Nitrate stable isotopes provide information about nitrate contamination and cycling by microbial processes. The Fischa-Dagnitz (Austria) spring and river system in the agricultural catchment of the Vienna basin shows minor annual variance in nitrate concentrations. We measured nitrate isotopes (δ15N, δ18O) in the source spring and river up to the confluence with the Danube River (2019-2020) with chemical and water isotopes to assess mixing and nitrate transformation processes. The Fischa-Dagnitz spring showed almost stable nitrate concentration (3.3 ± 1.0 mg/l as NO3--N) year-round but surprisingly variable δ15N, δ18O-NO3- values ranging from +5.5 to +11.1‰ and from +0.5 to +8.1‰, respectively. The higher nitrate isotope values in summer were attributed to release of older denitrified water from the spring whose isotope signal was dampened downstream by mixing. A mixing model suggested denitrified groundwater contributed > 50 % of spring discharge at baseflow conditions. The isotopic composition of NO3- in the gaining streams was partly controlled by nitrification during autumn and winter months and assimilation during the growing season resulting in low and high δ15N-NO3- values, respectively. NO3- isotope variation helped disentangle denitrified groundwater inputs and biochemical cycling processes despite minor variation of NO3- concentration.
Keywords: Agricultural catchment; biogeochemistry; denitrification; groundwater; isotope hydrology; mixing; nitrate; nitrogen-15; oxygen-18.