Abstract
Dispersed and unknown pollution sources complicate water management in large transboundary watersheds. We applied stable isotopes of water and nitrate together with contaminants of emerging concern (CECs: carbamazepine, caffeine, sulfamethoxazole, perfluorooctanoic acid and 2,4-dinitrophenol) to evaluate mixing and inputs of water and contaminants from tributaries into the mainstem of the transboundary Danube River. Stable isotope (δ18O, δ2H) variations from low values (− 13.3 ‰, − 95.1 ‰) in the Upper Danube after the Inn River confluence to high values (− 9.9 ‰, − 69.7 ‰) at the Danube River mouth revealed snowmelt dominated tributary mixing (~ 70%) in the mainstem. Stable isotopes of nitrate (δ15N-NO3) in the Danube River varied from lower values (+ 6.7 ‰) in the Upper Danube to higher values after the mixing with Morava River (+ 10.5 ‰) and showed that cold snowmelt can reduce biological activity and controls nitrate biotransformation processes in the mainstem up to 1000 km downstream. Data on emerging contaminants affirmed the low biodegradation potential of organic compounds transferred into the mainstem by tributaries. We found pollutant source tracing in large rivers is complicated by mixing of multiple sources with overlapping isotopic signatures, but additional tracers such as CECs improve the interpretation of hydrological processes (e.g., water transit time) and support tracing of nitrate pollution sources, and biogeochemical processes. Our approach can be applied to other watersheds to improve the understanding of dilution and mixing processes. Moreover, it provides directions for improving national and transboundary water quality monitoring networks.
Highlights
This study showed that a large river system has complex mixtures of water and nitrate sources which are often localized in origin and cannot be directly source traced, the larger riverine systems have the advantage of slower integrated responses to e.g., changes in hydrological conditions, which allows the application of stable isotopes to evaluate ranges of sources and processes
We found that tracers like CECs improved the interpretation of hydrological processes and supported the tracing of the nitrate sources, and the lack of biogeochemical processes
Our study represents a first-order synoptic approach that can be adapted to other large watersheds to better improve our understanding of dilution and mixing processes and to better focus remediation efforts, which are important for decision making regarding transboundary water management strategies
Summary
The aim of the study was to evaluate mixing and inputs of water and nitrate from tributaries to the mainstem of the transboundary Danube River and thereby to highlight directions for the improvement of national and transboundary water quality networks and monitoring
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