Environmental recovery from 137Cs contamination in Japanese coastal waters shown by comparison of temporal distributions with European seas

摘要

We compared the spatiotemporal distributions of 137Cs in the European Baltic Sea (semi-enclosed) and the North and Norwegian Seas (open to the ocean) after the Chernobyl Nuclear Power Plant (CHNPP) accident, with those in Japanese coastal regions, including the waters off Miyagi, Fukushima, and Ibaraki prefectures, open to the western North Pacific, after the Fukushima Nuclear Power Plant accident. The effective half-lives 1-9 years after each accident were shortest (1.6-4.7 y) in Japanese coastal waters, 4.9 y in the North Sea, and 14.4 y in the Baltic Sea, suggesting that decreases in 137Cs concentrations are largely dependent on the local geography, and that the dilution-diffusion effect of seawater was greater in the Japanese coastal waters. The effective half-lives of 137Cs in the surface waters of European seas, based on 30 years of data after the CNPP accident, became longer, ranging from 8.4 to 11.9 y. This may be due to the influence of rivers, and a delay in the decrease in 137Cs levels caused by the small difference in radioactivity concentrations between the seas and diluting waters. These results could contribute to the prediction of contamination levels in Japanese coastal waters in the future.