OBSERVATIONS ON PLUTONIUM IN THE OCEANS

This review describes various studies of marine plutonium distributions, behaviour and transfer carried out at the Marine Environment Laboratory of IAEA. Plutonium in sea water is primarily present in solution as Pu(V), probably as PuO2+, and on particles in reduced form as Pu(IV), most likely as species such as adsorbed Pu(OH)(4). In nearshore sediments and coastal soils, plutonium is complexed with high molecular weight humic and low molecular weight fulvic compounds. Results of oxidation state studies in the Black Sea and Norwegian fjords show that major reduction of dissolved Pu(V) occurs [to Pu(III)] in anoxic waters in association with complexation of reduced plutonium with these dissolved and colloidal humics and fulvics. The IAEA-MEL has also been active in developing and applying laboratory radiotracer methods to study rates and pathways of plutonium flux through marine organisms, particularly using the gamma-emitting tracer, Pu-237. Plutonium concentration factors spanning five orders of magnitude have been observed, with no significant dependence on oxidation state. Almost complete excretion of ingested plutonium by zooplankton makes their defecation an extremely important mechanism in the scavenging of plutonium from surface waters. This phenomenon has been well documentated in many field studies using time-series sediment traps to quantify and characterize elemental flux. Plutonium fluxes in the northwestern Mediterranean have recently been shown to correlate well with mass flux and suggest residence times of 24 and 3.5 years, respectively, for Pu-239,Pu-240 and Am-241 in the upper mixed layers, values which are rather longer than those measured previously. The IAEA-MEL has recently studied plutonium distributions in the vicinity of former waste dumpsites in the NE Atlantic Ocean and in the Kara Sea. The results of these surveys provide evidence of radiologically negligible but measurable leakage at the former site and of no leakage whatsoever at the latter. Computer predictions of potential future dispersion of plutonium-containing wastes from the Kara Sea suggest that the maximum possible global collective dose (30 manSv) would be extremely low. Finally, the review notes the increasing analytical contribution to environmental plutonium studies which can be made by mass-spectrometric techniques. At IAEA-MEL, the ETV-ICPMS technique along with low-level radiometric methods is being refined in order to maximize the possibility of using aquatic isotopic signals to characterize and identify nuclear source-terms.