Characterization of the Gacka River basin karst aquifer (Croatia): Hydrochemistry, stable isotopes and tritium-based mean residence times

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 Otočac 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 460ma.s.l. (above sea level). Three major springs (Pećina, Majerovo and Tonkovića) 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°C-328μS/cm, 9.6°C-350μS/cm and 8.9°C-312μS/cm) of the springs imply that they are fed from aquifers with similar mean residence times (MRTs). The mean δ18O contents of Majerovo, Tonkovića, and Pećina are around −10.1‰, −9.2‰ and −8.9‰, respectively, revealing differences in the mean recharge area elevations. Compared to the temporal amplitude of the18O signal of precipitation, the 18O 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 Pećina, Majerovo and Tonkovića are 5.48TU, 6.13TU and 6.17TU, 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 Pećina, Tonkovića and Majerovo of 12years (470Mm3), 12years (1190Mm3), and 12.2years (1210Mm3), respectively. Plug flow conditions dominate in about 90% of the total aquifer volumes. •The Gacka River karst aquifer is part of the strategic water resource of Croatia.•The discharge of springs indicates a long mean residence times of ground water.•Tritium-based models yield MRTs of ca. 12years and a large reservoir volume.•Areal recharge mainly in the cold season; plug flow conditions are dominant.