Spatial distribution of Rare Earth Elements in a transnational watershed: The case of the Danube River
Résumé
The distribution of Rare Earth Elements was investigated along the Danube River (from its sources in Germany to its delta in Romania/Ukraine) and in some of its tributaries for the first time during the fourth Joint Danube Survey (JDS4) in 2019. The negative Ce anomaly and the La/Lu ratio tend to decrease between Ulm (Germany) and the Black Sea, indicating an increasing trend of the anthropogenic influence as the Danube River runs toward the sea. This diffuse influence is also attested by the slow spatial variation of classical pollution parameters such as dissolved organic carbon (amount and optical characteristics) and nitrates. A strong gadolinium (Gd) anomaly (between 6 and 10), due to the release of anthropogenic Gd, was detected starting 10 km downstream the sources of the Breg River and the Brigach River, whose junction forms the Danube River. The anomaly remains larger than 10 until the Danube River leaves Croatia at Ilok. The anomaly pattern is related to the difference in the use of magnetic resonance imaging (MRI) in the countries sharing the Danube River watershed. The estimated annual flux of anthropogenic gadolinium in the Black Sea is 0.9 ton/year. An Ytterbium anomaly was detected in stations close to the delta and needs further investigation.
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