Monitoring of a small karst island aquifer as a prerequisite for its sustainable management (Vis island, Croatia)

The assessment of groundwater resources quality and quantity in arid or semi-arid climates is crucial for achieving long-term sustainable management due to their limited volume and scarce replenishment. The relevance of this topic is enhanced by their vulnerability to climate change. The Mediterranean region is considered to be a hot spot for climate change with a generally arid climate. Its groundwater resources are under increasing stress due to: growing population, agricultural and industrial development, seawater intrusion, and changing climate. Groundwater resources in islands are particularly stressed since they have small aquifer volumes and a limited extent of the recharge area. The island of Vis (Croatia) in the Adriatic Sea is a representative case study for stressed island groundwater resources. Vis is located 40 km from the mainland and depends exclusively on its local karst aquifer for public water supply. This groundwater resource is affected by a concomitant decrease of recharge, an increase of evapotranspiration, and a progressive anthropic impact due to tourism. Geochemical and hydrogeological monitoring were conducted from September 2019 to December 2022 in deep wells and coastal springs to assess the hydrochemical characteristics and the regime of the Vis groundwater resource. Although the monitoring period was characterised by low rainfall resulting in the decline in groundwater levels, the principal ion composition showed relative stability. The groundwater in wells generally showed predominant Ca-HCO₃ hydrochemical facies, while coastal springs or wells nearby the sea showed both Na-Cl and mixed Ca-Mg-Cl-SO₄ facies. Time series of in-situ measurements of groundwater temperature, pH value, and electrical conductivity have shown low variability, notwithstanding the low precipitation during the observed period. Groundwater temperature between 16°C and 18°C varied throughout the year following the air temperature variations. The groundwater pH was neutral to mildly alkaline with low annual variability. EC values were variable depending on the interaction between groundwater and seawater. However, all objects displayed relatively stable EC values despite the prolonged drought and the intensive exploitation during the summer period. These results evidenced the resilience of the aquifer, which owes to its favourable geological structure. Due to increasing natural and anthropic pressures on the resource, continuous monitoring and the establishment of an early warning system should be foreseen in the coming years.

Acknowledgments: This research was carried out within the framework of the INTERREG-CE project DEEPWATER-CE, funded by the European Regional Development Fund (ERDF).