Transport of plutonium (239/240Pu) and caesium (137Cs) in the Irish Sea: comparison between observations and results from sediment and contaminant transport modelling

A model for radionuclide transport in the marine environment is described and applied, in the Irish Sea, to the movement of caesium (137Cs) and plutonium (239/240Pu) discharged from Sellafield. The model resolves processes operating on tidal time scales, whilst being simple enough to run over decadal time periods. Modelled processes include transport in the water column, exchange of contaminants between dissolved and particulate phases, wave–current sediment resuspension, sediment transport and mixing of material within the seabed. Transport in the water column is by a combination of tide, wind and density-driven flows. Sediment transport is based on the erosion, advection and deposition of three sediment classes representing sand, flocculated mud/silt and fine background components. Transfer of radionuclides between the dissolved and sediment phases is implemented using rate equations. A layered seabed is incorporated with transfers between layers representing biological and physical mixing processes. We demonstrate that the model is able to reproduce observed concentrations of 137Cs and 239/240Pu in the water column, the build-up of 239/240Pu in the seabed over a 30-year time span, including recent changes in seabed inventory distributions. The model is used to identify and quantify the mechanisms responsible for this recent redistribution of 239/240Pu in the region. The results suggest that whilst sediment transport plays a role in redistributing contaminated sediment in the eastern Irish Sea, desorption, followed by transport in the dissolved phase and reabsorption onto particulate material, is the primary mechanism for redistribution of 239/240Pu.