Natural tracers - Indicators of the origin of the water of the Vrana Lake on Cres Island, Croatia

A complex hydrogeological exploration has been made of Cres island in Croatia. It included macrostructural analyses, geophysical exploration, exploratory drilling, exploration of lake bottom lithology and morphology by divers, groundwater tracing, various analyses of recent lake bottom deposits and hydrological and biological exploration. This paper deals with only one part of that complex work, exploration by means of natural tracers. As natural tracers, the heavy mineral composition in the recent lake sediments and the isotopic and chemical composition of lake water were used. Some hypotheses on the lake water origin were analyzed by means of geochemical models. The analyses indicate a predominant rainfall contribution in the recharge of lake water, with a possible influence of a deep karst underground and fresh/sea water interface on the dynamics of lake water and its composition.