Seasonal variation of 228Ra/226Ra ratio in seaweed: implications for water circulation patterns in coastal areas of the Noto Peninsula, Japan

To examine water circulation patterns of coastal water, 72 seaweed (Sargasso) samples and 27 coastal water samples were collected from coastal areas of the Noto Peninsula, Japan, during the period from December 1998 to June 2002. The 228Ra and 226Ra activities of those samples were measured by low-background γ-ray spectrometry. There was a wide range of activities of 228Ra (0.5–2 Bq/kg-fresh) and 226Ra (0.5–1.2 Bq/kg-fresh) in the Sargasso samples. The 228Ra/226Ra activity ratio of Sargasso samples exhibited seasonal variation with minimum values in June (228Ra/226Ra = 1) and maximum values in December (1.5–2.5), which was mainly governed by changes in 228Ra activity. It is also notable that the seasonal variation of the 228Ra/226Ra ratio of Sargasso is in approximate agreement with that of the ambient coastal water. Sargasso samples appear to have retained the 228Ra/226Ra ratio of the ambient coastal waters, and the temporal variations in that ratio provide insight into seasonal changes in water circulation in the Noto Peninsula coastal area.