Distribution Characteristics And Environmental Significance Of The Radionuclide Pu In The Sediments Of The Yangtze River Estuary And The North Jiangsu Tidal Flat

The estuary and the tidal flats are the most active region of land-ocean interaction and are most sensitive responded to the change of nature and human activities in the basin. They are also the zone which sensitivite to the global change. So the estuary and the tidal flats are one of the key points for the land-ocean interaction research in the coastal area. Radioactively isotopes such as Pu,137Cs and 210Pb are important tracers to evaluate the modern sedimentary environments. Based on the separation and analysis of radionuclide Pu, comparing with the 137Cs and the 210Pb, the sediments from the Yangtze River estuary and tidal flats of the north Jiangsu were analized for the characteristics of the Pu,137Cs and 210Pb. The methods concluded sector field ICP-MS,γandαspectrometry, respectively. The distribution characteristics and the geochemistry behavior of the Pu,137Cs and210Pb in sediments in the Yangtze River estuary, tidal flats of the north Jiangsu were analyzed. The reconstructing of the historical environmental information from the different radio isotopes was discussed. The results were useful to understand the geo-chemical process of these radionuclides in the region, to understand the sedimentation dynamics, the interaction process between the land and the ocean and the response of the regional environment to the anthropogenic acitivities and global changing. The main conclusions are summarized as follows:(1) For the ion exchange chromatography in the Pu separation, the nitric acid is better than the hydrochloric acid in the elution processes. However, at the same time ,the nitric acid solution give more 238U, which will disturb the ICP-MS analysis. The low concentration of the reductant of HF and Ticl3 has low Pu elution effect compared with the high concentration of the reductant. Using the reduced step and the oxidize step to adjust the valence of plutonium, the whole Pu yields seems to be better.(2) In the study area, an increasing trend for 239+240pu activities and 240Pu/239Pu atom ratios can be seen from the land to the sea and from the north to south in the estuary. The basic controlling factors were that, the Yangtze River runoff transported plenty of sediments to the estuary, at the same time, Taiwan Warm Curren(TWC), which separted from the Kuroshio current(KC), moving north forward. The TWC and the sediments were interact with eachother, the Pu from the Pacific Proving Gound(PPG) in the seawater were quickly scavenged to the sediments under suitable conditions. The PPG source of Pu with higher 239+240Pu activities and 240Pu/239Pu atom ratios(0.33-0.36), while the deposition of riverine sediments bearing much lower 239+24opu activity and lower 240Pu/239pu atom ratio (typical global fallout ratio,0.18) had scavenging effect, dilution and mixing effect, resulting in the spatial distribution characteristics of the 239+240pu activity and 240Pu/239Pu atom ratio in the estuary surface sediments.(3) A clear maximum peak for 239+240pu activities appears in the sediment profile of core 18 at the depth of 172 cm in the study area of the Yangzte River estuary. However, the maximum peak for 137Cs activities at the depth of 140 cm. One of the reasons for the different maximum peaks is that, migrated peaks of 137Cs activities in the Yangtze River estuary. Another reason for the discrepancy of the 137Cs and 239+240pu profiles lie in the main sources and the different chemical behaviours in the estuary,137Cs mainly coming form the terrestrial, whereas, the Pu in the seawater was dissolution, and extensively scavenged from the sea waters to parcticles when meets the particles materials. Based on the Pu chronology, the sedimentation rate of SC18 was 4.1 cm/a, which is different from the other chronology results. Intergrated the inconsistent of the sedimentation rates based on Pu,137Cs and 210Pb, they explained that sedimentation environment in the area were experienced different periods, and the sedimentation rates have show decreasing trend.(4) For sediment collected from the tidal flats of Xinyanggang in north Jiangsu, 137Cs and 239+240pu activities were increasing from sea to land, which constrolled by the grain size distribution in the different zones of the tidal flats. But the 210Pb activities on the superficial layer of the cores were varied spatially and the different organic matters sources may be the reason. Not all the profiles of 210Pb,137Cs and 239+240pu activities are suitable for chronology in the cores, because the strong tidal waves, storms and human activities in the tidal flats may disturb the originally radionuclides fallout deposition information. However, one core from the Suaeda salsa zone, which area the tidal wave was weak and the fine particles were aggregated, showed reliable 137Cs and239+240Pu peaks in the profiles that corresponded to the depth of 1963. Based on the information of sediment rates from the137Cs and 239+240pu profiles, the grain-size distribution, the texture of the cores and the topography, the vegetation evolution time in different zones of the tidal flats and the whole evolution lines of the tidal flat were reconstructed.(5) For sediment collected from the tidal flats of Linhong in north Jiangsu, based on the profiles of the 210Pb, 137Cs and 239+240Pu, the grain size distributions and the SEM surface microtextures of quartz grains from the sand of the cores, the different deposition areas were distinguished in the estuary and the adjacent areas. Depositional environment of Linhong estuary can be divided up as tide and wave intensive mixed deposition area in low tidal flat, tide mild dynamics deposition area in high tidal flat, tide and fluvial intensive mixed deposition area in river outlet, tide and fluvial mild mixed deposition area just inland of the river outlet.(6) In the north Jiangsu tidal flats of Xinyanggang and Linhong estuary, fine-particles deposits areas are important places for the deposition of 137Cs and 239+240pu(7) The relative contributions of the PPG close-in fallout in Yangtze River estuary are 41%-47%(average 44%). The land-origin global fallout transported by the river contributes were 45%, the remaining 11% attributed to direct global fallout. Most of the Pu in the sediments of the north Jiangsu tidal flat are attributed to the direct global fallout and the land-origin Pu transported by the rivers.(8) The research provide reliable baseline data set on the activity levels and isotopic composition of the anthropogenic radionuclides Pu at the area of Yangtze River estuary and the north Jiangsu tidal flats so that any further contamination from the nuclear power stations in the upper catchment basin of Yangtze River and the coastal areas can be identified, and the environmental distribution and migration of the radionuclides can be evaluated.