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Hydrochemical and isotope study of the upper part of the Kupa river drainage area - Croatia

Upper part of the Kupa river drainage area is typical karst terrain developed on part of the Dinaric carbonate platform. The terrain is made of rocks from under layered Palaeozoic deposits ; Mesozoic and Tertiary carbonate rocks, limestones and dolomites. The complex structural relations were formed as a consequence of subduction of the African under the European plate.Results are elements of overthrust tectonics that have special roles in shallow zones important for underground water flow paths. Main characteristics of the region is a presence of the large water dividing zones between the Adriatic sea and the Black sea catchments, which is situated in the mountain area of the Risnjak massive. That area is very rich in precipitation what together with lithologic and structural characteristics cause the presence of the large groundwater reserves and few large karst springs in the Kupa river drainage area: Čabranka spring, Kupa spring, Zamost, Velika and Mala Belica, Kupica spring and Zeleni Vir spring. Also, on the other side of the mountain massive, in the Adriatic Sea catchment, is a large perennial spring of the Rječina river outflow. That spring is located at the same altitude as the spring of Kupa river. But, annual precipitation regime with long summer dry period has influence on high groundwater recharges amplitude of those springs. During the 1997, 1998 and 1999, in different annual vegetation and hydrologic conditions were performed spring water sampling for hydrochemical and isotopic analysis and measurements of temperature, conductivity, pH and TDS on mentioned large springs and on the Rječina spring (Adriatic sea catchments). Hydrogeochemical facies of the study area mainly vary from Ca-HCO3, CaMg-HCO3 to MgCa-HCO3 main ionic composition as a result of dissolution of the carbonate rocks, limestones and dolomites. The presence of Palaeozoic clastic deposits in the drainage area of some springs causes increasing of magnesium, some heavy metals, non-metals, transition elements, actinide and lanthanide elements (Kupica spring, Zeleni Vir). Distribution of stable isotopes and tritium in the spring waters suggest that in that region, were distinguished a four different sub-catchments. The first one is the group of Rječina, Kupa and Kupari springs, which have, practically unique catchments area, situated in the Risnjak Mountain. The second group of the springs, Zamost, V. Belica, M. Belica and Kupica have the similar catchments situated in the middle part of the study area and they form tributaries of the north bank of the Kupa river upper part. Čabranka spring and Zeleni Vir spring drainage areas cover the larger part of terrains which borders are situated far from the springs. The Čabranka catchments is in the Slovenian mountains at higher altitudes while Zeleni Vir catchments lay deeply in the inland part of the Gorski Kotar region. Tritium activity of the spring waters corresponds with tritium activity of the recent precipitation of the study area. The high water potential suitable for the water supply is storaged in the deep karstified underground of the study area.

hydrochemistry; stable isotopes; Kupa river

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