Distribution And Sources Of Plutonium Isotopes In Lake Bosten And Its Upwind Area

Plutonium（Pu）is an important radionuclide on the environmental impact assessment from the human nuclear activities.Lake Bosten is located in the upwind area of Chinese Lop Nor nuclear test site and the downwind area of the former Soviet Union Semipalatinsk nuclear test site,previous study of Lake Bosten found that the vertical distribution of radionuclides ^239,240Pu in sediment core is significantly different from that in other lakes which far away from human nuclear activities.To further reveal the causes of the abnormal vertical distribution of plutonium isotopes in the sediments of Lake Bosten and preliminarily clarify the influence mechanism of the two nuclear test sites on the distribution of regional artificial radionuclides,he plutonium in sediments samples,water samples of main reivers and soil samples in the upwind area of Lake Bosten were analyzed using inductively coupled plasma mass spectrometry（ICP-MS）in this study.The mass balance model of plutonium isotope was used to analyze the source and contribution of plutonium isotopes in the sediment core of Lake Bosten.Meanwhile,with data assembled from published articles,the distribution of^239,240Pu in Lake Bosten and its upwind area are compared with those in the downwind area in northwest China,comparative study on the distribution characteristics and differences of Pu isotopes in the upwind and downwind environment of Bosten Lake.And the main conclusions are as follows:（1）The average values of ²⁴⁰Pu/²³⁹Pu atomic ratio（0.175±0.090 and 0.188±0.012）in Lake Bosten sediments and main river water samples,both are close to the average value of global fallout（0.180）,indicating that global deposition is the main source of ^239,240Pu.However,an extremely low ratio of 0.080 at 6 cm in BS10-2 core showed that this core has been contaminated partially originated from Chinese Lop Nor nuclear tests.The vertical distribution of ^239,240Pu flux anomaly in the sediment core shows that there is a continuous^239,240Pu input in Lake Bosten since the global subsidence peak in 1964.According to the1964 time marker of ^239,240Pu settlement,the contribution of ^239,240Pu carried into the lake through the river to the abnormal distribution of ^239,240Pu in the sediments of Bosten Lake is about 60%calculated by the watershed mass balance model.（2）The soil ^239,240Pu activity and ^239,240Pu inventory in the upwind area of Bosten Lake are 0.035～1.338 m Bq/g and 23.67～222.7 Bq/m²,respectively.Vegetation coverage may affect the distribution of ^239,240Pu activity in surface soil,and organic matter is an important factor affecting the distribution of ^239,240Pu in profile soil.The average values of ^239,240Pu inventory and ²⁴⁰Pu/²³⁹Pu atomic ratio in the soil are 92.77Bq/m²and 0.180±0.002,respectively,indicating that ^239,240Pu in the soil of the study area mainly comes from global deposition,and the former Soviet Union Semipalatinsk nuclear test and the Chinese Lop Nor nuclear test have little impact on the environment of the study area.The strong sandstorm and dust fall in Northwest China may carry Semipalatinsk preserved in the soil after atmospheric sedimentation into the sediments of Lake Bosten,resulting in the abnormal distribution of ^239,240Pu in the sediments.（3）The impact of Lop Nor nuclear test on lakes in Northwest China is small and accidental.The activity of ^239,240Pu in 84.8%of soil samples in Northwest China is less than1 m Bq/g,indicating that its radioactivity level is low.The average values of ^239,240Pu inventory and ²⁴⁰Pu/²³⁹Pu atomic ratio in the upwind and downwind soil of Lake Bosten are 90.61 Bq/m²and 0.175±0.002,161.27 Bq/m²and 0.154±0.001,respectively.It shows that the impact of Chinese Lop Nor nuclear test on Northwest China is mainly concentrated in the downwind of Lake Bosten,such as Dunhuang and Yumen in northwest Gansu.At the same time,compared with the impact of foreign nuclear tests or nuclear accidents on the surrounding environment,the Lop Nur nuclear test has little impact on Northwest China,mainly because only 6.64%of the fission products released enter the atmospheric troposphere and settle in the adjacent area of the nuclear test site.