Influence of the karst spring submergence on the karst aquifer regime

Abstract

With the construction of a 123 m high dam, a large and permanent karst spring was flooded by water, about 75 m deep. The annual average discharge of this zone is 80 m 3 s −1. The aquifer zone which discharges through this spring was formed under complex tectogenetic conditions of Dinaric karst. The most prominent structures are overthrusts and reverse faults. These structures have caused the formation of separate karst aquifers which are arranged stepwise behind an existing karst spring. Transformation of these aquifers into a single karst aquifer has taken place in the later phases of the karst evolutionary process. The velocity of the underground flows in this aquifer, which depends on the actual hydrologic situation and on the level of the water table, is between 0.9–14.0 cms −1. Water table fluctuations are very rapid with amplitudes up to 120 m. Photogeological analysis defines the tectonic zones along which underground karst channels of huge drainage capacity are formed. Extensive hydrogeological and geophysical research and hydrologic and hydrogeologic monitoring was carried out in the catchment area. In order to define the influence on the karst aquifer regime of flooding of springs a correlative analysis has been performed between the index of previous precipitation, the water level in the reservoir, the underground water level in the immediate vicinity of the spring and in the distant parts of the aquifer, and the inflow into the reservoir. The multiple correlation coefficients obtained show mutual dependence of most of parameters analysed. The analysis has shown that the flooding of springs has some influence on the hydrological conditions of the upstream polje and the surface in the immediate tributary area. There is also an evidence of formation of an underground reservoir in the karst aquifer. It is connected with the downstream aquifers which are under the influence of the reservoir being formed deep in the banks, and has some influence on the layout of the natural watersheds. As a consequence of flooding of spring and reservoir formation there is induced seismicity in the catchment of the spring.