The role of near-shore industrial waste releases in the dispersion of radionuclides in the NE Irish Sea

Abstract

Over the past 27 years, through the use of autoradiographic methods combined with field observations and laboratory studies, I have concluded that the behaviour and distribution of the α-active actinide radionuclides in the estuarine and marine sediments of the NE Irish Sea are significantly influenced by the releases of other non-radioactive industrial wastes. Since the 1700s, the various industrial activities in the Cumbrian coastal region have included: haematite mining, diverse non-ferric metal extraction industries, coal mining and a large number of blast furnaces for the manufacture of iron. More recently (1954–92), the Albright and Wilson phosphoric acid factory at Whitehaven, Cumbria, has discharged large quantities of phosphogypsum slurries into the NE Irish Sea. Iron wastes and slag products, together with phosphogypsum and its associated by-products containing the rare earth elements, are extremely reactive towards the actinides. These wastes are now slowly being removed from the region by natural processes following the rapid decline of heavy industry in the area. These wastes have been present since BNFL first started to discharge radionuclides into the NE Irish Sea and have not, so far, been considered in any models for the dispersion of radionuclides in the region. It is shown that sediments of the NE Irish Sea and local estuaries contain a significant part of the actinide content as coatings on two iron minerals, magnetite and haematite; there is also a significant diffuse distribution associated with hydrated iron oxides attached to quartz grains. However, not all magnetite and haematite grains from a given site show α-activity. Relative to the intensity of the α-activity of the constituent minerals in sediments, the two iron minerals can be regarded as hot particles and are associated with a further set of far more intense hot particles that either may be totally derived from BNFL Sellafield or may also include a contribution from the Albright and Wilson effluents. The retention of actinides in sediments of the NE Irish Sea is believed to be influenced by debris derived from redox recycling. Redox debris can be distributed to depth in sediments through bioturbation. For surface sediments, the distribution is likely to be controlled by sediment mobility. Actinide-enriched magnetite and haematite placer deposits are common in the region, especially in sands. Fractionation and differentiation of iron minerals takes place within the sands as a consequence of energy inputs associated, for example, with tides and storms. Historic data for NE Irish Sea and estuarine sediments provide evidence for a change in facies from muds to sandy silts and a decrease in the concentration of iron over the past 30 years. This will influence the potential behaviour and distribution of actinides in the sediments irrespective of any reductions in discharges. The worst scenario would be continued loss of fine-grained material from the NE Irish Sea and the selective retention of iron minerals and related redox debris, such that the inventory of actinides present in the NE Irish Sea (∼95% of that discharged) becomes associated with surficial sediments. At present, evidence to support this hypothesis is meagre, but it should not be ignored as the iron mineral re-concentration processes are already well-developed.