The island of Susak differs from other Croatian islands in its geological and hydrogeological properties, because it is, for the most part, covered with fine-grained loess-type sands. At the outset of the research it was unclear whether the main aquifer lies within the superficial deposits or within the underlying carbonate rocks. This gave rise to two alternative conceptual models, depending on whether the interface between the superficial cover and the carbonates occurs above or below sea level. Hydrogeological and geophysical investigations were conducted. The primary objective of the geophysical exploration was to determine optimum positions for exploratory boreholes. The results showed that the main aquifer is contained within the carbonate rocks, and that this carbonate aquifer is overlain by a second, confined, aquifer within the superficial sandy deposits. Groundwater within the carbonate aquifer is hydraulically connected to the sea by fault zones. Measurement of electrical conductivity in boreholes established that the mixing zone of fresh and salt water is at depths from 22 to 30 m below ground level. It is necessary to ensure constant control of pumping quantities and control of the brackish water lens by monitoring of the groundwater salinity. For inclusion in public water supply, the groundwater should be treated by desalination.