Vertical Distributions of Plutonium and ¹³⁷Cs in Lacustrine Sediments in Northwestern China: Quantifying Sediment Accumulation Rates and Source Identifications

Abstract

We investigated the vertical distributions of 239+240Pu and 137Cs activities and 240Pu/239Pu atom ratios in two lakes in northwestern China. The 240Pu/239Pu atom ratio from the Chinese nuclear test (CNT) site at Lop Nor is too complicated to identify the regional fallout source. However, radionuclides from the test site may be resolved mainly by the vertical distribution of radionuclides because there is no overlap between the global fallout peak and the local fallout peak. We analyzed Pu isotopes and 137Cs in sediments from two lakes (Sugan and Shuangta) located in northwestern China, near the CNT, and Lake Sihailongwan located in northeastern China to date recent lacustrine sediments and resolve global/local fallout sources. The apparently low 240Pu/239Pu atom ratio of 0.103 +/- 0.010 at Lake Sugan was likely representative of the fallout from the Lop Nor site. Our results also demonstrated that the 239+240Pu activity was more useful for recent chronology of lacustrine sediments, compared to 137Cs. Sediment accumulation rates of 0.651, 0.058-0.061, and 0.015 g x cm(-2) x a(-1) for sediment cores of Lake Shuangta, Lake Sugan and Lake Sihailongwan, respectively, were obtained by using the 239+240Pu fallout peak as a discrete time marker for 1964. The respective contributions of 137Cs and 239+240Pu from the CNT were estimated to be about 40 and 27% for Lake Sugan and 36 and 20% for Lake Shuangta. Plutonium isotopes in lacustrine sediments were proven to be useful for quantifying sediment accumulation rates and for source identification of the radioactive contamination.

Pu isotopes in soils collected downwind from Lop Nor: regional fallout vs. global fallout

“…Two extremely low 240 Pu/ 239 Pu atom ratios of 0.059 and 0.085 were observed for YM1 (20–25 cm) and YM2 (6–8 cm) locations, respectively. Similar results were also reported in lake sediments collected in northwest China and the CNTs were suggested to contribute to the Pu contamination in these lakes 18 19 . The present results for the soil core samples collected downwind from the CNT site further revealed that the 240 Pu/ 239 Pu atom ratios derived from the regional fallout of the CNTs could be lower than the global fallout value.…”

“…For example, Jin et al 16 reported a 240 Pu/ 239 Pu atom ratio of 0.16 in the soil of Gansu Province. More recently, Wu et al 18 and Liao et al 19 found 240 Pu/ 239 Pu atom ratios of 0.103 and 0.080 in deep sediments collected from Lake Sugan in the downwind direction from the CNT site and from Lake Boston in the upwind direction, respectively. These results suggested that the 240 Pu/ 239 Pu atom ratio from the CNTs could also be lower than the global fallout value.…”

“…1200 km from the CNT site) in Gansu Province and the determined 240 Pu/ 239 Pu atom ratio was 0.188 ± 0.009, revealing that global fallout was the main source for Pu contamination in their sampling place. Two other studies focused on the characterization of Pu in sediments from lakes in Gansu and Xinjiang Provinces and low 240 Pu/ 239 Pu atom ratios down to 0.08 were observed 18 19 . These studies suggested that the freshwater lakes around the CNT site received Pu deposition from the CNTs and the 240 Pu/ 239 Pu atom ratio from the CNTs could be lower than that of global fallout.…”

Plutonium in Soils from Northeast China and Its Potential Application for Evaluation of Soil Erosion

“…Similar 240 Pu/ 239 Pu atomic ratios of 0.157 ~ 193 with an average of 0.182 ± 0.012 (1σ) have also been reported in a sediment core collected in Lake Sihailongwan, northeast China (42°20′N, 126°50′E)33. In addition, the measured 239 + 240 Pu/ 137 Cs activity ratios (Supplementary Table S1 online) in all surface soil samples (~0.045, decay corrected to 1 st Sept. 2009) agreed well with the reported values (0.030 ~ 0.043) in soils contaminated only by global atmospheric fallout34.…”

“…Based on these facts, it might be concluded that the direct contribution of the close-in fallout from Chernobyl accident to the Pu in soils in the northeast China should be negligible. Moreover, the reported 240 Pu/ 239 Pu atomic ratios of close-in fallout Pu in the Semipalatinsk NWT (0.04 ~ 0.07)14 (more than 3400 km distance to the present sampling locations), the Chinese NWT (0.08 ~ 0.103)3338 and from the Kyshtym accident (0.0282 ± 0.0002)45 were all obviously lower than those (around 0.18) observed in this work, indicating that any close-in fallout contribution from these two NWT sites as well as from Kyshtym site could also be negligible in our sampling sites.…”