Element transport in marine coastal ecosystems – modelling general and element-specific mechanisms

Detta är en avhandling från Stockholm, Sweden : Stockholm University

Sammanfattning: Understanding the behaviour of naturally-occurring and anthropogenically-derived radionuclides (isotopes) in the marine environment is important, because there is a need for their effective utilization either as in situ tracers or for industry, medical applications, the planning of waste-disposal facilities, and estimations of human and animal health risks. Radionuclides generally have the same chemical properties as their stable element analogues and thus show similar environmental behaviour and cycling to their stable counterparts. However, they are distinguished by potentially enhanced impacts on organisms due to their radioactivity emitted as alpha, beta or gamma radiation. There are many nuclear facilities, such as power reactors, research reactors, waste handling facilities and fuel production facilities located in the drainage area of the Baltic Sea which discharge directly or indirectly into the Baltic Sea. Thus knowledge about factors and processes in space and time that control the fate of radionuclides in the aquatic ecosystem provide the basis for dose and risk assessment for human and biota. Radionuclides’ stable analogues are often elements that have numerous industrial applications and are used widely in society (e.g., metals, rare earth metals). These are often released in aquatic environments and can be toxic. Consequently, they also require better control in order to protect the environment from pollution and provide quick responses in case of accidents. Understanding how such elements are distributed in the freshwater, seawater, sediments and aquatic organisms is therefore important. Modelling is a useful approach for the estimation of possible element concentrations and inventories in the aquatic ecosystem.In this thesis a dynamic stochastic compartment model (K-model) for radionuclide transfer in a marine coastal ecosystem was implemented for a coastal area in Öregrundsgrepen, Baltic Sea, Sweden, in order to determine the fate of radionuclides released in the sea water (Paper I). Radionuclide-specific mechanisms such as radionuclide uptake via diet and adsorption of radionuclides to organic surfaces were connected to the ecosystem model. Using the model, we estimated concentration ratios (CR; the ratio of the radionuclide concentration in an organism to the concentration in water) in the ecosystem and compared these with measured data for grazers, benthos, zooplankton and fish for 26 elements. The uncertainty variations of CRs were reduced when the model was parameterised with site data and elements with higher sorption capacity had higher CRs for all organism groups. The K-model was also validated with a 3D hydrodynamic spatial model (D-model) (Paper II). Despite differences in temporal resolution, biological state variables and partition coefficients, the accumulation of Th-230, Cs-135 and Ni-59 in biological compartments was comparable between the models and with site measurements. Both models provided confidence limits for their modelled concentration ratios, an improvement over models that only estimate mean values. The models developed here are useful tools for assessing the degree to which organisms can be affected by the distribution and fate of pollutants in the marine environment. 

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