Distribution Of Multi-radionuclides In Surface Sediment Of Chongming Tidal Flat In The Changjiang Estuary And Their Implication Environmental Signification

To evaluate the source of sediment in East Chongming Tidal Flat and further more understand the deposition characteristic of sediment in the Changjiang Estuary, particle-reactive radionuclides such as 7Be, 137Cs,228Th,210Pb and 210Po were used in this article by studying the distribution of radionuclides and their activity ratios from atmospheric deposition and river input, and the grain-size of sediment from high flat to low flat were also considered. The main results are showed below:(1) The temporal and spatial distribution of sediment grain-size:Under the influence of vegetation, sediment source and hydrodynamics, the sediment grain-size in the Tidal Flat showed temporal and spatial variations. The mean size of surface sediment increased from north section to south and increased from high flat to low flat. Compared to mean size of surface sediment in autumn and winter, the mean size is much smaller in spring and summer. By using a water elutriation apparatus, sediment in the Tidal Flat was devided into 4 grades called A(the most thick), B, C and D(the most fine) according to size of the sediment particle. From A to D, the content of clay increased while the grain-size and content of silt decreased. In the same grade of sediment the thick part exhibited an offshore increasing trend, and the fine part showed a decreasing trend.(2) The activities of radionuclides:7Be had a high value in activity in spring; 137Cs had a low value in activity in the whole year and its variation with the size of sediment particle is same as 210Pb. The 210Po/210Pb ratio was greater than 1, which meant 210Po was excess. The 210Po activity and 210Po/210Pb ratio correlated well with the loss on ignition meant a highly correlation between 210Po activity and organic material content.(3) Correction analysis between grain-size and radionuclide activities: Correlation analysis between grain-size and activities were done and it showed that all the correlation were significant at the 0.01 level. The activities of radionuclides decreased as the size of surface sediment increased. The correlation coefficient of 210Pbex,137Cs,228Thex and 7Be were 0.82,0.78,0.63 and 0.50 separately.(4) The radioactivitie ratios:The average 210Po/210Pb ratio of Chongming tidal flat from high to low flat were 1.07,1.08 and 1.04 separately, which were closed to those of suspended particulate matter from XuLiuJing (0.90). The average values from atmospheric deposition of Shanghai was 0.22. The 7Be/210Pbex and 137Cs/210Pbex ratio of surface sediment were both closed to XuLiuJing. These revealed that the sediment of the Chongming tidal flat was mainly controlled by the supply of the Changjiang River. Further study on 7Be/210Pbex and 137Cs/210Pbex ratio showed that the terrial characteristic of high flat and middle flat were stronger than low flat, which had less value of nuclide ratio due to the influence of hydrodynamics.