Exploring the potential of the north-eastern Adriatic basin to host freshened groundwater using onshore-offshore 3D hydrogeological modelling.

Due to their long-term storage capacity, coastal aquifers are an important source of freshwater for industry, agriculture, and human consumption. However, these onshore groundwater aquifers are under enormous stress mostly due to the increase in usage. Thus, in the last decades, there has been a growing interest in alternative freshwater sources, such as offshore freshened groundwater (OFG). This study focuses on the Venetian Friulian Plain (north-eastern Italy) and the neighbouring northern Adriatic Sea and aims to quantify the potential extent and the genesis of the complex aquifer system hosted in the Mid-Late Quaternary siliciclastic sediments. The main objective is to provide an updated hydrogeological overview of the study area, showing the offshore (OFG) extent of the onshore aquifer system. The objective of this paper is achieved through the development of four three-dimensional models of the study area: 1) a deterministic model based on lithostratigraphic data using Leapfrog GEO; 2) a discrete stochastic geostatistical model based on lithostratigraphic data using Petrel 2017; 3) an aquifer geometry trend model unlinked to lithological data; 4) a model of sedimentary sequences associated with Plio-Quaternary sea level fluctuations (Cyclothems). The input for the geological models is a large regional dataset that includes borehole stratigraphies, screens depth, well logs and seismic reflection profiles. Our results demonstrate that the Quaternary glacio-eustatic fluctuations in the northern Adriatic basin are primarily responsible for the formation of the groundwater aquifer system in the study area.  The sediments hosting these waters are mainly composed of sandy coastal deposits, which are a relict of the northward and southward migration of the coastline in relation to marine sea level changes. Previous studies show that similar sedimentary systems extend in the entire northern Adriatic Basin. Our hydrogeological models confirm the offshore continuum of the onshore groundwater aquifers, and suggest that vast OFG resources are potentially widespread in the entire northern Adriatic basin.