Isotopic and Chemical Constraints on the Biogeochemistry of Dissolved Inorganic Carbon and Chemical Weathering in the Karst Watershed of Krka River (Slovenia)

The hydrogeochemical and carbon isotope characteristics of the Krka River, Slovenia, were investigated to estimate the carbon transfer from the land ecosystem in the watershed. During the 3-year sampling period (2008–2010), temperature, pH, electrical conductivity, major ion content, dissolved inorganic carbon (DIC) and dissolved organic carbon content, and the isotopic composition of DIC (δ13CDIC) were monitored in the main stream of the Krka River and its tributaries. The major solute composition of analysed waters is dominated by an input of HCO3−, Ca2+ and Mg2+ originating from carbonate dissolution. The Mg2+/Ca2+ and Mg2+/HCO3− molar ratio values ranging from 0.24 to 0.71 and 0.05 to 0.30, respectively, indicate a high degree of dolomite dissolution relative to calcite. Dissolved CO2 concentrations in the river were up to tenfold supersaturated relative to the atmosphere, resulting in supersaturation with respect to calcite and degassing of CO2 downstream. The δ13C values in river water range from −15.6 to −9.4 ‰ and are controlled by the input of tributaries, exchange with atmospheric CO2, degradation of organic matter, and dissolution of carbonates. The mass balance calculations for riverine DIC suggest that the contribution from carbonate dissolution and degradation of organic matter have major influence, whereas the exchange with atmospheric CO2 has minor influence on the inorganic carbon pool in the Krka River.