Diagenetic reactivity of the plutonium in marine anoxic sediments (Cumbrian mud patch - eastern Irish Sea)

¹ Laboratoire de Radioécologie de Cherbourg-Octeville, IRSN, 50130 Cherbourg-Octeville, France
² UMR CNRS 6143 Morphodynamique Continentale et Côtière, Université de Caen, 14000 Caen, France
³ Laboratoire de Chimie Analytique et Marine, FR CNRS 1818 et UMR CNRS 8013, Université de Lille, 59655 Villeneuve d'Ascq Cedex, France
⁴ Department of Exprimental Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁵ Laboratoire de Mesure de la Radioactivité de l'Environnement, IRSN, 91400 Orsay, France
⁶ Université du Littoral Côte d'Opale, UMR 8013 ELICO, 62930 Wimereux, France
⁷ The Centre for Environment, Fisheries & Aquaculture Science (CEFAS), Lowestoft, UK

Abstract

An extensive diagenetic study carried out on a sediment core collected in the Cumbrian mud patch off the Sellafield nuclear reprocessing plant, is presented. Sequential leaching data, using a thoroughly validated protocol specifically designed to prevent any resorption of the released plutonium in the course of the extraction, demonstrate that a significant proportion of the plutonium is loosely bound to sites that readily exchange with seawater, oxidise upon oxygenated water contact (reactive sulphides) or easily dissolve upon changes in pH. Such a result contradicts many previous sequential extraction studies which have reported that little of the plutonium in Irish Sea sediments is in a readily available form. The profile of dissolved plutonium in pore waters indicates an active uptake process at depth, probably linked to sulphide (Acid Volatile Sulphide) precipitation. These reactive sulphides are liable to act as source of plutonium to the overlying water if they are brought close the interface by bioturbation or in contact with oxygenated seawater by burrowing activity.