Geochemical Journal, Vol. 51 (No. 5), pp. 391-407, 2017
Liang-Feng Han,1 Zvjezdana Roller-Lutz,2 Tamara Hunjak,2 Hans O. Lutz,2,3* Takuya Matsumoto1 and Pradeep Aggarwal1
1Isotope Hydrology Section, Division of Physical and Chemical Sciences, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, P.O. Box 100, A-1400 Vienna, Austria
2Stable Isotope Laboratory, Medical Faculty, Rijeka University, 51000 Rijeka, Croatia
3Faculty of Physics, Bielefeld University, 33615 Bielefeld, Germany
(Received September 9, 2015; Accepted August 6, 2016)
Due to their extreme anisotropy and heterogeneity, the hydrogeological characterization of karst terrains is notoriously difficult. Approximately 50% of Croatia consist of karst aquifers, in particular the touristically important region along the Adriatic coast. Being an important water reservoir for this area, we have chosen the Gacka area of Croatia as a typical example. We studied the mixing of groundwater and the responses to recharge in several karst springs by using environmental tracers, including stable isotopes in water (18O, 2H), tritium (3H), chlorofluorocarbons (CFCs) and noble gases (3He, 4He, Ne, Ar, Kr, Xe). The variation of the stable isotopic signal (δ18O and δ2H) in the spring discharges provides qualitative information on the mixing of waters with different transit time. Assuming piston flow in the system, the CFC and 3H data indicate that the waters contain mainly post-1980 recharge. The CFC concentrations in the spring waters were found to be significantly different between wet and dry weather periods. The higher CFC values during the wet weather period indicate that the waters contained some water components that have relatively short mean residence times. An important finding of the study was that the sole application of tritium-helium methods does not permit an accurate age determination. This is due to mass exchange between liquid and gaseous phases occurring in karst aquifer systems particularly in dry weather periods. During such a dry weather period, more space in the karst groundwater system is not filled with water compared with a wet weather period. It is shown in this study, however, that the combined use of stable isotopes (18O, 2H), tritium, CFCs and noble gases is able to account for groundwater mixing and responses to recharge in this specific scenario. Compared to single-tracer studies, multi-tracer techniques will allow a much better understanding and characterization of these systems. This information is vital for the assessment of resources and potential contamination of the groundwater in this special environment.
Key words: groundwater dating and mixing, environmental tracers, multiple tracer techniques, karst, Croatia