Tracing biogeochemical processes and pollution sources with stable isotopes in river systems: Kamniška Bistrica, North Slovenia

Abstract. Biogeochemical processes were investigated in the Kamniška Bistrica River (Slovenia), which represents an ideal natural laboratory for studying pollution sources in catchments with high weathering capacity. The Kamniška Bistrica River water chemistry is dominated by HCO₃⁻, Ca²⁺, and Mg²⁺ and Ca²⁺/Mg²⁺ molar ratios indicate that calcite weathering is the major source of solutes to the river system. The Kamniška Bistrica River and its tributaries are oversaturated with respect to calcite and dolomite. pCO₂ concentrations were on average up to 25 times over atmospheric values. δ¹⁸O values in river water ranged from −10.4 to −7.7‰ and plotted near the local meteoric water line, δ¹³C_DIC values ranged from −12.7 to −2.7‰, controlled by biogeochemical processes in the catchment and within the stream; carbon dissolution is the most important biogeochemical process affecting carbon isotopes in the upstream portions of the catchment, while carbon dissolution and organic matter degradation control carbon isotope signatures downstream. Contributions of DIC from various biogeochemical processes were determined using steady state equations for different sampling seasons at the mouth of the Kamniška Bistrica River; results indicate that: (1) 1.9 to 2.2% of DIC came from exchange with atmospheric CO₂, (2) 0 to 27.5% of DIC came from degradation of organic matter, (3) 25.4 to 41.5% of DIC came from dissolution of carbonates, and (4) 33 to 85% of DIC came from tributaries. δ¹⁵N values of nitrate ranged from −5.2‰ at the headwater spring to 9.8‰ in the lower reaches. Higher δ¹⁵N values in the lower reaches of the river suggest anthropogenic pollution from agricultural activity.