Spatial variations of 226Ra, 228Ra, and 228Th activities in seawater from the eastern East China Sea

A total of 74 surface and subsurface water samples at depths shallower than 200 m were collected in the eastern East,,a, China Sea (ECS), and spatial and seasonal variations in Ra-226, Ra-228 and Th-228 activities were examined by low-background gamma-spectrometry. The Ra-228 and Th-228 activities (as well as Ra-228/Ra-226 and Th-228/Ra-228 activity ratios) on the surface exhibited notable seasonal and lateral variations ((228)/Ra/Ra-226 = 0.2-3.4; Th-228/Ra-228 = 0.1-0.4). From the Ra-228 activity (0.2-6 mBq/L) and salinity (27.2-34.5), fractions of the Kuroshio, shelf, and Yangtze River waters on the surface of the ECS were estimated to be 50-95%, 3-50%, and 2-8% in July, and 95-100%, 1-5%, and 0-1% in April, respectively, exhibiting a lower fraction of shelf water in winter spring and on the Japan side of the ECS. While the surface waters had high Th-228 activity (similar to 0.1-0.6 mBq/L) in July, resulting from the high activity of the parent Ra-228, their Th-228/Ra-228 ratio (similar to 0.1) was lower compared with that of waters in February-April (0.15-0.35). Th-228 is considered to be preferentially removed from the surface in summer, reflecting a high flux of biogenic particles.