Geochemical and stable isotopic variability within two rivers rising under the same mountain, but belonging to two distant watersheds

Complementary geochemical and stable isotope investigations of the Gorski Kotar karst aquifer system in western Croatia were obtained for the first time, to answer the question whether both studied rivers drain the same aquifer system or not. The two main rivers, the Kupa and the Rječina, rise under the same mountain range, but belong to two different watersheds (Black Sea and Adriatic Sea). The karst aquifer of Gorski Kotar is a potentially important source of drinking water for two neighboring countries, Croatia and Slovenia (Central and South Europe), and is strongly influenced by both Mediterranean and continental weather conditions. It is a part of the Dinaric karst, which is “locus typicus” for karst worldwide and one of the most typical karst areas in the world.To answer the main question of our research, baseline data were thoroughly collected comprising stable isotopes, concentration of dissolved and total trace metals in water, and multi-elemental analyses of river sediments, together with other physical-chemical parameters (pH, dissolved oxygen, electric conductivity and temperature). Total dissolved solid (TDS) was not measured but estimated as (EC) 0.67. Such multi-technique approach was applied for the first time in the Dinaric karst systems.Multi-elemental analysis of fine sediment fraction (<63 μm) of eight samples was performed by ICP-MS. Elemental composition of sediments is quite different in the two studied river valleys, which indicates different origin of their waters. Also, concentrations of selected metals were compared with existing sediment quality criteria and anthropogenic influence is evaluated and discussed.Analytical results of major ions determined in three springs (Kupa spring, Rječina spring, Zvir spring in the City of Rijeka) were used to construct a Piper diagram, which showed that they are of a Ca-Na-HCO3-Cl type. The highest concentration of Mg is present in the Kupa spring, while the highest concentrations of Na and Cl are present in the Zvir spring. Groundwaters are underlain by limestone, less by dolomite and are under a maritime influence. There is a big difference with the main groundwater types reported in North Africa, i.e. in the south Mediterranean Sea, where two main groundwater types are Na-Cl and Ca-SO4-Cl.Concentrations of total and dissolved fractions of trace metals in surface water samples were determined by voltammetry and compared to the European Water Framework Directive values. Concentrations of total Pb and Zn are significantly higher in the water of the Kupa spring than the Rječina spring, suggesting that the karst water bodies of the two springs are not of the same origin.In addition, stable isotope composition (deuterium and oxygen-18) was determined in the spring waters sampled during low and high water conditions. Due to the significant difference in oxygen isotope-ratios it was concluded that the karst reservoir for the Kupa and Rječina springs is not identical.The results obtained from the combination of physical-chemical, geochemical (water and sediment) and isotope multi-technique analyses pointed out that even though the springs of the Kupa and Rječina rivers originate from the same mountain range, they do not drain the same karst aquifer.