Modelling surface radioactive spill dispersion in the Alboran Sea

The Strait of Gibraltar and the Albora´n Sea are the only connection between the Atlantic Ocean and the Mediterranean Sea. Intense shipping activities occur in the area, including transport of waste radionuclides and transit of nuclear submarines. Thus, it is relevant to have a dispersion model that can be used in an emergency situation after an accident, to help the decision-making process. Such dispersion model re quires an appropriate description of the physical oceanography of the region of interest, with simulations of tides and residual (average) circulation. In this work, a particle-tracking dispersion model that can be used to simulate the dispersion of radionuclides in the system Strait of GibraltareAlbora´n Sea is de scribed. Tides are simulated using a barotropic model and for the average circulation a reduced-gravity model is applied. This model is able to reproduce the main features of the Albora´n circulation (the well known Western Albora´n Gyre, WAG, and the coastal circulation mode). The dispersion model is run off-line, using previously computed tidal and residual currents. The contamination patch is simulated by a number of particles whose individual paths are computed; diffusion and decay being modelled using a Monte Carlo method. Radionuclide concentrations may be obtained from the density of particles per water volume unit. Results from the hydrodynamic models have been compared with observations in the area. Several examples of dispersion computations under different wind and circulation conditions are presented.Junta de Andalucía RNM-41

Measuring and modelling temporal trends of 226Ra in waters of a Spanish estuary affected by the phosphate industry

The presence and temporal evolution (1990–2001) of 226Ra in a tidal estuary affected by the phosphate industry has been investigated. Water samples collected in the course of four sep arate sampling campaigns were analysed for 226Ra content using a gas flow proportional coun ter following Ba coprecipitation. Two 226Ra sources have been identified: direct discharges from the industrial complex and run-off from a phosphogypsum pile. Although activity levels are similar, or even higher, than those found in other environments affected by the phosphate industry, there has been a general decrease in contamination since direct discharges ceased in 1998 due to new regulations from the EU. However, sediments are now acting as a source of Ra to the water column due to redissolution processes. A numerical model of the estuary has been developed to describe quantitatively the experimental results. The model solves the hydrodynamics and the dispersion equation of 226Ra including interactions with sediments. Model results are, in general, in good agreement with observationsMinisterio de Educación y Ciencia 1FD97-0900-CO2-01European Union (EU) FIGE-CT-2000-0008

Viewpoint on the Integration of Geochemical Processes into Tracer Transport Models for the Marine Environment

Two types of models are used to describe the interactions of tracers dissolved in the ocean with marine sediments: equilibrium and dynamic models. A brief description of these models is given in this opinion paper, and some examples are presented to show that preference should be given to the dynamic descriptions in modern pollutant transport models to be used in emergencies as well as in the case of chronic tracer releases to the sea

Models for radioactivity dispersion assessments in Andalusian coastal waters: Gulf of Cádiz and Alborán Sea

Rapid response models for the assessment of the consequences of a radioactive spill in the coastal waters of Andalusia (south of Spain) have been developed. These waters comprise the Gulf of Cadiz, GoC, (Atlantic Ocean) and the Alborán Sea, AS, (Mediterranean Sea) and two models are described, covering both sectors. This subject is of high relevance since these waters are the only connection between the Atlantic and the Mediterranean. Thus, there are intense shipping activities that for instance include the transit of nuclear submarines. The models include hydrodynamic sub-models appropriate to describe the water circulation in each area. Results from the hydrodynamic models have been carefully tested through comparisons with observed tides and currents. The dispersion models are based upon a particle-tracking technique. Thus, the radioactive spill is simulated by a number of discrete particles, each one equivalent to a number of units, whose paths are computed. Turbulent diffusion and radioactive decay are calculated using a stochastic Monte Carlo method. The radionuclide concentrations may be obtained at the desired time from the density of particles per water volume. Some applications and examples of results are given

Modelling the physico-chemical speciation of plutonium in the eastern Irish Sea: a further development

A numerical full three-dimensional model previously developed to simulate the physico-chemical speciation of plutonium in the eastern Irish Sea has been improved. The model solves simultaneously the hydrodynamic equations, the suspended matter equation and the equations that give the time evolution of Pu concentrations in water, suspended matter and bottom sediments. It is considered that Pu may exist in each phase in two different oxidation states. Redox reactions are also considered. In the earlier version of the model, a one-step kinetic model was used to describe the transfers of radionuclides between the dissolved and solid phases. Although with this kind of model the contamination of the sediments can be properly simulated, it is clearly not able to describe the re-dissolution of radionuclides from a contaminated sediment once the external source to the sea is reduced. Thus, the model has been improved by substituting the one-step model with a two-step kinetic model consisting of two consecutive reversible reactions. Now it is possible to simulate both the sediment contamination and the re-dissolution processes.European Commission FIGE-CT-2000-0008

A modelling study on 137Cs and 239,240Pu behaviour in the Alborán Sea, western Mediterranean

A model for simulating the dispersion processes of 137Cs and 239,240Pu in the Alborán Sea is described. The model consists of two hydrodynamic models: a 2D depth-averaged model and a two-layer model which provide tidal and geostrophic currents, respectively; a sediment transport model which provides suspended particle concentrations and sedimentation rates over the domain; and the radionuclide dispersion model including interactions of dissolved radionuclides with suspended particles and bed sediments. These processes are formulated using kinetic transfer coefficients. The hydrodynamic and sediment models are run and validated in advance, and their results are then used to simulate the dispersion of 137Cs and 239,240Pu, which are introduced from atmospheric fallout. Radionuclide concentrations in the water column and distributions in bed sediments have been compared with measurements in the sea. Both set of data are, in general, in agreement. The model has also been applied to calculate radionuclide fluxes through the Strait of Gibraltar. These computed fluxes have been compared with previous estimations as well.Junta de Andalucía RNM-41

A Lagrangian oil spill transport model for the Red Sea

An oil spill model was developed for the Red Sea. It uses detailed data on water circulation: instantaneous tidal currents, tidal residuals (both from a tidal model specifically applied to the Red Sea) and baroclinic circulation derived from HYCOM ocean model. The model can also incorporate forecasts of local (in the spill area) winds. The transport model is Lagrangian and includes advection/diffusion plus specific processes for oil, as buoyancy, decomposition and evaporation. Results of the tidal model were compared with observations in the sea. The transport model was first applied to passive particles to compare the significance in advective processes of the components of the current (tidal, tidal residual, baroclinic currents) over several areas and seasons and then it was applied to some oil spills: the model output was compared with results from previous simulations and it was also applied to hypothetical oil spill

Redissolution and long-term transport of radionuclides released from a contaminated sediment: a numerical modelling study

A numerical model based upon residual circulation has been developed to simulate the long-term (years) transport of non-conservative radionuclides in the English Channel. The transfer of radionuclides between water, suspended matter and bottom sediments has been described in terms of kinetic transfer coefficients, so that the model can be applied in situations out of equilibrium. The model has been used to study and compare the effects of direct discharges of radionuclides, the case of a continuous release and the redissolution of radionuclides from a contaminated sediment. These numerical experiments have been carried out for two radionuclides with a different geochemical behaviour: 137Cs and 239,240Pu. It has been found that natural equilibrium in the partition of radionuclides between the liquid and solid phases is reached in the case of instantaneous releases and in the case of redissolution from a contaminated sediment. However, in the case of a continuous release such partition remains out of natural equilibrium. Also, the behaviour of 137Cs and 239,240Pu is similar, but the time scales of the processes are clearly different due to the different affinities of both radionuclides to remain fixed to the solid phases.European Commission FIGE-CT-2000-0008

Environmental modelling in the Gulf of Cadiz: Heavy metal distributions in water and sediments

The Gulf of Cadiz (GoC) connects the Atlantic Ocean and the Mediterranean Sea. An environmental study of the GoC is carried out through numerical modelling. First, a 3D baroclinic model is used to obtain the residual circulation and a 2D barotropic model is applied to calculate tides. The results of these models are used by a 3D sediment transport model which provides suspended matter concentrations and sedimentation rates in the GoC. Then heavy metal dispersion patterns are investigated using a 3D model which includes water–sediment metal interactions and uses the outputs of the hydrodynamic and sediment transport models. The metal transport model has been applied to simulate the dispersion of Zn, Cu and Ni introduced into the GoC from three rivers draining the Iberian Pyrite Belt, in the southern Iberian Peninsula. Results from the hydrodynamic, sediment and metal transport models have been compared with measurements in the GoC. In particular, the contamination of sediments collected along the southern coast of Spain is well reproduced by the model. Metal plumes reach the Strait of Gibraltar, thus the three rivers constitute a source of pollutants into the Mediterranean Sea.Junta de Andalucía RNM-41

The enhancement of Ra-226 in a tidal estuary due to the overation of fertilizer factories and redissolution from sediments: Experimental results and a modelling study

The presence of 226Ra in a tidal estuary formerly affected by direct discharges from a phosphate fertilizer complex has been investigated. In general, activity levels are lower than those detected when direct discharges were carried out. However, there is still a clear contamination that can be attributed, presumably, to the disposal of Ra-containing phosphogypsum to the river and to the redissolution of radionuclides from the contaminated sediments. A numerical model of the estuary has been developed to investigate these hypothesis. The hydrodynamics are first calibrated and standard tidal analysis is carried out. The dispersion model computes instantaneous currents from the so obtained tidal constants to speed up simulations. The exchanges of 226Ra between water and the bottom sediment have been described in terms of kinetic transfer coefficients. Model results are, in general, in agreement with observations. The simulation results support the previous idea of sediments acting as a source of 226Ra to the water column.Ministerio de Educación y Ciencia 1FD97-0900-CO2-01European Commission FIGE-CT-2000-0008