Offshore Freshened Groundwater Reserves Identification As Revealed by Geophysical and Stratigraphic Data: Insights from the Northern Adriatic Shelf (Italy)

The analysis of twenty geophysical well logs covering a shelf area of about 3,500 km2 in front of the Emilia-Romagna coast (Italy), has shown apparent resistivity (ρa) values consistent with important Offshore Freshened Groundwater (OFG) reserves stored in the first 450 m of the Middle-to-Upper Pleistocene succession, and extending seaward about 60 km from the modern shoreline. Four classes with different ρa intervals (i.e., different salinities) were identified. The first three (1, 2 and 3) classes are characterized by ρa ranges of 7-28 Ω m, 4-7 Ω m and 2-4 Ω m, respectively. These values are higher than seawater resistivity (< 2 Ω m - i.e., class 4) and, based on the OFG definition (i.e., “the water stored in the sub-seafloor with a total dissolved solid concentration below that of seawater”), they have been used for OFG identification. Class 1 ρa is coherent with fresh-to-brackish water content, whereas classes 2 and 3 have been interpreted as transitional to seawater. The correlation of offshore wells (spontaneous potential and ρa profiles) with onshore data (stratigraphic and lithological) from water wells and additional geophysical well logs, led to the stratigraphic architecture reconstruction of the Plio-Pleistocene siliciclastic succession along onshore-offshore transects, up to 60 km-long, from the Apennine front to the Adriatic shelf. The uppermost (first 450 m) Middle to Upper Pleistocene interval displays a vertical alternation of high-permeability (amalgamated and laterally continuous gravel to sand bodies) and low-permeability (mud-dominated) strata made of fluvio-deltaic, coastal and shelfal deposits. The high-permeability bodies represent the offshore extension of the onshore aquifer systems, whereas the low-permeability units make the aquitards. Along the transects, different stratigraphic intervals characterized by the four ρa classes have been identified. The highest ρa values (class 1) have been documented in the first 300 m of the succession, despite its deposition mostly occurred in deltaic to marine (i.e., saline water) conditions. This interval wedges out seawards, with ρa progressively decreasing down to class 3 values at about 35 km from the coast. Similarly, ρa decreases vertically, between about 300 and 450 m depth. Such a vertical gradual decrease may suggest that locally aquitards do not completely prevent water exchange, and transitional classes 2 and 3 likely resulted from freshwater and seawater mixing through space and time. Below 450 m depth, ρa drops to < 2 Ω m (class 4), thus defining the lowermost limit of the OFG reserves. Onshore-offshore reconstructions additionally revealed how OFG aquifers are actively recharged in correspondence of the Apennine front, where the topographic gradient is higher and permeable units are subaerially exposed. Their extremely high degree of amalgamation even allows the topographically-driven recharge of the deeper (and marine) strata.The relatively shallow depth (< 350 m) of the northern Adriatic aquifers and the presence of several and abandoned oil&gas platforms in the area, provide a good opportunity to further investigate these OFG reserves that are strategic for the densely populated Emilia-Romagna coastal plain.