The Gusić reservoir is a compensation basin constructed in the karstfield. The reservoir lies on a Quaternary overburden free of ponors(sinkholes) and suffosions. After the reservoir filling the water losse swere up to <6.4 m3/s. The bottom impermeability has been ensured with a 0.4 m thick clay base blanket. During reservoir exploitation,suffosions and ponors developed through which 2 m3/s water waslost. Such conditions required reservoir repair within a short time frame of approximately 10 days during which the power plant was shut down. When the reservoir was emptied, a resistivity sounding(Wenner electrodes arrangement at five depth levels) was conducted on the reservoir bottom and clay blanket. An alternative concept has also been considered - the construction of a new reservoir area that has similar hydrogeological conditions. The solution for the possible impact of groundwater (air) "pressure" could be the construction of a horizontal "event" in the deepest part of the palaeodepression, wherethe bedrock karstification is the most intensive and the interrelation between groundwater and air pressure on the overburden is possible. The reservoir bottom impermeability could be resolved with a clay blanket or with synthetic materials.