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

This work focuses on the occurrence of (236)U 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 (236)U (as well as (238)U and (137)Cs). Our results indicate that (236)U concentrations in Danish seawater are distributed within a relatively narrow range of (3.6-8.2) × 10(7) atom/L and, to a certain extent, independent of salinity. (236)U/(238)U atomic ratios in Danish seawater are more than 4 times higher than the estimated global fallout value of 1× 10(-9). The levels of (236)U/(238)U 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 (236)U in Danish seawater. However, unexpectedly high (236)U/(238)U ratios as well as high (236)U concentrations were observed at low-salinity locations of the Baltic Sea. While this feature might be interpreted as a clue for another significant (236)U input in the Baltic Sea, it may also be caused by the complexity of water currents or slow turnover rate.