EFFECTIVE ENVIRONMENTAL HALF-LIVES OF SR-90 AND CS-137 IN THE COASTALSEAWATER OF JAPAN

The effective environmental half-lives of Cs-137 and Sr-90 together wi th their variations in concentrations in the coastal seawater of Japan were examined based on results of long-term and systematic measuremen ts during 1984-1994. Concentrations of these radionuclides in surface water from 12 areas in the region of the Kuroshio and the Tsushima Cur rents along Japan were similar and higher than in surface waters of th e cold current of Oyashio. Relatively constant concentrations were obs erved in seawater above 200 m depth that appears to be well mixed, as generally assumed, with sharp gradient below 200 m depth. The observed mean concentrations of Cs-137 in coastal waters of Japan decreased fr om 3.1 +/- 0.2 mBq/L in the surface mixed layer (0-200 m depth) to 1.8 +/- 0.2 mBq/L at 500 m depth (a decrease of 58%). The observed mean c oncentration of Sr-90 in coastal waters of Japan decreased from 2.2 +/ - 0.2 mBq/L at the surface mixed layer to 1.4 +/- 0.2 mBq/L at 500 m d epth (a decrease of 64%). The vertical distribution of Cs-137/Sr-90 ac tivity ratio ranged from 1.45 at surface to 1.31 at a depth of 450-500 m, suggesting a possible difference in the behavior between Sr-90 and Cs-137 in coastal seawater. Concentrations of Sr-90 and Cs-137 in the surface mixed layer were 3.3 +/- 0.4 mBq/L (n = 40) and 4.3 +/- 0.5 m Bq/L (n = 40), respectively, in 1984. The concentrations dropped fairl y steadily from 1984 to 1994, except for a clear increase of Cs-137 in the spring of 1986 (5.0 +/- 1.1 mBq/L, n = 34) due to Chernobyl depos it. The concentrations of Sr-90 and Cs-137 in 1994 were 2.2 +/- 0.2 mB q/L (n = 51) and 3.1 +/- 0.2 mBq/L (n = 51), respectively. Effective e nvironmental half-lives of Sr-90 and Cs-137 of the surface mixed layer in the Tsushima Current system are calculated to be 17.3 years (95% c onfidence interval 16.1-18.7 years) during 1984-1994 and 18.7 years (1 3.4-31.2 years) during 1987-1994, respectively.