Authors: Petra Kovač-Konrad and Nenad Buzjak
The focus of study in this work is a result of an exploration of a cave system Pećine-Veliko Vrelo. The cave-diving exploration took place during 10-days expedition in 2009 organized by Caving section “Željezničar”, Caving club “Samobor” and Croatian biospeleological society guided by Petra Kovač-Konrad and Vedran Jalžić.
The main characteristic of Ogulin-Plaški valley is fluviokarst and karst geomorphology. The whole area of Ogulin-Plaški valley is an example of a typical karst hydrogeology characterized by permanent springs at western margin of the area. In this area we can distinguish true hydrogeological barriers, relative barriers, zones of shallow water in fractures and zones consisting of deep permeable rocks. Pećine and Veliko Vrelo are two of the numerous springs situated on contact of a true barrier and zone of permeable rocks. True barrier zone is situated on direction Vitunj Zagorska Mrežnica spring – Plaški and it is 30 kilometres long. On this barrier many karst springs represent output of underground water which flows from the underground of Mt. Mala Kapela. Although cave explorations started in 19-th century, cave-diving explorations started recently in 2006. The result of cave-diving exploration is that 1350 meters of submerged passages were explored from which main passage still continues. From that 725 meters were mapped and cave map was made as well as video and photo documentation.
It took 47 dives to accomplish this result. It was established that Pećine and Veliko Vrelo are dammed types of karst springs developed in carbonate sediments and they make a unic cave system. The entrance passage to Veliko Vrelo is vertical and 30 meters deep. After that passage continues in horizontal direction. Together with Pećine passages it can be classified as a combined speleo-feature. Many underground animal species were also noticed. Pećine spring represents level speleological object because cave passages are developed on two levels. One level is in vadose zone and it is situated higher than lower level (the main passage) that is in phreatic zone. The higher level is in later stage of development, and the lower channel is in first stage of development. Because this speleological object has two entrances it can be classified as a cave system. Directions of underground waters are still quite unexplored and this is why this exploration is significant for future protection of this area. The underground waters are connected and since they are used for drinking from local population, any pollution can have serious consequences. During the exploration pollution with bulky waist was noticed at the entrance of Veliko Vrelo. Construction of buildings, railroads and roads above or near cave passages may result in their permanent pollution or collapse of channels. Exploration of cave passages and their mapping can insure adequate protection of water and its unique underground fauna. Results of this exploration will be represented to the local population especially to the children in elementary and high schools to achieve preventive protection of underground waters of this area.