| 其他摘要 | Plutonium isotopes and radio-cesium are anthropogenic radionuclide with strong chemical toxicity and high irradiation risk respectively. Their enrichment in environment and transfer through food-chains will raise threat to human health. Some researches about plutonium and radio-cesium have been carried out on the analytical method, radiation dose and lacustrine geology. The fact is, however, that these works were just on the threshold of studies on artificial radionuclide in China. Moreover, it is of great importance to recognize the source, distribution and destination of anthropogenic radionuclide in lakes or reservoirs as a result of both lakes and reservoirs are the main sources of drink-water or agricultural irrigation water. Since lake sediment which is regarded as the destination of all pollutants could effectively preserve the contaminating history of drainage area, it can be used to elucidate the polluting process and provide the basic information for water quality controlling. The plutonium and radio-cesium can provide significant recent chronology data for these environmental researches due to their exclusively characteristic.
In this paper, some special lakes and reservoirs were selectively studied as different location, type and disturbance to understand the distribution and transportation of plutonium and 137Cs in China and elucidate their sedimentary mechanisms in lakes by using the traditional 137Cs analytical method based on gamma spectrometry and plutonium analytical method which was developed in this work for freshwater lake sediment. Furthermore, we also discussed the availability of 239+240Pu as a type of recent chronology for lakes. The main conclusions are as follows.
1、We measured Pu isotopes in freshwater lake sediments by ICP-SF-MS after separation and purification using AG1-X8 resin followed by a strong base resin of AG MP-1M. The high U decontaminated factor, suficient recovery and the high sensitivity of ICP-SF-MS demonstrated that this analytical method for Pu isotopes is suitable for studies on recent lake sedimentation. Moreover, the interference of PbCl+ on plutonium isotopes was effectively eliminated by using the concentrated HBr as the final eluent for Pu elution, so this analytical method is also suitable for the analysis of 241Pu.
2、The 239+240Pu inventory, 137Cs inventory, 240Pu/239Pu atom ratio, 239+240Pu/137Cs activity ratio and vertical distribution of 239+240Pu activity and 137Cs activity in lakes demonstrated that Lake Bosten which is located at the west of Lop Nor have been contaminated partially by the re-suspending particle of soluble aerosol radionuclide and hot-particle which originated from China’s Nuclear Tests, and whole Lake Bosten may have also been contaminated by those radionuclide which were transported by surface runoff from Lop Nor. The plutonium isotope ratio in sediment core of Lake Bosten is close to the value of global fallout, suggesting that CNT’s Pu has similar 240Pu/239Pu atom ratio to that of global fallout. This phenomenon was also supported by results of Lake Sugan and Shuangta reservoir, both them are close to the east of Lop Nor. Lake Sugan and Shuangta reservoir have also been polluted by aerosol particles of radionuclide re-suspended from Lop Nor. About 40 percent of 137Cs and 27 percent of 239+240Pu in Lake Sugan were originated from CNT, and CNT contributed about 30 percent of 137Cs and 27 percent of 239+240Pu to Shuangta reservoir.
3、As for Lake Qinghai, it is hard to resolve the Pu pollution of CNT from global fallout due to its extremely low sediment rate. However, the evidently low 240Pu/239Pu atom ratio around 0.06 in sediment layer under the 1964 marked layer indicate that Lake Qinghai may have gotten Pu contamination from the Chinese nuclear weapon producing base named “Atomic City” which was built in 1957 and is about 10km far from the northwest bank of Lake Qinghai.
4、Regarding this case for lakes in southwest China, part of 137Cs in sediment core of Lake Chenghai may from CNT by long-term transportation. More evidences are, nevertheless, extremely required to confirm this hypothesis, such as the source of Pu and 137Cs in lakes which locate in Tibet Plateau and Sichuan Basin. Both the Pu isotope ratio and the Pu vertical distribution in lakes, including other lakes in southwest China, Lake Sihailongwan (Northwest China), Lake Taihu and Lake Poyang (East China) demonstrated that all radionuclide in these lakes were originated from global fallout.
5、A maar lake is a broad, low relief crater that is caused by a phreatic eruption or explosion caused by groundwater contact with hot lava or magma. Under the condition that the lake has never been disturbed, maar lake could keep the best record of depositional history, in which inapparent diffusion of deposit radio-cesium and plutonium is the main transfer pattern. As for shallow lakes, frequent human activities, hurricane and inter-convection between surface water and bottom water of lake may cause intensive re-suspending of sediment and strengthen the diffusion of deposit radio-cesium and plutonium. So as to those shallow lakes have been given inhomogeneous loss on 137Cs and Pu, and the vertical distribution of 137Cs and Pu in sediment core became out-of-order.
6、Lakes which have a little disturbance presented well linear correlation between 137Cs activity and 239+240Pu activity in sediment core, indicating that the 239+240Pu accumulation peak in such lakes can provide effective recent chronology information for lacustrine science. However, 137Cs and Pu couldn’t be used to date sediment in those intensively disturbed lakes. The sedimentation rates of different core in Lake Qinghai are evidently different from each other, suggesting that sedimentation rates or sedimentary accumulation rates in different locations of the same lake are changed. |
修改评论