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Abstract. Intertidal sediments from a range of depositional environments in the eastern Irish Sea have been analysed with regard to their radionuclide content, ...
R E L A T I O N S H I P S B E T W E E N R A D I O N U C L I D E C O N T E N T AND T E X T U R A L P R O P E R T I E S IN I R I S H SEA I N T E R T I D A L S E D I M E N T S

J. CLIFTON ~: P. McDONALD t A. PLATER : and F. OLDFIELD : Princess Royal Building, Westlakes Research Institute, Moor Row, Cumbria CA24 3LN, U.K. : Department of Geography, University of Liverpool, PO Box 147, Liverpool L69 3BX, U.K.

Abstract. Intertidalsediments from a range of depositional environmentsin the eastern Irish Sea have been analysed with regard to their radionuclide content, particle size distribution and magnetic properties. Concentrations of 2akAmand I~Cs are highly influencedby the abundanceof sediment finer than 32btm in fine-grained sedimentaryenvironments,whilst radionuclideactivity in coarser sediments is less dependenton particle size. Investigationof the magnetic properties of these sediments highlights a similar association between this size fraction and magnetic remanence,the latter being shown to offer strong potential as a grain size proxy in monitoring ~37Csconcentrationsin fine-grainedsediments. Analysis of particle size fractions indicates the extent to which ~37Csmay be enriched in the clay size fractionsof both fine and coarse sediments.

Key words: americium-241,caesium-137, ffactionation, grain size. magnetic properties, normalisation, Sellafield

1. Introduction

Low-level radioactive liquid waste has been discharged to the Irish Sea from the British Nuclear Fuels (BNFL) nuclear reprocessing plant at Sellafield in Cumbria since 1952 (Gray et al., 1995). Particle-reactive isotopes such as americium-241 (24~Am, halflife=432.7y) tend to adsorb rapidly onto sediment following discharge, whilst isotopes of the alkali metals such as caesium-137 ('37Cs, half-life=30.2y) remain predominantly in the dissolved ionic state in saline water and undergo wider dispersion by tidal currents. Both radionuclides, in common with trace metals, exhibit preferential sorption with respect to fine grained sediment (Carpenter et al., 1991; Livens & Baxter, 1988); the greater surface area of fine particles enhancing the extent to which radionuclide adsorption may take place. Micaceous minerals such as illite and coatings of organic matter and sesquioxides in fine sediment fractions serve to enhance the sorption of ~37Cs and : ~ A m (Comans & Hockley; 1992, McDonald et al., 1990). A range of techniques have been developed to minimise particle size-induced variations in radionuclide content in order to elucidate the underlying spatial and temporal trends in activity. These have traditionally included normalising radionuclide data to the abundance of a specific particle size class such as the fraction finer than 63gm (Assinder et al., 1993), although fractions such as the percentage less than 75/am and 50btm have been used (Aston & Stanners, 1982; Clifton & Hamilton, 1982). Physical separation of particle size fractions by settling techniques has been employed to identify the dominant fraction contributing to bulk sample activity (Ramsay & Raw, 1987).

Water, Air and Soil Pollution 99: 209-216, 1997. 9 1997 Kluwer Academic Publishers. Printed in the Netherlands.

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J. CLIFTON, E T AL.

Alternative methods make use of sediment properties which are dependent upon particle size. The abundance of elements such as aluminium (AI) or lithium (Li), which are structurally incorporated within clay minerals, has been utilised to reflect the presence of fine sediment in studies of trace metal pollution (Daskalakis & O'Connor, 1995; Loring, 1991). Investigation of the magnetic properties of sediments has indicated significant associations with the presence of fine mineral sediments 'Bonnett et al., 1988; Oldfield et al., 1993). This study undertakes detailed analysis of the composition and radionuctide content of a wide variety of sediments from the eastern Irish Sea in order to identify the correct grain size class to utilise in normalisation procedures. The potential of magnetic properties to act as a grain size proxy measurement in radionuclide monitoring will be investigated. The effectiveness of particle size separation techniques and the nature of radionuclide concentration in size fractions will be examined.

2. Methodology

Surface intertidal sediments (0-1 cm depth) were collected using a plastic trowel from 57

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