Anthropogenic 236U in Danish Seawater: Global Fallout versus Reprocessing Discharge

This work focuses on the occurrence of U-236 in seawater along Danish coasts, which is the sole water-exchange region between the North Sea-Atlantic Ocean and the Baltic Sea. Seawater collected in 2013 and 2014 were analyzed for U-236 (as well as U-238 and 137Cs). Our results indicate that U-236 concentrations in Danish seawater are distributed within a relatively narrow range of (3.6-8.2) X 10(7) atom/L and, to a certain extent, independent of salinity. U-236/U-238 atomic ratios in Danish seawater are more than 4 times higher than the estimated global fallout value of lx 10(-9). The levels of U-236/U-238 atomic ratios obtained are comparable to those reported for the open North Sea and much higher than several other open oceans worldwide. This indicates that besides the global fallout input, the discharges from the two major European nuclear reprocessing plants are dominating sources of U-236 in Danish seawater. However, unexpectedly high U-236/U-238 ratios as well as high U-236 concentrations were observed at low-salinity locations of the Baltic Sea. While this feature might be interpreted as a clue for another significant U-236 input in the Baltic Sea, it may also be caused by the complexity of water currents or slow turnover rate.