Modelling radioactivity dispersion in the Alborán Sea, western Mediterranean

Abstract

A numerical model which simulates the dispersion of non-conservative radionuclides in the Alboran Sea has been developed. Water currents are obtained from two hydrodynamic models working on the same domain: a 2D depth-averaged model is used two calculate tides and a 2-layer has been applied to obtain the residual (mean) circulation. The output of the hydrodynamic models is first applied to calculate suspended particle concentrations and sedimentation rates over the domain, which are also required by the radionuclide transport model. This assumes that radionuclides may be present in three phases: dissolved, suspended matter and bed sediments. Exchanges between the liquid and solid phases are described through kinetic transfer coefficients. The model has been applied to simulate the dispersion of 137-Cs and 239,240- Pu introduced in the sea from atmospheric fallout. Measured and computed radionuclide concentrations in the water column and in bed sediments are, in general, in good agreement. The Alboran Sea is the only connection between the Atlantic Ocean and the Mediterranean Sea and has a high ecological and economic interest. Moreover, it is one of the most productive areas of the whole Mediterranean Sea. Thus, a considerable effort has been made in the last years to study sedimentation processes in the Alboran Sea, radionuclide distributions in waters and sediments, and metal and radionuclide fluxes through the Strait of Gibraltar since they affect the biogeochemistry of the entire Mediterranean Sea. These processes are studied by means of numerical modelling, for the first time, in the present work. The water circulation is studied by means of two models: a depth-averaged model is used to calculate tides and a two-layer model is applied to obtain the geostrophic (mean) flow. Computed currents are used to calculate sediment transport and sedimentation rates over the area. Then, results of the hydrodynamic and sediment transport models are used to simulate the dispersion of 137 Cs and 239,240 Pu. A mathematical description of the interactions of dissolved radionuclides with the solid phases (suspended matter and bed sediments) in a two-layered sea has been developed. Uptake/release reactions have been described by means of kinetic transfer coefficients. Computed concentrations of radionuclides in the water column and in the bed sediments have been compared with measurements in the Alboran Sea. Both set of data are, in general, in good agreement. In particular, the model predicts very different distributions in the bed sediment depending on the geochemical behaviour of the radionuclide. The computed partition of plutonium between water and suspended sediments is also correctly calculated by the model. Radionuclide inventories in the Alboran Sea and fluxes through the Strait of Gibraltar have been calculated and compared with previous experimental estimates. Finally, some numerical experiments have been carried out to determine flushing-times of the system. This is the first time that such a complete study of radionuclide dynamics in the Alboran Sea has been carried out.