Hydrogeological model of the Venetian-Friulian Plain and North Adriatic: Assessing Onshore Dynamics and Offshore Availability

Abstract Groundwater is the primary source of freshwater for nearly half of the global urban population, yet aquifers worldwide are increasingly threatened by climate change and non-sustainable extraction rates. Coastal aquifers are particularly vulnerable, facing risks of saline intrusion and water quality degradation. As a result, research efforts to evaluate the status and temporal response to climate challenges and water demand are becoming increasingly common. Furthermore, offshore freshened groundwater (OFG) has been detected along most passive continental margins, suggesting an unconventional freshwater resource that could help mitigate future water scarcity risks. This study investigates the groundwater dynamics in the Venetian-Friulian Plain (VFP), northern-eastern Italy, and the adjacent North Adriatic Basin (NAB). The VFP features a complex system of layered confined aquifers in its southern coastal area (Low Plain) and a thick phreatic aquifer in its northern part (High Plain). Both areas presents signs of a declining water table. The region is heavily exploited, with approximately 50,000 water wells, many of which operate with uncontrolled 24-hour extraction. The study aims to characterize the geometry, distribution, and genesis of aquifers; assess the status and evolution of onshore water resources over the past 23 years; evaluate the impact of water well extraction on the water table and budgets; and investigate the potential of the NAB to host OFG. The objectives of this study are achieved through hydrogeological modelling conducted in two main phases: geological modelling and numerical groundwater modelling. The geological modelling phase focuses on the development of four three-dimensional (3D) models using various approaches to define the onshore-offshore distribution of aquifers and aquicludes, the geometric trends of the main exploited aquifers, and aquifer relationship with eccentricity-dominated cyclostratigraphy associated with past sea-level fluctuations in the NAB (cyclothems). The results show that the region's aquifers are likely onshore-offshore connected, that they are south azimuth dipping toward the NAB, and stratigraphically linked with cyclothems. Aquifer genesis research revealed, supported by aquifers' lithological composition, marine fossil evidence, and sediment anisotropy consistent with coastal processes, that the majority of aquifers likely originated from marine coastal deposits, relics of past sea level fluctuations. A groundwater numerical model, based on one of the modelled geological frameworks and calibrated with data from 180 monitoring wells, simulated water table trends from 2000 to 2023. This model incorporated key parameters, including meteoric recharge, river leakage, and groundwater extraction from wells. The model was run with two scenarios: an active well scenario with currently projected well extraction rates, and an inactive well scenario without well extraction, to determine the influence of groundwater exploitation on groundwater heads and budget. The model results confirm an overall declining water table trend, exacerbated by well extraction, which induce an increase on the yearly decline water table rate of up to an order of magnitude. Additionally, they reveal the NAB is an integral component of the underground aquifer system functioning as both a recipient of offshore-directed water fluxes and a provider of onshore-directed water fluxes. The findings of this study highlight the region's vulnerability to aquifer depletion, and saline intrusion, exacerbated by well extraction, underscoring the critical need for sustainable groundwater management in the VFP, including enhanced monitoring and regulation of well extraction. Furthermore, the study suggests that the NAB holds significant potential to host offshore freshened groundwater, derived both from active onshore meteoric recharge and emplacement during periods of marine sea-level low stands.