Hydrology shapes the Danube River pollution in Vienna

The Danube at Vienna drains a catchment of ~100,000 km2 and is an alpine river with an average discharge of 1900 m3/s that can vary by orders of magnitude. Flood events commonly result from snowmelt during the spring and early summer, heavy rainfalls during late summer and autumn, and ice burst events during winter. The most important tributary is the Inn, draining a large portion of the Austrian Alps. This catchment consists of the northern Calcareous Alps, the Palaeozoic Greywacke zone, and the Crystalline zone; elevations range from 310 to 3800 m. The mean discharge of the Inn at the confluence is 750 m3/s. Downstream of the confluence, southern tributaries from high rainfall areas in the Calcareous Alps and northern tributaries from areas with less rainfall and granitic geology enter the Danube. The Danube catchment is not only geologically, topographically, and climatologically diverse, but also contains diverse land use: forests; intensive and extensive agriculture sites; industrial and urbanized centres. Along the Inn, agricultural activity is focused on the Inn valley. Flatter regions upstream of Vienna, are used for crops, horticulture, and intensive livestock farming. Nitrate concentrations in the groundwater of these areas is often near the Austrian drinking water limit of 50 mg/L. The International Commission for the Protection of the Danube carries out synoptic sampling campaigns every few years. Water stable isotopes trace water origin and mixing. Nitrogen and oxygen isotopes of nitrate together with compounds of emerging concern (CECs) can delineate pollution sources and biogeochemical cycling processes. Using these tools revealed that tributaries contributed nitrate from different sources and CECs, while the mainstem generally mixed and diluted these contributions. Snowmelt derived water fractions from the Inn catchment, controlled Danube water chemistry but also diluted pollutants and influenced nitrate processes. An international study on CECs documented high CEC cumulative concentrations in the Vienna Danube. These results, however, also included samples taken downstream from the Vienna Wastewater treatment plant.None of these studies have investigated the temporal component, considering the highly variable flow regime of the upper Danube. Here we present the results of a monitoring program carried out in collaboration with a public high school located on the Vienna Danube. We took monthly field samples of the Danube together with the pupils and discussed the results. The hydrological year described here was exceptional from a historical point of view but might show how hydroclimatic circumstances could emerge in a global warming scenario: A very dry and warm winter in the Alps was reflected in isotope ratios and CEC concentrations from November-April. Variability in the isotopic ratios and CECs increased in May likely due to higher contributions from the Alps, as compared to local groundwater. A recorded flooding event in late summer showed completely different pattern with regards to CECs and isotopes. Some CECs were diluted, while previously not detected ones appeared. Besides the importance of monitoring to understand the impacts of an accelerated hydrological cycle on river, this study shows the fruitful integration of schools into environmental monitoring.