Hydrological Modeling of a Fractured Volcanic Aquifer to Analyze Interactions Between Anthropogenic Water Resource Usage and the Natural System

The Alcantara River basin, located in Sicily (Italy), encompasses an area of 606 km², including the northern slopes of Mount Etna, Europe's highest active volcano (3357 m a.s.l.). The river stretches for 55 km, originating from the Nebrodi Mountains at 1400 m a.s.l. and discharging into the Ionian Sea approximately 5.5 km south of Taormina (Messina). Groundwater significantly sustains the river’s flow. Irrigation and drinking water wells extract an average of 0.23 m³/s, while a drainage gallery collects water from three springs, supplying the Alcantara aqueduct with an average flow rate of 0.48 m³/s, as measured from January 2009 to December 2022.
To better understand the aquifer-river interactions and assess the impacts of groundwater extractions, the aquifer system was modeled using MODFLOW 6, a finite-difference numerical code developed by the U.S. Geological Survey (USGS). The study covered 14 years (2009–2022), leveraging monthly groundwater withdrawal records provided by the aqueduct operator. Hydraulic conductivity was calibrated using PEST, a software tool for parameter estimation and uncertainty analysis. Two scenarios were considered: (1) the current condition, including all known groundwater extractions, and (2) a hypothetical scenario without extractions. The model was validated by comparing observed and simulated discharge trends from the drainage gallery.
Simulation results revealed that groundwater extractions reduce natural spring discharge to the river by an average of 22%. This reduction shows significant seasonal variability, with the most pronounced impacts in spring and less severe effects in winter. Interestingly, despite this reduction, the midstream section of the river did not experience zero discharge, even during the driest periods (e.g., the summers of 2020 and 2021). This discrepancy suggests the influence of additional unaccounted factors, such as unauthorized or unrecorded water withdrawals, or potential hydrogeological changes induced by seismic activity.
These findings emphasize the need for systematic monitoring of groundwater and surface water resources. Enhanced monitoring would provide a deeper understanding of aquifer-river interactions, identify drivers of hydrological regime alterations, and inform strategies to optimize groundwater use and mitigate its impacts on river systems. Such efforts are essential for protecting the natural environment and ensuring the long-term availability of water resources.