Hydrogeological Exploration of the Rjecina River Spring in the Dinaric Karst

The Rjecina spring is one of the major springs in the Dinaric Karst. It appears at the contact between permeable carbonate and impermeable clastic rocks, with a discharge of up to 120 m3/s but it dries up during the dry summer seasons. The spring occurs close to the town of Rijeka, 325 m above sea level and offers an outstanding opportunity to cover gravitationally the public water demand of a town of about 200,000 inhabitants, and the touristic needs of the whole region. This hydrogeological research project is a part of efforts to solve the problems of water deficiency during the dry summer seasons up to a maximum of three months. It was necessary to enter the parts of a karst aquifer that are active even in time of any outflow from the Rjecina spring by complex geological, hydrogeological and geophysical exploration accompanied with deep exploratory boreholes. During earlier exploration, it was determined that there are no active inflows in the immediate hinterland of the spring and that it is necessary to discover the inflows from other karst structures, that behave as retentions of karst springs in the zones of permanent discharge. The presence of multiple overthrusted structures in the zone around the spring site suggest the existence of deep zones of water retention, which may be reached by an access gallery from the Rjecina canyon. This work represents a substantial change in the exploration methodology for Dinaric Karst aquifers, because it directs the researchers toward deep, unknown retention spaces which contain large reserves of high-quality groundwater outside urban areas.