Distributions of 210Pb and 210Po in surface water surrounding Taiwan: A synoptic observation

A total of 85 large-volume surface samples were simultaneously collected for the determination of dissolved and particulate 210Pb (210Pbd and 210Pbp) and 210Po (210Pod and 210Pop) during the Joint Hydrographic Survey (JHS) on board four research vessels from 31 May to 17 June 2007. Generally, the spatial distributions of the two radionuclides in the surface water surrounding Taiwan are controlled by the current system and the extent of particle scavenging. Except in the nearshore waters along the coast of China and Taiwan, a higher proportion of 210Po (~25%) is associated with particles than that of 210Pb (2–8%) in the surface water of the study area, demonstrating the different affinities to particles of the two radionuclides. The results show large geographic variations in terms of scavenging and removal rates of the two radionuclides. Along the shelf-edge of the marginal seas of the western North Pacific, where Kuroshio Water mixes with shelf water, 210Pb and 210Po results in a linear correlation between the two radionuclides. This shelf-edge exchange phenomenon is also found in the region off southeastern Taiwan where the spreading of the mixture of Kuroshio Branch Water and South China Sea Surface Water occurs. The residence time of 210Pb increases from 0.2 years in coastal waters to 2.6 years in Kuroshio Water, and the residence time of 210Po ranges from 0.2 to 0.3 years. An atmospheric 210Pb flux of 0.6–2.1 dpm cm−2 a−1 in the seas surrounding Taiwan is estimated.