Display omitted •A karst island brackish water lens was investigated via diverse methods.•Hydrogeological mapping, geophysics, bore-drilling, and pumping tests were used.•Electrical tomography determined the groundwater level and transition zone.•Hydrochemical data supported hydrogeological and geophysical research.•Pumping tests revealed hydraulic characteristics in karst environment. Ilovik is one of the smallest inhabited islands (5.51 km2) in the Adriatic Sea. It is composed of karstified Cretaceous and Paleogene limestones. Despite its small size, the island was the subject of an extensive research program. The main objective was setting up a groundwater extraction site to extract brackish groundwater at a rate of least 1.0L/s with chloride concentrations below 5000 mg/L. The research was carried out in three phases using various methods: geological and hydrogeological mapping, geophysical profiling (electrical tomography and seismic refraction), investigatory core drilling, test-well design, pumping tests, groundwater level monitoring, and hydrogeochemical analyses (in situ and laboratory tests). The underground spread of electrical resistivity mapped the groundwater level and transition zone from fresh to brackish water, and seismic velocities indicated fractured zones. After the pumping tests, hydraulic parameters were calculated based on calculations for similar karstic aquifers, and hydraulic conductivity was within orders of magnitude of 10−4–10−6m/s. The results have great applicability, as the water quantity and quality were determined. Additionally, parametric estimation of the hydrogeological properties was useful even in heterogeneous Dinaric karst environments. Such karst environments have to be studied via several different methods, with common interpretation, to avoid ambiguous conclusions.