An assessment of hydrogeological parameters on the karstic island of Dugi Otok, Croatia

SummaryExtensive explorations have been performed in the northern part of the island of Dugi Otok in the Adriatic Sea (Croatia) with the purpose of extraction of fresh and/or brackish water. The island is predominantly composed of karstified carbonate rocks of the Cretaceous age. The explorations included geological and hydrogeological mapping of the area, geophysical surveys (by means of the geoelectric tomography method, electrical resistivity sounding, and surface seismic refraction), exploratory core drilling, hydrochemical in situ and laboratory explorations, and the pumping test with the calculation of rock mass parameters. Based on the obtained results, it was concluded that it is possible to exploit the groundwater with a pumping quantity of 5 l/s without intrusion of salt water into the fresh-water lens. Choosing a pumping regime based on the parameters of the aquifer (rock mass hydraulic conductivity order of magnitude 10−4–10−6 m/s), and knowing the parameters of the well, as well as the tidal efficiency, should reduce the possibility of intrusion to the minimum. Because of the systematic quality of the performed explorations and obtained results, the island of Dugi Otok can serve as a typical example for such kinds of exploration works on other karstic islands. Hydrogeological composition behaves partly according to Ghyben–Herzberg‘s law, but because of the nature of the karstic hydrogeological system, such a statement needs to be accepted approximately because of the highly complicated relationships.