In one part of the groundwater flow in the Vrelić cave (near the village Donje Dubrave), during the explorations in 2003, oil pollution was perceived. During field prospection and contact with local population, few possible pollution sources were located. There was a strong possibility that the oil traces in the cave are the result of the railway accident in 1970. Because of the railway accident on Rijeka-Zagreb railroad, tank carriage sliped off from the tracks and the content of the dangerous cargo leaked onto the nearby valley, 800 meters away from the cave entrance. A spring, used sometimes for water supply by the local population is also located nearby. Sampling of the water from the cave and the spring was carried out several times during the next period with the intention of monitoring the pollution impact on water quality in different seasonal (climatic) conditions. In the course of the laboratory analysis of the samples the following parameters were determined: Total hardness; Concentrations of Calcium, Magnesium, Iron, Chlorides and Nitrates; pH-value; TOC; colony-forming unit (CFU); total Coliform; fecal Streptococus; Proteus bacteria; Salmonella bacteria and Clostridium perfringens bacteria. Several of other parameters were also measured by mobile devices: Conductivity (EC), TDS, Redox-potential, pH-value and water temperature. Water tracing with Na-fluorescine was carried out before the analysis and the connection between groundwater flow in cave and the spring water was confirmed. After the creation of a topographical (speleological) map of the cave and thanks to the surface (field) measurements, the distance between the place of Na-fluorescine spill in the cave and the Vrelić spring was defined (the paper is published in Croatian).