Fri Oct 03, 2014
A 3D numerical model designed to track the dispersion and fate of 90-Sr in the waters and biota of the northwest Pacific Ocean was published by Maderich and colleagues in the peer-reviewed journal Science of the Total Environment. The authors used a dynamic model including the marine food chain to assess the fate of 90-Sr in the northwest Pacific from 1945-2010 and the radiological health risk from Fukushima through marine 90-Sr exposure pathways from 2011-2040. The model is designed to predict the dispersion of 90-Sr derived radioactivity in the water, sediments and the transfer of the isotope through the marine foodweb resulting in doses to humans through the consumption of marine products. The model accounts for transfer of 90-Sr from the terrestrial environment to the ocean over time and tracks the transfer of the isotope from phytoplankton, zooplankton, molluscs, crustaceans to fish as shown schematically in the following figure.
The calculated activities of 90-Sr in water, bottom sediment and marine biota are in good agreement with measurements made in the coastal area around Fukushima before the accident. Fewer direct measurements of 90-Sr exist compared to Cs isotopes because the analysis of 90-Sr requires significantly more sample processing and handling to separate it from other beta-emitters. For this reason the model assumes that 90-Sr releases from Fukushima to the ocean are related to 137-Cs releases in ratios determined by direct measurement after the disaster. While releases are ongoing the model determined that the activities in sediments and marine foodstuffs depend primarily on the initial releases in March and April 2011 when rates of release were greatest. Using a conservative estimated release of 640 TBq (TBq = 10^12 Bq) the model predicts the following individual dose rates to Japanese consumers of sea organisms where the average consumer eats 23.4 kg of fish, 2 kg of crustaceans, 1.3 kg molluscs and 3.7 kg of macroalgae per year. It was also assumed that 50% of the fish consumed included the organisms bones.
While the modeled activities of 90-Sr in fish agrees well with limited measurements made in fish the model tends to slightly underestimate the activity of 90-Sr in fish. More measurements of this radionuclide in seawater, sediments and biota will improve our understanding of how 90-Sr moves through the environment. Ongoing releases of 90-Sr from the Fukushima site also dictate that monitoring of the levels in the marine environment are necessary and prudent to determine the radiological health risk to seafood consumers.