The distribution and transport of Sellafield derived 137Cs and 241Am to western Irish Sea sediments

Sellafield derived 137Cs and 241Am were analysed in surficial sediments at 23 sites in the western Irish Sea. Concentrations varied between 3–175 Bq 137Cs/kg and 1–147 Bq 241Am/kg. There are two distinct basins in the western Irish Sea separated by an area of restricted depth and this bathymetry is reflected in the radionuclide concentrations. Highest concentrations were found in the northerly samples and substantially lower concentrations further south. This pattern of distribution can be explained by the predominantly northerly residual flow and the existence of seasonal stratification in this area of weak tides. 137Cs/241Am ratios were also highest in the northern basin, and it is suggested that this is caused by differences in the mechanisms of transport of the two elements. The direct physical migration of contaminated particles from the eastern Irish Sea is the dominant transport mechanism in the north of the study area, whereas transport in the dissolved phase is most important to the southern region. Concentrations of both elements were best correlated with the percentage <15 μm particulate fraction suggesting that particles within this range are most important for direct transport and scavenging from the soluble phase.