Distribution and migration characteristics of 137Cs, 239+240Pu, and natural radioactive nuclides in the soil of Leye Tiankeng, China

Abstract

To investigate the distribution characteristics of radioactive nuclides in the soil of typical karst Tiankeng areas, two soil core samples and thirty surface soil samples were collected from the Leye Tiankeng Group, northwest Guangxi province in China. The soil's vertical distribution of ²³⁸U, ²³²Th, ²²⁶Ra, and ⁴⁰K is uniform. Furthermore, the distribution trends of ²³⁸U, ²³²Th, and ²²⁶Ra in the surface soil are similar. The activity range of ^239+240Pu and ¹³⁷Cs in surface samples was 0.037 ± 0.008 to 1.680 ± 0.037 Bq/kg and 0.86 ± 0.09 to 17.25 ± 0.38 Bq/kg, respectively. The specific activity ranges of ¹³⁷Cs in two core samples were 0.12–4.36 Bq/kg and 0.29−4.76 Bq/kg, respectively, while those of ^239+240Pu in core samples were 0.001–0.321 Bq/kg and 0.004−0.390 Bq/kg, respectively. The atomic ratios of ²⁴⁰Pu/²³⁹Pu and the activity ratios of ¹³⁷Cs/^239+240Pu both indicate that the artificial radioactive nuclides in this region originated from atmospheric fallout. The sedimentation rates derived from the distribution curve of ²¹⁰Pb_ex and distribution of ¹³⁷Cs and ^239+240Pu in soil exhibit considerable parallel. The distribution of ^239+240Pu and ¹³⁷Cs in soil exhibits a high degree of similarity, and the deviations are mainly attributed to the influence of the physical and chemical properties of the soil. Finally, the distribution of two nuclides, ^239+240Pu and ¹³⁷Cs, in the research region after 100 years was predicted using the CDE model.