Anthropogenic 236U in the North Sea - A Closer Look into a Source Region

In this study we present new seawater data of U-236 and U-238 sampled in the North Sea in 2010. The North Sea has been and is still receiving a considerable input of anthropogenic radionuclides from nuclear reprocessing facilities located in La Hague (France) and Sellafield (Great Britain). It therefore represents an important source region for oceanographic tracer studies using the transient signal of anthropogenic U-236. A proper knowledge of the sources of U-236 is an essential prerequisite for such tracer studies. The U-236 data set presented in this study covers the transition regions of the North Sea to the Atlantic Ocean, to the Baltic Sea, and upstream the Elbe River. It is discussed in the context of available U-236 data from the literature. Our results show that both U-236 concentrations U-236/U-238 ratios in surface waters of the North Sea can be explained by simple binary mixing models implying that U-236 behaves conservatively in seawater. We further show that the input of U-236 by the Elbe River is negligible, while there might be a maximum input of 12 g/yr via the Baltic Sea. The results of the mixing models suggest that this still unidentified U-236 contamination could be supplied by fresh water input.