The regime of inflow and runoff from Vrana Lake and the risk of permanent water pollution

Nevenka Ožanić
Josip Rubinić

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Vrana Lake on the Cres Island (Croatia) is a specific natural phenomenon of 220 million m3 of fresh water on the island's karst. Cres is small island with the total area of only 405, 78 km2 and the lake is only 2 to 3 km away from the island's coast. The Lake is kryptodepression which has neither directly measurable inflow nor runoff. It is functioning as balance relation between salt and fresh water in karst aquifer. The dynamic of water exchange besides through lakes surface is happening through its underground aquifer also. The Lake is the only source of water supply for islands Cres and Losinj. Because of the noticed trend of declining of the water level at the end of eighties, complex hydrological, hydrogeological, hydrochemical, limnological and ichtiological researches started. Main goal of mentioned researches was the Lakes protection from overpumping, e.g. from over declining of the water level that could permanently result in salination of fresh water. On the other hand attention should also be focused on possible contamination of water with pollutants that could enter the Lake through underground. Inside of the Lakes catchment area is 5 to 6 km long part of the magistral road and it presents potential contamination source. Present work analyses mechanisms of the potential contamination of the Lake's system. Interrelation between dynamic of fluctuations of underground and Lake's water level and the regime of functioning of the Lake's karst aquifer were performed. Results show that single pollution incident could permanently devastate lake with the resulting loss of water supply.