reports the preliminary results obtained for the geochemical features of the waters and gases collected over the epicentral area from boreholes drilled at different depths, thus intercepting water and gases with different origins and circulation. The aim of the investigations was to improve our knowledge of the fluids circulating over the seismic area (e.g. origin, provenance, interactions, mixing of different components, temporal changes). This was achieved by collecting samples from both shallow and deep-drilled boreholes, and then, after the selection of the relevant sites, we looked for temporal changes with mid-to-long-term monitoring activity following a constant sampling rate. This allowed us to gain better insight into the relationships between the fluid circulation and the faulting activity. The sampling sites are listed in Table 1, along with the analytical results of the gas phase. […]</p>
Highlights
A periodic sampling of the groundwaters and dissolved and free gases in selected deep wells located in the area affected by the May-June 2012 southern Po Valley seismic sequence has provided insight into seismogenic-induced changes of the local aquifer systems
Considering together the information of the geochemical features of the gas phase, we argue that they originate from deep gas reservoirs, for which we exclude contributions from the mantle or mantle-derived sources, considering the isotopic ratio of He
The CO2-rich component is the main carrier for He; considering the radiogenic origin for the He component, we argue that the deep crustal component is represented by a CH4-dominated (>80% by volume), CO2-rich (1% to 5% by volume) gas phase
Summary
A periodic sampling of the groundwaters and dissolved and free gases in selected deep wells located in the area affected by the May-June 2012 southern Po Valley seismic sequence has provided insight into seismogenic-induced changes of the local aquifer systems. Waters appear to distribute along a progressive dilution pattern for saline (65 g/l) waters found in fractured Mesozoic carbonates or in Pliocenic or pre-Pliocenic layers This characterizes the structural high in the Modena-Ferrara low-plain area, which suggests possible admixing of a deep-seated component at different extents. In terms of the oxygen and hydrogen isotopic compositions (Table 2), the collected waters deviate from the line of meteoric precipitation reported for Apennine catchments and the easternmost part of the Region (Ravenna Plain), while they plot close to the meteoric water line for northern Italy [Longinelli and Selmo 2003] This indicates a meteoric origin, as already suggested by Martinelli et al [1998] and a long circuit for the local groundwaters.
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