On the Appearance of a Brackish Spring 30 m Above Sea Level Near Trogir (Southern Croatia)

Abstract

The intermittent karst spring Slanac east of the town of Trogir is a unique karst phenomenon by its appearance at an altitude of 30 m above sea level. This is a consequence of the areal hydrogeology and the development of karstification and karst groundwater flow paths from the most recent glaciation to the present. There are at least three factors controlling the discharge of brackish water at such altitudes during the highest groundwater tides: (a) a considerably greater permeability of calcareous rocks lying below the present sea level in comparison to the permeability of the upper parts, (b) the presence of almost pure sea water upstream of a hanging hydrogeological barrier at the spring site at the end of a dry season, (c) ever higher hydraulic resistance to groundwater flow below the hanging hydrogeological barrier due to ever longer siphonal flows and their gradual colmatation.