Source, Spatial Distribution and Migration of <sup>239+240</sup>Pu in Chinese Soils

Abstract

The source, spatial distribution, typical vertical distribution and migration of 239+240 Pu in Chinese 647 surface soils and 66 soil cores are investigated in this study. To provide a complete spatial distribution of 239+240 Pu in Chinese surface soil, an overall pattern of and mechanism for 239+240 Pu migration in Chinese core soil, we summarize and analyze the 239+240 Pu surface and vertical distributions in literature. Comprehensive statistical analysis suggests four typical distributions in Chinese soil cores. It is found that the 239+240 Pu vertical distribution is closely related to the soil type and the 239+240 Pu migration. The result indicates that global fallout is the major source of 239+240 Pu in Chinese soils, with the average 240 Pu/ 239 Pu atom ratios around 0.18. Abnormally low 240 Pu/ 239 Pu ratios are detected in Jiuquan region and northern Xinjiang province in Northwest China, which can be attributed to nuclear weapons tests. To further investigate the migration behaviors of 239+240 Pu in soil cores in China, we apply the Convection Dispersion Equation (CDE) model and find that 239+240 Pu apparent convection velocities ranges from 0.00032±0.00031 cm/a to 0.473±0.083 cm/a. As shown in four typical vertical 239+240 Pu activity distribution soil cores, the deepened 239+240 Pu activity maximum value position implies a fast 239+240 Pu migration rate or apparent convection velocity. This study for the first time suggests a significant linear correlation ( R 2 =0.847) between depth of 239+240 Pu maximum value (cm) and apparent convection velocity (v, cm/a), and we speculate that v usually does not exceed 0.5 cm/a. A significant linear correlation is also identified between apparent dispersion coefficient (D, cm 2 /a) and v 2 in Chinese soil cores.