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 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 ma.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°C-328 µS/cm, 9.6°C-350 µS/cm and 8.9°C-312 µS/cm) of the springs imply that they are fed from aquifers with similar mean residence times (MRTs). The mean δ(18)O contents of Majerovo, Tonkovica, and Pecina are around -10.1‰, -9.2‰ and -8.9‰, respectively, revealing differences in the mean recharge area elevations. Compared to the temporal amplitude of the(18)O signal of precipitation, the (18)O 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 (1,190 Mm(3)), and 12.2 years (1,210 Mm(3)), respectively. Plug flow conditions dominate in about 90% of the total aquifer volumes.

Stable isotope composition of the meteoric precipitation in Croatia.

The precipitation is the input into the water system. Its stable isotope composition has to be known for the proper use and management of water resources. Croatia is not well represented in the Global Network of Isotopes in Precipitation (GNIP) database, and the geomorphology of the country causes specific local conditions. Therefore, at the Stable Isotope Laboratory (SILab), Rijeka, we monitor the stable isotope composition (δ(18)O, δ(2)H) of precipitation. Since δ(18)O and δ(2)H are well correlated, we concentrate the discussion on the δ(18)O distribution. Together with GNIP, our database contains 40 stations in Croatia and in the neighbouring countries. Their different latitudes, longitudes and altitudes give information of great detail, including the influence of the topographic structure on the precipitation in the south-eastern part of Europe, as well as the complex interplay of the different climate conditions in the area. Within a few hundred kilometres, the stable isotope values display a significant change from the maritime character in the south (mean δ(18)O around-6 to-8‰) to the continental behaviour in the north (mean δ(18)O around-8 to-11‰). Depending on the location, the mean δ(18)O values vary with altitude at a rate of approximately-0.2‰/100 m and-0.4‰/100 m, respectively. Also the deuterium excess has been found to depend on location and altitude. The data are being used to construct a δ(18)O map for the entire area.

Note on the spring region of Gacka River (Croatia).

We present the first results of a study of the Gacka River spring area, an important Croatian drinking water resource. Stable isotope data (obtained at the Stable Isotope Laboratory SILab in Rijeka) are used in combination with hydrological and meteorological data. The delta18O values give information on the mean catchment altitudes of the three main springs of Gacka River (Majerovo vrelo, Tonkovica vrelo and Pecina). Hydrological, meteorological and stable isotope data indicate good mixing of infiltrated and ground waters. The precipitation d-excess shows a seasonally varying influence of continental and maritime air masses.