Studies of anoxiC conditions in Framvaren fjord, Gullmaren fjord and Byfjorden and of mixing between seawater and freshwater at the Kalix river and estuary

The sediments in the anoxic Framvaren fjord acts as a source for actinides to the overlaying water column. The remobilisation process is most likely linked to early diagenetic alteration of the marine organic material in the sediments. This is indicated by the close correlation between Pu, Am and dissolved organic carbon depth profiles in the water column. Speciation studies of the plutonium and americium in the water column shows that both to a large degree are associated to colloidal material in the size range 0.01-0.45 μm. Less than 2% is retained by a 0.45 μm filter which is reflected in the low KD-values obtained of about 20 000, which is at least a factor of 10 lower than in typical coastal waters. It is also proven that the plutonium exist almost entirely in the trivalent state in the anoxic water column. This study is the first ever to show extensive remobilisation of plutonium and americium from sediments in anoxic marine basins. Similar remobilisation from sediments most likely occur in other anoxic marine waters where early diagenesis results in humic and fulvic acid production. Although the remobilised actinides in the Framvaren fjord at present don't pose any radiological hazard due to the lack of fish in anoxic waters, it is of great concern to identify processes involved in the remobilisation of actinides from anoxic sediments as such sediments likely will be a major source for actinides in the Baltic Sea and other oxygen sensitive basins in the long term perspective. In such basins the remobilised plutonium may reach oxygenated and biological productive waters by convection. Results from the temporarily oxygen deficient Gullmaren fjord on the Swedish west coast shows that remobilisation from sediments can not be identified during short (a few months) periods of oxygen deficient water. The rapid bioturbation (quantified by tracer studies) in this fjord results in that sedimenting organic material rapidly is buried and distributed within the upper 10-15 cm of sediments. This means that the organic material is diluted with large amounts of inorganic material which may act as traps for the mobilised actinides. It is likely that anoxic conditions must persist for a long enough time to allow organic rich sediment to accumulate in sufficient amounts before substantial amount of humics are produced that may mobilise the actinides. This time depends on the organic matter flux to the sediments. Laminated sediments and suspended matter collected in the estuary of the Kalix river shows that the input of plutonium to the Baltic Sea from river run-off at present is of rather small importance but may in the long term perspective become one of the more important sources as the residence time of plutonium in the the Baltic Sea water column is much shorter than the residence time in the river drainage basins. Drainage basins containing large percentage of mire and wetland will be of particular importance. (EHS, LN)