226Ra, 210Pb, 210Po Distributions at the Sea off Southern Taiwan: Radioactive Disequilibria and Temporal Variations

Abstract

(superscript 210)Pb and (superscript 210)Po in both particulate and dissolved phases were measured at the sea off southern Taiwan, where the Kuroshio intrudes as a branch in winter and mixes with the South China Sea water and coastal waters. Although the effects of mixing and circulation, based on hydrographic data, were significant, the removal rates of these two nuclides due to scavenging by sinking particles were evaluated. Dissolved and total (superscript 210)Pb and (superscript 210)Po in surface water increased but particulate (superscript 210)Pb decreased away from the coast in September, 1994, as well as in January, 1995. On the other hand, both particulate and dissolved (superscript 210)Po in surface water varied temporally and were higher in January, 1995. The mean residence time calculated for the surface water (superscript 210)Pb with respect to particulate scavenging was generally longer in January, 1995 (0.57-0.71 yr) than in September, 1994 (0.25-0.41 yr) if a constant atmospheric flux of 2 dpinlcm^2/y in the area was assumed. Similarly, for (superscript 210) Po it was also longer in January, 1995 (0.56-7.7 yr) than in September, 1994 (0.23-2.0 yr). The (superscript 210)Pb and (superscript 210)Po profiles measured from three stations in different periods showed that their concentrations varied temporally and spatially, therefore probably reflecting changes in hydrography and particulate scavenging. In the upper layer where (superscript 210)Pb was in excess of (superscript 226) Ra, the mean residence time for (superscript 210) Pb with respect to the removal by sinking particulates was about 1.3-3 yrs. In the lower layer where (superscript 210)Pb was deficient relative to (superscript 226)Ra, the mean residence time ranged from 26 to 44 yrs resulting from variations in (superscript 210)Pb and (superscript 226)Ra inventories or varying (superscript 210)Pb deficits. The mean residence time calculated for (superscript 210)Po in the upper, intermediate and lower layers as defined by (superscript 210)Po deficit or excess relative to (superscript 210)Pb also varied in the three sampling periods. These variations were due to the fact that both (superscript 210)Po and (superscript 210)Pb profiles varied in response to particulate scavenging and the mixing of several water masses in variable proportions at different times. (superscript 210)Pb was not recycled, but at two stations in January, 1995, (superscript210)Po was recycled over 100%, requiring a lateral input. This feature was also observed at the sea off northeastern Taiwan.