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New data are presented which highlight to the behaviour of Pu upon discharge into the open sea from nuclear reprocessing plants at Sellafield and Cap La Hague and Sellafield. Specifically, certain physico-chemical characteristics of Pu in sea water and sea-bed sediments are shown to be independent of both the source and near-field environmental conditions. For example, in filtered sea water the discharge-normalized Pu concentrations are similar at both sites (ca. 20-40μΒςΓ' per GBq discharged). The oxidation state distribution of Pu is also similar, with Pu(V,VI) predominating. A significant fraction of the Pu(III,IV) present in filtered water is associated with > 1 kDa colloids. In each zone, tide energy would appear to control the long-range dispersion of Pu in both the filtered and particulate fractions.

Several sampling campaigns were carried out in the Seine estuary in order to study the behaviour of plutonium isotopes versus salinity ( 23 "Pu and 239 ' 240 Pu), in 0.45 μηι filtered waters. During estuarine mixing, the plutonium has a non-conservative behaviour: plots of Pu activities versus salinity display an excess of dissolved 238 Pu (RI = «"p u / 2! " 4°P u > 1) in low salinity waters (0.5 and 1.5 g Γ 1 ) and a depletion (RI < 1) in the zone of intermediate salinities (8 to 15 g Γ')> with respect to conservative dilution.Laboratory experiments showed that Pu(V) present in seawater is reduced to the III, IV states and sorbed onto particles with a maximum in low salinity waters which could explain the depletion observed at intermediate salinity in the Seine estuary. Reduction occurs prior to sorption and is connected with the Seine river colloidal and dissolved species (humic acids). The Pu associated with estuarine sediments desorbs from the "oxidehydroxide" fraction in contact with river water, thus partly explains the plutonium excess measured in low salinity filtered waters of the Seine.

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