Accumulation, solid partitioning and remobilisation of (9(9Tc in subtidal and intertidal sediments in the Irish Sea

• Published in: Continental shelf research Vol. 29; no. 16; pp. 1995 - 2010
• Main Authors: Finegan, P, Vintro, L L, Mitchell, P I, Boust, D, Gouzy, A, Kershaw, P J, Lucey, J A
• Format: Journal Article
• Language: English
• Published: 15.09.2009
• ISSN: 0278-4343
• DOI: 10.1016/j.csr.2009.02.005

Recent studies have shown that accumulation of (9(9Tc in seabed sediments labelled by authorised radioactive liquid discharges into the NE Irish Sea from the Sellafield reprocessing complex is greater than previously thought. In this paper, new data on (9(9Tc concentration profiles in subtidal and intertidal sediments from the eastern and western Irish Sea are provided with a view to elucidating the processes responsible for the incorporation and retention of (9(9Tc in the seabed. The data show that substantial amounts of (9(9Tc have accumulated in the fine-grained subtidal sediments off the Cumbrian coast, particularly after increased releases from Sellafield following the commissioning of the Enhanced Actinide Removal Plant (EARP) in 1994. In all the cores taken in this area, (9(9Tc has been found to be present to depths in excess of 30cm. Analysis of (1(3(7)Cs and (2(4(1)Am profiles, together with other supporting geochemical data, show a high degree of homogenisation of the sediments down to these depths as a result of physical and biological processes, and confirm that incorporation of (9(9Tc into the sediment compartment is actually the result of mixing and reworking, rather than active sediment accumulation. In contrast, active deposition of material transported from this mixed pool of sediment appears to be the dominant mechanism controlling (9(9Tc profiles in intertidal areas close to the Sellafield discharge outfall. Data obtained from the analysis of subtidal sediment cores from the western Irish Sea mud basin suggest that similar mixing processes to those occurring in the subtidal sediments of the eastern Irish Sea are also active in this area. Time-series data on (9(9Tc concentrations in surficial sediments from this basin, gathered in the period 1988-2004, inclusive, show a clear increase in concentrations, by a factor of 2, between samples collected pre-EARP and post-EARP. The constancy of (9(9Tc concentrations in surface sediments throughout the 1980s and the early-1990s suggests that little redissolution and export of (9(9Tc occurred over this extended period. A similar observation applies to the post-EARP period, when concentrations remained relatively constant despite the reported steady decrease in (9(9Tc concentrations in the overlying waters. This apparent lack of remobilisation is consistent with data from sequential extraction analyses, which indicate that the bulk of the (9(9Tc is strongly bound to non-labile geochemical phases, with only a small proportion associated with exchangeable and acido-soluble phases. Further, these analyses show that (9(9Tc is not associated with oxygen-sensitive and highly-reactive acid-volatile sulphides (AVS) to any significant extent.