3-D Modelling technique of time-series 137Cs concentration in coastal organisms in the case of short term introduction

For the development of the modelling technique in predicting 137Cs concentrations in marine biota, temporal and space distribution of 137Cs concentrations in marine organisms are important especially in case of short-term introduction to coastal area. We developed a 3-dimensional model composed of nuclide transfer both from seawater and the food chain in an imaginary coastal area, expressing the temporal 137Cs concentrations in marine organisms accompanied with the habitat location. The result of simulation by 3-D model in the case of short term introduction exhibits the following information: 1) the introduced 137Cs in seawater is diluted and disappeared quickly by current advection and diffusion, while 137Cs in organisms slowly increased even after the seawater is cleaned, mainly from the contribution from 137Cs transfer through the food chain, 2) the 137Cs concentration peak in fish appears approximately 3 months later, and the concentration ratio (137Cs in organism / 137Cs in seawater) at that time is larger than the biological concentration factor (BCF) in equilibrium state. This result indicates that i) monitoring of 137Cs concentration in fish must be continued until 1 year after a 1 week acute input, and ii) the 137Cs increase in seawater may have occurred during the previous 3 months in the case that a higher 137Cs concentration ratio (organism / seawater) than 100 is observed in fish, because of the 137Cs memory effect of local food chain ecosystem. (author)