Spatial and seasonal variations in δ18O and δD values in the River Sava in Slovenia

Stable isotope measurements of oxygen and hydrogen (delta O-18 and delta D) in stream waters and precipitation were used to investigate hydrological pathways and residence times in the River Sava catchment in Slovenia. delta O-18 and delta D values of precipitation exhibited strong seasonal variations. Precipitation is of mixed Atlantic-Mediterranean origin which is also reflected in the isotopic composition of River Sava surface waters. The isotopic content of the river water is dependent on temperature, elevation of the recharge area and amount of precipitation. Spatially, rivers water is O-16-enriched in the high mountain areas due to higher amount of precipitation, lower temperature and higher elevation of the recharge area, white in the tower part of the River Sava catchments the delta O-18 values are higher. It is also observed that the River Sava responds quickly to precipitation which is reflected in its delta O-18 and delta D values and the low residence time estimated in the river. Using an exponential flow model it was calculated that mean stream water residence times varied between 0.4 and 2.1 years. The shortest residence time determined at Savica is connected mainly to the higher amount of precipitation in the upper Sava watershed and the mountains carbonate karst terrain characterized by a higher runoff. The mean residence time in the main stream water of the River Sava was estimated to be 1.32 years and is in a good agreement with the residence time determined using H-3 measurements. These data extend our knowledge in understanding the hydrological cycle of the River Sava, the interactions between precipitation, surface water and groundwater and, at the same time, demonstrate the utility of isotope tracers in determining catchments characteristics. (C) 2008 Elsevier B.V. All rights reserved.