Stable isotopes of water, dissolved inorganic carbon and nitrate in the Danube River – results from the Joint Danube Survey in 2025

Within the frame of the Joint Danube Survey 5 in July 2025, we explore the source and fate of water and nutrients (namely carbon, nitrate, phosphate and sulphate) along the Danube river, major tributaries and selected groundwater wells using stable isotope techniques.

Both hydrogen and oxygen isotope composition of the water (²H-H₂O and ¹⁸O-H₂O) were determined using a laser-based isotope analyser (Picarro 2140i). Among the nutrients, we measured carbon isotope composition of dissolved inorganic carbon (¹³C-DIC) and the oxygen and nitrogen isotope composition of nitrate (¹⁸O-NO₃ and ¹⁵N-NO₃) by Gasbench- and a Precon-Isotope Ratio Mass Spectrometer (MAT253Plus, Thermo Fisher Scientific) after H₃PO₄ addition and Ti(III)reduction respectively.

The δ²H-H₂O and δ¹⁸O-H₂O values range from -88.7‰ to -25.0‰ and from -12.4‰ to -7.5‰. The tributary Inn has the lowest isotope value of all samples and is the main water source even further downstream. After the Inn-Danube confluence the isotope values continuously increase along the river length due to influence of other tributaries with comparatively higher isotope values. Groundwater samples have similar isotope values compared to river water indicating surface water - groundwater interactions. The δ¹³C-DIC values also increase with the distance from the source to the sea from -11 ‰ to around -9‰, but the increase levels off after around 1200 km and remains constant between -8 and -9‰. The tributary Inn is high (-8.5‰) in contrast to the Danube (-10.5‰) before the confluence. Low δ¹³C-DIC values in tributaries are only seen for the Morawa and Ipel. Distinct lower and higher values of δ¹³C-DIC are found downstream of Vienna and Budapest respectively. DIC values well separated carbonate dissolution and photosynthesis dominated versus organic matter mineralisation driven systems. Preliminary results of δ¹⁵N-NO₃ and δ¹⁸O-NO₃ values indicate various sources of nitrate e.g. relevant wastewater input downstream of the major cities; low δ¹⁵N-NO₃ values in Lim and Inn tributaries indicating a higher proportion of soil derived nitrogen; and distinct higher δ¹⁸O-NO₃ values common for NO₃ fertilizers in the tributary Vah.

We gain a first glimpse on the source and fate of water and nutrients in the Danube river. Pending measurements of sulphate and phosphate isotopes, implementation of mixing models and comparison with earlier surveys will complement these findings and increase its relevance.