Studies of actinides in a superanoxic fjord

Abstract: Water column and sediment profiles of plutonium, americium, thorium and uranium have been obtained in the superanoxic Framvaren fjord, southern Norway. The concentrations of bomb test fallout Pu, Am as well as 'dissolved' Th in the bottom water are the highest recorded in the marine environment. The behaviour of the actinides in the anoxic water mass is to a large extent governed by the behaviour of the colloidal material, probably consisting of humic substances. Ultrafiltration reveals that 40-60% of the actinides are associated to the large colloids (10kD-0.2um), surprisingly this is valid also for uranium. Determination of oxidation state by chemical methods show that plutonium exist almost entirely as a trivalent species, while uranium to at least 40% exist as U(IV). The uranium in the 10 kD permeate fraction was to a somewhat lesser extent in the reduced state indicating uranium to be stabilised in the reduced valence state by complex formation with humic colloids. The sediments acts as a source for Pu, A