An ecosystem model of the environmental transport and fate of carbon-14 in a bay of the Baltic Sea, Sweden

The environmental transport and fate of a hypothetical discharge of radioactive 14C from the Swedish final repository for radioactive operational waste (SFR) was investigated using an ecosystem modelling approach. It involved identification, quantification and dynamic modelling of the main flows and storages of carbon both in the physical environment and in the food web of a bay in the Baltic Sea. In the model, 14C was introduced into the food web via photosynthesising organisms. Contamination of the modelled ecosystem was assessed assuming a release of 51.3 MBq per year for 1000 years. The implications of changes of two parameters on the 14C fate were examined: route of 14C entry in the food web and water exchange. Modelling results were also used to estimate steady-state 14C-concentrations in biota, to investigate the time needed to reach steady-state and to calculate the ecological half-life of the radionuclide for the modelled compartments and the ecosystem. Since the modelled area is characterised by a fast water exchange, most of the discharged 14C was flushed out of the system (99.8%), and diluted in a much larger recipient. However, a small fraction of the discharge was assimilated by primary producers, which enabled subsequent transfer of 14C to organisms at higher trophic levels (e.g. fish, seals and humans). In general, the highest 14C-concentrations were observed in benthic plants and benthic macrograzers followed by fish and other organisms. An assumption of 14C entry into the food web via benthic primary producers was found to lead to increased concentrations in biota (especially benthic organisms) and reduced rates of water exchange were also observed to significantly increase the 14C exposure of the organisms.