Temporal variation of 137Cs and 239,240Pu in the Sea of Japan

Concentrations of the anthropogenic Cs-137 and Pu-239.240 in surface water of the Sea of Japan were in the ranges of 2.7-3.8 mBq l(-1) and 1.3-8.0 mu Bq l(-1) respectively, in 1993-1994, which are in the same order of magnitude as those in the Nor th Pacific. The time-series data indicated a marked increase of surface Cs-137 in 1986 and 1987 after the major radioactive dumping and the Chernobyl fallout in 1986 and then rapidly decreased thereafter. The apparent half-residence time of Cs-137 in the surface water was estimated to be about 3 years for excess Cs-137 and 16 years for a rather long time-scale transport, respectively. For Pu-239.240 in the surface water, no systematic temporal variation was observed over the past two decades, which may reflect rapid recycling of deeper Pu. The results revealed that most of the recent radioactivity in water columns of the Sea of Japan was of global fallout origin from atmospheric nuclear testing and partly the Chernobyl fallout. No clear signal about the effect of radioactivity from Russian dumping was observed. (C) 1998 Elsevier Science Ltd. All rights reserved.