All traffic marine centers and cities with developed industry must carry out preventive maintenance procedures in order to avoid unwanted leakage of various pollutants and thus pollution of the surrounding area. Therefore, in this paper, the movements of pollution are simulated if adequate preventive maintenance is not carried out in the case of tanker breakdowns and oil spills from the Urinj refinery. Numerical simulations were performed in the OpenFOAM open source program, where the pollution movement was defined as the passive scalar movement. The results of the use of fixed boundary conditions and real boundary conditions taken from the ROMS database are presented. It can be seen that the ROMS data describe the flow of the sea and the movement of pollution much better, because such an approach simulates real conditions. In the case of tanker breakdowns, it can be seen that in fixed boundary conditions the pollution moves towards the Istrian coast and partly exits the bay through Vela vrata, while in ROMS boundary conditions the pollution in winter mostly stays in the center of the bay, while in summer it moves towards the island Cres. In the case of oil spills from the Urinj refinery, fixed boundary conditions show the spread of pollution along the shore of Rijeka, while ROMS boundary conditions show the circular spread of pollution due to certain vortices located near the refinery. This paper proves the importance of using ROMS data on boundary conditions, because it's the only way can we get closer to the real conditions appearing in the Kvarner Bay. Based on the presented simulations, it is possible to predict the time of arrival of contamination to a certain area and thus prepare equipment to prevent further spread.