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 460 ma.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-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, 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 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 Pećina, Majerovo and Tonkovića 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 Pećina, Tonkovića 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.
Department of Geological Engineering, Hacettepe University, 06800 Ankara, Turkey.
Nur N
NN
Ozyurt
Stable Isotope Laboratory, Medical Faculty, Rijeka University, 51000 Rijeka, Croatia; Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany.
Hans O
HO
Lutz
Stable Isotope Laboratory, Medical Faculty, Rijeka University, 51000 Rijeka, Croatia.
Tamara
T
Hunjak
Stable Isotope Laboratory, Medical Faculty, Rijeka University, 51000 Rijeka, Croatia.
Diana
D
Mance
Stable Isotope Laboratory, Medical Faculty, Rijeka University, 51000 Rijeka, Croatia. Electronic address: roller@medri.uniri.hr.
Zvjezdana
Z
Roller-Lutz
D017523:Croatia; D004784:Environmental Monitoring; D060587:Groundwater / Q000737:chemistry*; D008956:Models, Chemical; D045483:Rivers / Q000737:chemistry; D014316:Tritium / Q000032:analysis*; D014872:Water Movements; D014875:Water Pollutants, Radioactive / Q000032:analysis*
D014875:Water Pollutants, Radioactive; D014316:Tritium
Croatia; Environmental isotopes; Gacka River; Karst; Lumped parameter modeling