Temporal variations of 90Sr and 137Cs concentrations in Japanese coastal surface seawater and sediments from 1974 to 1998

Abstract

Abstract 90 Sr and 137 Cs concentrations were determined in surface water and bottom sediments collected at 11 sites offshore from Japan during the period 1974–1998, to investigate their temporal variations and behaviour in the coastal marine environment. The concentrations of 90 Sr and 137 Cs in surface water have decreased with time since 1974. After the period of atmospheric nuclear weapons tests, the mean residence times of 90 Sr and 137 Cs were about 41 and 51 years, respectively. The 137 Cs/ 90 Sr activity ratios in coastal seawater during the atmospheric nuclear weapons tests (up until 1980) were lower than those after the tests due to the inflow of 90 Sr in river water. A sharp increase in 137 Cs levels was observed in airborne dust, in precipitation on the Japanese islands, and in coastal surface seawater in 1986 following the Chernobyl accident. However, the 137 Cs levels in surface water returned to pre-1986 levels quickly, indicating rapid removal of Cs from the surface to deeper water. Concentrations of 90 Sr in sediments were generally much lower than those for 137 Cs, reflecting the more effective scavenging of Cs from the water column. In Ca-rich sediments, consisting of corals and shells, higher 90 Sr levels and 90 Sr/ 137 Cs activity ratios were found, reflecting higher accumulation of Sr than Cs in marine organisms. Higher accumulation of 90 Sr than 137 Cs was also found in seaweed (gulfweed and wakame).