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The article presents findings of a two-year systematic study of stable isotope content in two karst groundwater resources in Primorsko-goranska county (Croatia): the Martinšćica wells (MWs) and the Dobrica spring (DBC). The temporal and spatial variation of hydrogen and oxygen isotopes is commonly studied in conjunction with hydrogeological conditions such as groundwater dynamics and discharge conditions. However, since this information was incomplete, we were forced to work with limited data and rely on analyses of stable isotope monitoring results. The obtained results show that winter precipitation is the most common recharge source for the systems, and the average residence time of water in the subsurface is less than a year. Furthermore, the MWs system is a typical dual-porosity system with dominant base flow. The results of the nonparametric regression analysis show that the possibility of seawater intrusion into the spring affecting DBC isotope content cannot be ruled out. We believe that the results presented in the paper demonstrate that when combined with statistical analyses, environmental stable isotopes are a powerful tool for gaining insights in karst hydrogeology.

Complexity of karst groundwater networks requires implementation of standard hydrogeological monitoring and unconventional methods of investigation. We present the results of isotope composition analysis for three karst springs and rain water collected in their hinterland. During two years, spring water samples were collected on a weekly and rain samples on a monthly basis. The main findings are: winter precipitation of Mediterranean origin dominates springs' recharge, a dual porosity model is a fit for the studied systems, and hinterlands of the springs have different retention capabilities.