Groundwater flow mechanism in the well-developed karst aquifer system in the western Croatia: Insights from spring discharge and water isotopes

The spring hydrograph and the stable isotope (δ18O, δ2H) compositions in the water samples collected from the Rječina and Zvir springs and precipitation were analysed and used to characterize the karst aquifer. The recession coefficient obtained from the hydrograph analysis indicates only a fast-flow spring component at the Rječina spring. The lack of the base-flow spring component is the primary reason for the spring drying out during the dry periods. The low recession coefficient of the Zvir spring indicates a base-flow and discharge from well-drained fissures and fractures in the spring catchment area during the low water stage. A mean residence time (MRT) of groundwater was calculated for stable isotope δ18O using lumped parameter approach by applying the exponential model, combined exponential-piston and dispersion models to isotopic input (rainfall) and output (spring) data sets during 2011-2013. The MRT of 3.24 and 3.6 months for the Rječina spring and 7.2 months for the Zvir spring suggests recent groundwater recharge from precipitation.