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

Abstract 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.

[1]  C. B. Vreugdenhil,et al.  A depth-integrated model for suspended sediment transport , 1985 .

[2]  M. G. Lyons,et al.  Developments in the Cumbria Model and its Application to Radiological Assessment in the Irish Sea , 1998 .

[3]  A. M. Davies,et al.  The response of the Irish Sea to boundary and wind forcing: Results from a three‐dimensional hydrodynamic model , 1994 .

[4]  I. Young,et al.  The growth of fetch limited waves in water of finite depth. Part 1. Total energy and peak frequency , 1996 .

[5]  D. M. Nelson,et al.  Measurements of the oxidation state and concentration of plutonium in interstitial waters of the Irish Sea , 1980 .

[6]  D. Mccubbin,et al.  Photochemical dissolution of radionuclides from marine sediment , 1996 .

[7]  José-María Abril,et al.  A 2D 4-phases marine dispersion model for non-conservative radionuclides. Part 1: Conceptual and computational model , 1993 .

[8]  D. Swift,et al.  Evidence of recent sedimentation in the eastern Irish Sea , 1988 .

[9]  J. Durbec,et al.  A New Approach for the Modelling of Sediment Reworking Induced by a Macrobenthic Community , 1997 .

[10]  Vladimir S. Maderich,et al.  THREETOX - A Computer Code to Simulate Three-Dimensional Dispersion of Radionuclides in Stratified Water Bodies , 1997 .

[11]  L. R. Johnson.  The transport mechanisms of clay and fine silt in the north Irish Sea , 1983 .

[12]  J. Aldridge.  Hydrodynamic Model Predictions of Tidal Asymmetry and Observed Sediment Transport Paths in Morecambe Bay , 1997 .

[13]  I. Young,et al.  The growth of fetch limited waves in water of finite depth. Part 2. Spectral evolution , 1996 .

[14]  P. Smolarkiewicz.  A Fully Multidimensional Positive Definite Advection Transport Algorithm with Small Implicit Diffusion , 1984 .

[15]  Jørgen Fredsøe,et al.  Turbulent Boundary Layer in Wave‐current Motion , 1984 .

[16]  A. Ansell,et al.  Sediment bioturbation by the echiuran worm Maxmuelleria lankesteri (Herdman) and its consequences for radionuclide dispersal in Irish Sea sediments , 1996 .

[17]  A. Steele,et al.  Further studies on the distribution of 137Cs in British coastal waters—I. Irish Sea , 1982 .

[18]  L. León Vintró,et al.  Trends in plutonium, americium and radiocaesium accumulation and long-term bioavailability in the western Irish Sea mud basin , 1999 .

[19]  Lawrence P. Sanford,et al.  Assessing the paradigm of mutually exclusive erosion and deposition of mud, with examples from upper Chesapeake Bay , 1993 .

[20]  E. Fraga,et al.  Some physical and chemical features of the variability of kd distribution coefficients for radionuclides , 1996 .

[21]  R. Periáñez,et al.  Three-dimensional modelling of the tidal dispersion of non-conservative radionuclides in the marine environment. Application to 239,240Pu dispersion in the eastern Irish Sea , 1999 .

[22]  P. McDonald,et al.  The influence of mixing timescales and re-dissolution processes on the distribution of radionuclides in Northeast Irish Sea sediments , 1998 .

[23]  Raúl Periáñez,et al.  Modelling the dispersion of non-conservative radionuclides in tidal waters—Part 1: Conceptual and mathematical model , 1996 .

[24]  A. M. Davies,et al.  A numerical model study of parameters influencing tidal currents in the Irish Sea , 1993 .

[25]  I. Pulford,et al.  Sediment radionuclide profiles: Implications for mechanisms of Sellafield waste dispersal in the Irish Sea , 1994 .

[26]  Helen Mitchener,et al.  Erosion of mud/sand mixtures , 1996 .

[27]  E. Gmitrowicz,et al.  Observations of the transverse structure and dynamics of the low frequency flow through the North Channel of the Irish Sea , 1995 .

[28]  L. Fernand,et al.  The summer gyre in the Western Irish Sea: Shelf sea paradigms and management implications , 1997 .

[29]  D. Myrhaug,et al.  Wave-current interaction within and outside the bottom boundary layer , 1993 .

[30]  Spectral Estimates of Bed Shear Stress at Subcritical Reynolds Numbers in a Tidal Boundary Layer , 1992 .

[31]  L. Fernand,et al.  Seasonal evolution of the cold pool gyre in the western Irish Sea , 2000 .

[32]  R. Pingree,et al.  Sand transport paths around the British Isles resulting from M 2 and M 4 tidal interactions , 1979 .

[33]  P. Blowers,et al.  Dissolved plutonium and americium in surface waters of the Irish Sea, 1973–1996 , 1999 .

[34]  D. Swift.  The macrobenthic infauna off Sellafield (north-eastern irish sea) with special reference to bioturbation , 1993, Journal of the Marine Biological Association of the United Kingdom.

[35]  S. Hawkins,et al.  Benthic disturbance by fishing gear in the Irish Sea: a comparison of beam trawling and scallop dredging , 1996 .

[36]  P Ciffroy,et al.  Kinetics of the adsorption and desorption of radionuclides of Co, Mn, Cs, Fe, Ag and Cd in freshwater systems: experimental and modelling approaches. , 2001, Journal of environmental radioactivity.

[37]  J. Abril,et al.  A 2D 4-phases marine dispersion model for non-conservative radionuclides. Part 2: Two applications , 1993 .

[38]  T. Barnett,et al.  Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP) , 1973 .

[39]  J. N. Aldridge.  CSERAM: A Model for Prediction of Marine Radionclide Transport in both Particulate and Dissolved Phases , 1998 .

[40]  R. Luettich,et al.  Dynamic behavior of suspended sediment concentrations in a shallow lake perturbed by episodic wind events , 1990 .

[41]  P. Kershaw,et al.  Observations on the redistribution of plutonium and americium in the Irish Sea sediments, 1978 to 1996: concentrations and inventories , 1999 .

[42]  J. Simpson,et al.  The relationship between concentrations of suspended particulate material and tidal processes in the Irish Sea , 1993 .

[43]  S. Boudjelas,et al.  The distribution of fine suspended sediments in the surface waters of the Irish Sea and its relation to tidal stirring , 1998 .

[44]  R. Periáñez.  Three-dimensional modelling of the tide-induced dispersion of radionuclides in the sea , 1998 .