Rethinking unconventional groundwater resources from continental shelves: Insights from the Po-Adriatic system

Offshore freshwater resources hosted within continental shelves constitute a valuable resource for coastal regions experiencing increasing water scarcity. This study identifies and characterizes significant fresh and brackish groundwater resources within the Western Mediterranean by integrating and reinterpreting multi-proxy datasets available from public repositories. Onshore-offshore correlation of well logs revealed laterally continuous gravel and sand bodies, with resistivity locally exceeding 30 Ω·m, indicative of fresh water within the uppermost 300 m beneath the northern Adriatic seabed. These permeable bodies are part of the Pleistocene (last 870 kyr), multi-layered succession composed of vertically-stacked aquifers and aquitards/aquicludes deposited in fluvio-deltaic, coastal, and shelfal environments. A conservative volumetric assessment indicates approximately 26.5 km³ of fresh water (>15 Ω·m) within ~35 km of the modern shoreline, with an additional ~33.4 km³ of brackish water (<5-15 Ω·m) extending ~15 km further offshore and reaching depths of ~500 m below the seabed. The stratigraphic architecture, governed by tectonic activity and glacio-eustatic oscillations, primarily control the geometry, distribution, and connectivity of these offshore aquifers. Integration of stratigraphic reconstructions with hydrogeological modelling indicates that several offshore aquifers are hydraulically connected to inland systems, implying active recharge and potential renewability.This study demonstrates how geological methodologies originally developed for hydrocarbon exploration can be effectively repurposed to identify offshore groundwater, quantify volumes, and distinguish renewable from non-renewable aquifer systems. Furthermore, the systematic reuse of existing datasets provides an efficient, low-impact, and cost-effective strategy for preliminary offshore groundwater investigations across continental shelves worldwide.