Effective environmental half‐lives of 90 Sr and 137 Cs in the coastal seawater of Japan

Abstract

The effective environmental half‐lives of 137 Cs and 90 Sr together with their variations in concentrations in the coastal seawater of Japan were examined based on results of long‐term and systematic measurements during 1984–1994. Concentrations of these radionuclides in surface water from 12 areas in the region of the Kuroshio and the Tsushima Currents along Japan were similar and higher than in surface waters of the cold current of Oyashio. Relatively constant concentrations were observed 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 137 Cs in coastal waters of Japan decreased from 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 concentration of 90 Sr 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 depth (a decrease of 64%). The vertical distribution of 137 Cs/ 90 Sr activity 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 90 Sr and 137 Cs in coastal seawater. Concentrations of 90 Sr and 137 Cs in the surface mixed layer were 3.3 ± 0.4 mBq/L ( n =40) and 4.3 ± 0.5 mBq/L ( n =40), respectively, in 1984. The concentrations dropped fairly steadily from 1984 to 1994, except for a clear increase of 137 Cs in the spring of 1986 (5.0 ± 1.1 mBq/L, n =34) due to Chernobyl deposit. The concentrations of 90 Sr and 137 Cs in 1994 were 2.2 ± 0.2 mBq/L ( n =51) and 3.1 ± 0.2 mBq/L ( n =51), respectively. Effective environmental half‐lives of 90 Sr and 137 Cs of the surface mixed layer in the Tsushima Current system are calculated to be 17.3 years (95% confidence interval 16.1–18.7 years) during 1984–1994 and 18.7 years (13.4–31.2 years) during 1987–1994, respectively.