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The vertical profiles of (239+240)Pu and (137)Cs activities and (240)Pu/(239)Pu isotopic ratios are determined for three sediment cores of Lake Qinghai from the Qinghai-Tibetan Plateau, China, and compared with those in sediments of another three lakes (Lakes Bosten, Sugan, and Shuangta), the only existing ones closest to Lop Nor area, China's nuclear weapons test site in the northwestern part of the country. The mean inventory of 47.7 ± 18.7 MBq km(-2) for (239+240)Pu activity in Lake Qinghai is comparable to the average value of global fallout expected at the same latitude, yet the mean inventory of 1112.0 ± 78.0 MBq km(-2) for (137)Cs is slightly lower than that of global fallout. Anomalously low (240)Pu/(239)Pu isotopic ratios (0.038-0.125) were found in the 3-6.5 cm deep sediment layers, indicating the trace Pu input from early nuclear weapons research activities at Atomic City in the lake's watershed during the 1950-60s. Model calculation indicated that the Pu input accounted for approximately 5-16% of the total Pu inventory. The observation of low (240)Pu/(239)Pu ratio in the deep sediment layer provided a new time marker for recent sediment dating in the lake and around the area. The results are of great significance to the further understanding of sources, records, and environmental impacts of global and regional nuclear activities in the environment and provide important chronological information for further studies on the water eutrophication process and climatic change, and reconstruction of pollution history of organic contaminants and heavy metals in the watershed of Lake Qinghai.

[1] Previous global models generally simulated high concentrations of 210 Pb in the surface air over China. Measured data of 210 Pb are needed in order to examine the accuracy and performance of the models. We collected the first-ever set of surface air radionuclide measurements of 210 Pb and 7 Be at Guiyang (26.57°N, 106.72°E, 1080 m above sea level) on the east of the Himalayas in Guizhou Province and at Mt. Waliguan (36.29°N, 100.90°E, 3816 m above sea level), a Global Atmosphere Watch station for the World Meteorological Organization, on the Qinghai-Tibetan plateau extending from the Himalayas in Qinghai Province, China. These measurements were carefully analyzed for data assurance and quality control and were used for validating models. The patterns for the variations in both measured 210 Pb and 7 Be at the Guiyang site have been shown to be consistent. In this case the increase of 7 Be concentration was the result of the weak subsidence of air developed at the periphery of the high-pressure system. Because of the weak subsidence the boundary layer turbulence could mix the upward transport of 210 Pb, thus increasing 210 Pb concentration. The ratio of 7 Be/ 210 Pb indicated a seasonal variability with rough monthly oscillation. The ratio peaks were in spring and summer when the convective activities were significant. The 210 Pb levels measured at Mt. Waliguan, located at the peak of the mountain, could be attributed to the long-range transport of 222 Rn that decays to 210 Pb, whereas the 7 Be concentration varied directly with the local airflow passing over the site. We found that it was not appropriate to use the ratio of 7 Be/ 210 Pb for analysis of vertical transport exchange processes at the high-altitude Mt. Waliguan site because of the long-range transport of 210 Pb that occurred at the site.

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