The Gacka River basin aquifer is a highly-developed karst system, located in the Croatian Dinarides. It is mostly composed of permeable Jurassic and Cretaceous carbonate rocks, and clastic sedimentary rocks of Paleogene age. Gacka River provides high quality water for the town of Otocac and several villages; together with the neighboring Lika River, the water is used for the Hydroelectric Power Plant at Senj on the coast About 10 perennial and over 20 seasonal springs are located at 450 to 460 m a.s.l. (above sea level). Three major springs (Pecina, Majerovo and Tonkovica) provide 57% of the mean annual river flow. Similarities between the average groundwater temperatures as well as between the average specific electrical conductivity values (9.0 degrees C-328 mu S/cm, 9.6 degrees C-350 mu S/cm and 8.9 degrees C-312 mu S/cm) of the springs imply that they are fed from aquifers with similar mean residence times (MRTs). The mean delta O-18 contents of Majerovo, Tonkovica, and Pecina are around -10.1 parts per thousand., -9.2 parts per thousand. and -8.9 parts per thousand., respectively, revealing differences in the mean recharge area elevations. Compared to the temporal amplitude of the O-18 signal of precipitation, the O-18 signal variations of the springs are substantially attenuated because the recharges occurring at different times are well mixed within the aquifers. This indicates MRTs of more than just a few years. The average tritium contents of Pecina, Majerovo and Tonkovica are 5.48 TU, 6.13 TU and 6.17 TU, respectively. Serially connected exponential-plug type unsteady lumped-parameter models run on an annual time scale resulted in rather satisfactory matches between the observed and calculated tritium contents for all studied springs. The models revealed similar MRTs (and corresponding reservoir volumes) for Pecina, Tonkovica and Majerovo of 12 years (470 Mm(3)), 12 years (1190 Mm(3)), and 12.2 years (1210 Mm(3)), respectively. Plug flow conditions dominate in about 90% of the total aquifer volumes. (C) 2014 Elsevier B.V. All rights reserved.